Search

The high-performance Al12Si4Cu3NiMg alloy with massive transition elements has been developed recently for piston of diesel engines. The influences of microstructures on the tensile and low-cycle fatigue (LCF) behaviors were investigated with conventional and in situ testing technologies in this study. Upon the tensile loading, the microcracks initiate from the primary Si near the surface when the tensile stress reaches approximately 280 MPa (close to the tensile strength); then, the microcracks grow rapidly through eutectic Si and intermetallics (Al3CuNi) for the gravity casting (GC) alloy. With regard to the LCF, the microcracks initiate from the shrinkage pores regularly because of the localization of fatigue damage in the GC alloy. The shrinkage pores, usually accompanied by the brittle intermetallics (Al3CuNi), typically exhibit more complex geometry. The lamellar Al3CuNi provides a preferred path for the crack propagation after fatigue crack initiation. The ultrasonic melt treatment (UT) was used to optimize the microstructures and reduce casting defects (shrinkage pores), thereby resulting in improvement in tensile and LCF properties. For the UT alloy (pore-free), the microcrack initiates from some coarser primary Si rather than from the shrinkage pores because of the accumulation of micro-scale plastic deformation. Furthermore, higher resistance of crack propagation can also be realized with refinement of microstructures in the UT alloy. Increase in crack initiation and propagation resistances is the main reason for improvement in the LCF life of the UT alloy.

Microstructures, mechanical properties and creep behavior of a gravity casting Mg−3Yb−0.6Zn−0.4Zr (wt%) alloy were investigated. The results indicate that the studied alloy has smaller grains compared with the conventional Mg−RE based casting alloys, and the dominant intermetallic phase is Mg2Yb although minor Mg41Yb5 was also found in grain interior. In addition, the studied alloy exhibits medium tensile strength but excellent ductility at room temperature. Simultaneously, a complicated serrated flow phenomenon was observed in its flow curves at 150–250 °C. Finally, we found that the studied alloy exhibits poor creep resistance compared with the conventional Mg−RE based alloys. This could be mainly attributed to both poor thermal stability Mg2Yb phase and discontinuous intermetallic phase skeleton. Also, based on the analyses of the stress exponent and activation energy as well as microstructural observation, dislocation climb was proposed as the dominated creep mechanisms of the studied alloy at 180–220 °C and 40–80 MPa.

This study investigates the low-cycle fatigue properties of AZ31 sheets with different thicknesses of 1, 1.5, 2, and 3 mm—which are fabricated by twin-roll casting and subsequent hot rolling—through fully reversed strain-controlled fatigue tests. As the thickness of the sheets decreases, their average grain size decreases, texture intensity increases, and tensile yield strength and elongation gradually increase. At strain amplitudes of greater than or equal to 0.6%, {10–12} twinning in compression and detwinning and subsequent slip in tension occur repeatedly, which forms asymmetric hysteresis loops. The different grain sizes of the sheets result in different compressive peak stresses during the fatigue tests. However, the overall cyclic deformation behavior is similar in all the sheets, and consequently, their fatigue lives exhibit an insignificant difference. The loading direction also has a negligible influence on both the cyclic deformation behavior and the fatigue life, which implies that the sheets exhibit in-plane isotropic fatigue properties. The stress amplitude and plastic strain amplitude vary considerably during the fatigue test. In contrast, the variation in total strain energy density is insignificant over the entire fatigue life, and therefore, it is a proper fatigue damage parameter for predicting fatigue life. A unified fatigue life prediction equation using the total strain energy density is established, and the fatigue lives predicted using the equation are found to be in good agreement with the experimentally determined values, regardless of the strain amplitude, sheet thickness, and loading direction.

In this investigation, the effect of pouring temperature and the surface angle of vortex casting on microstructural evolution and mechanical properties of 7050Al-3 wt%SiC composite was studied. Two pouring temperatures of 700 °C and 800 °C were employed during the vortex casting. As well, two surface angles of 45° and 90° were used for the vortex casting. Cast microstructures were analyzed by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and optical microscope. Mechanical properties were measured by uniaxial tensile test, Vickers hardness and Charpy impact test. Microstructural studies showed a uniform distribution of SiC particles in the 7050 Al matrix is achievable by increasing the pouring temperature from 700 °C to 800 °C and also by decreasing the surface angle from 90° to 45°. This uniform distribution of particles was coincided with the breaking of the dendrite arms in the microstructure of 7050 Al-3 wt% SiC. Mechanical properties measurements showed that the yield strength, hardness and fracture energy of the composite were increased by increasing the pouring temperature and reducing the surface angle during the vortex casting.

The influence of melt flow in the gating system on microstructure and mechanical properties of high pressure die casting AZ91D magnesium alloy was investigated. Experimental and theoretical results showed that the externally solidified crystals (ESCs) were broken and eliminated by the shear stress caused by the turbulent melt flow in gating system. In comparison with the casting fabricated with conventional gating system, a simultaneous increase in both strength (~ 16% higher ultimate tensile strength) and ductility (~ 70% higher elongation) were achieved in the casting fabricated with modified gating system. Furthermore, different modes of crack initiation and propagation were found. For the one fabricated with the conventional gating system, the crack initiated at the shrinkage pore and propagated in a quasi-cleavage fracture form. In contrast, for the other one fabricated with the modified gating system, micro-voids appeared between the α-Mg grains and β-Mg17Al12 phase, and crack propagated in a ductile form.

Compact endless cast and rolling mill (CEM) is an innovative process to manufacture a hot-rolled steel strip by combining casting and hot-rolling processes. However, the yield point phenomenon (YPP) of low carbon steel strip-processed by CEM induces a negative effect in the steel product and an additional post-process is required to remove the YPP. In this study, the influence of the dislocation density and the grain interior solute atoms on the yield point elongation (YPE) of low carbon steels were quantitatively investigated using 3-dimensional atom probe tomography analysis and X-ray convolutional multiple whole profile fitting. The YPE of low carbon steel is suppressed as the dislocation density increases and carbon atom content decreases. Because the dislocation density of low carbon steel by the CEM process is increased by lowering the processing temperature, the yielding behavior of the CEM products can be eliminated without any additional post-processing. The quantitative study on the carbon/dislocation density and YPP of low carbon steel not only represents the theoretical basis of the role of carbon-dislocation interaction on YPP but also provides an effective solution to optimize the CEM process for superior products.

Precipitation-strengthened Ni-based superalloys are leading candidate materials for advanced ultra-supercritical (A-USC) plants with steam conditions up to 760 °C (1400 °F) and 35 MPa (5 ksi). This study evaluates representative specimens from a large casting of Haynes 282 to study the effect of microstructural heterogeneity on the mechanical behavior of this alloy. The tensile test results of cast Haynes 282 over the temperature range 20–816 °C exhibited lower tensile strength and ductility in comparison with the reference wrought Haynes 282. However, the creep rupture tests of the cast alloy below 704–788 °C and 190–431 MPa presented a similar stress-Larson-Miller parameter to that of the wrought material. Microstructural and dislocation characterizations using scanning electron microscopy, conventional transmission electron microscopy, and scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy were performed to understand the microstructural evolution before and after the mechanical tests. The heterogeneous microstructures of the cast Haynes 282 material, including the coarse grains, potential casting defects, and a bimodal size distribution of γ′ precipitates, were detrimental to the tensile behavior, whereas the coarse-scale grains had a positive effect on the creep performance because diffusional creep was the dominant creep mechanism.

This work examines the processing of a hot-work tool steel using laser-based powder bed fusion of metals (PBF-LB/M). The hot-work tool steel was produced using a low-cost powder mixture consisting of pure iron and other elemental powders as well as ferroalloys. Furthermore, a prealloyed starting powder with the same nominal chemical composition as the powder mixture was produced by inert-gas atomization. Besides, a reference steel was produced by casting to compare the microstructures and mechanical properties resulting from the different processing routes. The first step examined the application of a chemically homogeneous and dense layer of the powder mixture prior to PBF-LB/M densification. In addition to evaluate suitable process parameters for PBF-LB/M processing of the starting materials, the microstructure formation was comprehensively examined using electron microscopy and the processes adapted to it. To eliminate defects (cracks, pores) and chemical inhomogeneities, thermal posttreatments, namely supersolidus liquid phase heat-treatment (SLPHT) and hot isostatic pressing (HIP) were performed. Suitable heat-treatment parameters were evaluated. Finally, the obtained microstructures and the associated properties of the post-processed PBF-LB/M samples were compared with those in the reference states. As a main result, it was possible to achieve full redensification and simultaneous chemical homogenization of the PBF-LB/M-processed powder mixture by SLPHT post-processing. The hardness of the additively manufactured and SLPHT-post-processed specimens exceeds that of the cast reference.

The low cycle fatigue response of a single crystal γ ′ -containing CoNi-base superalloy has been investigated in air and in vacuum at 950 ° C . The fatigue performance is demonstrated to be similar to the 1st-generation single crystal Ni-base superalloy AM1. Fracture surface imaging and cross-sections on both alloys reveal that oxidation-assisted surface cracking is responsible for fatigue crack initiation in air. The fatigue performance improves under vacuum for both alloys as crack initiation transitions to internal casting pores retained from the solidification process. Recrystallized regions on the fracture surfaces of the vacuum-tested samples of both alloys are identified by electron backscattered diffraction, which has not been previously reported for single crystal Ni-base alloys at these test conditions. The role of the base element (Ni- vs. CoNi- vs. Co-base) on the fatigue performance is discussed.

As a promising material for application in lightweight automobile, Al–Si alloys are subjected to complex cyclic thermo-mechanical loading when employed in engines. This study examined the high-cycle fatigue behavior of Al–12Si–4Cu-1.2Mn-T6 cast aluminum alloy at room temperature (RT) and 350 °C and evaluated the effect of different second phases by microstructure observation and nano-indentation characterization. The high-cycle fatigue strength of the studied alloy at RT and 350 °C is 125.0 MPa and 47.5 MPa, respectively, exhibiting excellent fatigue resistance compared with other heat-resistant Al alloys. Fatigue cracks originate from casting defects at RT, but nucleate from primary Al15Mn3Si2 phase on the specimen surface at 350 °C. The modulus difference between α-Al15Mn3Si2 and α-Al phases is higher at 350 °C than at RT, allowing cracks to initiate and propagate more likely along the phase interface at 350 °C. The hardness and reduced modulus of α-Al15Mn3Si2 phase are greatly higher at 350 °C than at RT, both in the suspended specimen and the fractured specimen. The significant difference in residual stress between α-Al15Mn3Si2 and α-Al phases indicates that α-Al15Mn3Si2 phase bears more stress loading than α-Al phase during fatigue process, especially at 350 °C. The findings presented in this study may shed some light on the design of high-performance heat-resistant aluminum alloys.

It is crucial to clarify the intrinsic deformation and damage behaviors of casting materials under different loading conditions for establishing the material damage model and accurately predicting the security performance of cast components. In this study, uniaxial tensile and compressive tests were performed to investigate the complete mechanical behavior of cast A356-T6 aluminum alloy at room temperature, and the corresponding microstructure evolution were analyzed. The tensile and compressive specimens were prepared at porosity-free locations on a low pressure die cast A356 wheel subjected to T6 heat treatment to avoid premature fracture caused by initial cast pores. The results show that negligible asymmetry in yielding and hardening behaviors is observed between tension and compression, but obvious differences in ductility exhibit. Specifically, the elongation for tension is limited to 15%, but reaches over a strain of 1.2 for compression. Additionally, microcracks during tension are prone to be initiated at the interface between Si and Al matrix, which mainly propagates in the Al dendrites, and finally leads to the fracture. While the dominant strain accommodated mechanism shifts with strain levels in compression. It evolves from the incipient plastic deformation inside grains at initial strain stage to the sliding of fragmented dendrite assisted by eutectic silicon particles at later stage, which may be the main reason for the superior ductility during compression.

© 2021 The Authors. This article unmasks the probabilistic nature of high-pressure die casting; specifically, the cause of scatter in the tensile ductility of die-cast Al8Si0·4Mn0·3Mg (wt.%) alloy. Scatter in tensile ductility is related to the size of large pores and non-metallic inclusions. We propose that these non-metallic inclusions form during the pyrolysis of commercial plunger lubricants, and that these large pores derive from dilatational strains introduced during semi-solid deformation. The apparent randomness of pore formation is thus ascribed to the heterogeneous nature of the semi-solid network. Reducing heat loss in the shot chamber is shown to promote a more homogeneous grain structure, leading to a decrease in the maximum pore size from 1.32 mm to 0.37 mm, and an increase in the minimum tensile ductility from 6.8% to 9.4%. Engineering and Physical Sciences Research Council and Jaguar Land Rover Ltd. [project reference 2043200].

Ternary Mg 94 Ni 2 Y 4 (at%) alloy was prepared by conventional gravity casting or direct squeezing casting and then subjected to T6 heat treatment. The influence of squeeze casting pressure and heat treatment on microstructure evolution and mechanical properties of Mg 94 Ni 2 Y 4 alloy containing long period stacking ordered (LPSO) structure was studied. The results show that gravity cast Mg 94 Ni 2 Y 4 alloy consisted of primary α-Mg, 18 R LPSO and eutectic structure containing α-Mg, LPSO and Mg 2 Ni phases. The application of pressure cannot change phase constituents and the type of LPSO phase. With the increase of squeeze casting pressure, microstructure was refined, the volume fraction of LPSO was decreased while that of eutectic structure was increased. Under the highest pressure of 320 MPa, kinking of LPSO phase was induced and the best mechanical properties were obtained, with the ultimate tensile strength of 250 MPa, the yield strength of 140 MPa and the elongation of 8%. They were improved by 20.2%, 22.8% and 45.5%, respectively, than those of gravity cast Mg 94 Ni 2 Y 4 . After heat treatment, bulk-shaped LPSO phase transformed into rod-shaped one, but no lamellar 14 H LPSO was precipitated in the matrix. Besides, the application of squeeze casting pressure prevented LPSO from delaminating during heat treatment.

Commercially pure titanium was produced using laser engineered net shaping (LENS) and selective laser melting (SLM) processes. The SLM and LENS processing parameters as well as critical aspects including densification and balling effect were investigated. The resulting properties were studied and compared with those from traditional casting. Investigation of the processing parameters showed that significantly higher laser power and energy density is required in LENS compared to SLM in order to obtain near full density (99.5%). The microstructural investigations revealed an α microstructure with mixed morphologies including plate-like and widmanstatten for LENS somewhat similar to the serrated and fine acicular α obtained from casting. In contrast, the SLM samples showed only martensitic α′ phase mainly with a lath-type morphology. The difference between SLM and LENS microstructures was discussed based on interrelated aspects including energy density, solidification rate and specific point energy. Differences in their microstructures are mainly associated with differing rates of cooling and differing energy densities during SLM and LENS processing. Compression and hardness tests indicated that SLM titanium possesses better mechanical properties due to a fine grain size and martensitic phase composition, whereas LENS and cast titanium with α microstructures show similar mechanical properties.

The low-cycle fatigue test and the compression creep-fatigue test of the cast Al-9Si-CuMg alloy for cylinder heads were carried out at high temperature. The cyclic mechanical response characteristics of the material under two kinds of test loads were analyzed. The microstructural evolution process and fracture mechanism of the material in the process of failure were studied, and the damage modes of the material under two kinds of loads were summarized. The results showed that, the stress-strain response (hysteresis loop) of the cast Al-9Si-CuMg alloy exhibited cyclic stability under high temperature low-cycle fatigue loading at 200 ℃, and the crack occurred at the holes and defects. When the temperature was raised to 350 ℃, the voids and defects in the material would no longer be the crack source, and the cyclic stress-strain response of the material would also show the characteristics of continuous softening. When coupled with the effect of compressive creep, the yielding radius and elastic modulus of the cyclic stress-strain response were almost unchanged, but the yield center moved upward significantly. Microscopic observation indicated that, to some extent, the effect of compressive creep at high temperature could prevent the initiation and propagation of the large-sized crack within the material, and prevent the rapid development of damage under low-cycle fatigue loading. The eutectic silicon particles inside the material firstly cracked under the compressive creep-fatigue load and formed many tiny cracks or cavities. These tiny cracks extended in the aluminum matrix and connected with the neighboring cracks until they broke.

The microstructures and mechanical properties of three Mg-Zn-Gd-Zr alloys with specific Zn/Gd mass ratios to form different types of phases are systematically investigated by casting and subsequent indirect hot extrusion. With the Zn/Gd ratio increases from 0.27 to 1.4, second phases change from W (Mg3Gd2Zn3), X (Mg12GdZn) and Mg5(Gd,Zn) to W, I (Mg3Zn6Gd) and MgZn2. After extrusion, the needle-like and dispersively distributed I-phase provides effective strengthening effects to the experimental alloys. The lamellar-like X-phase in the as-extruded condition is hard and fragile to start cracks. W-phase owns superior ductility in these alloys. A high performance I-phase containing Mg-7Zn-5Gd-0.6Zr alloy is prepared by conventional casting and subsequent hot extrusion. It owns an ultimate and yield strength of 350 MPa and 285 MPa, respectively, and its elongation is 13.6%.

Casting is one of the preferred routes to manufacture net shape parts. Major concern in this process is to obtain suitable microstructure in the cast parts and thereby achieving desired mechanical properties. Recently, a new approach in metal casting called ‘in-situ microwave casting’ (MWC) has been reported in which microwave energy is used during melting, pouring and solidification of the cast material. The complete process is carried out inside the applicator cavity which adds more flexibility due to control over the mold temperature and solidification inside the applicator. In the present work, some studies/analysis on microstructure and mechanical properties of the developed in-situ microwave casts of aluminum 7039 alloy has been presented. The alloy was cast in-situ in an ambient atmosphere inside a microwave applicator using microwave energy at 2.45 GHz and 1400 W. Characterization of the in-situ casts was carried out to study the grain structure, phases and their distribution, porosity and mechanical properties. A dense cast was obtained with porosity less than 2%. The in-situ cast consists of MgZn 2 , Mg 2 Si, Al 3 Fe and Al 8 Fe 2 Si precipitates and intermetallic phases. Ultimate tensile strength (UTS) of the in-situ microwave cast was observed to be 148.46±10 MPa and the average micro indentation hardness was observed higher near the gain boundaries (maximum 132 HV) than inside the α-Al grains (maximum 120 HV).

This paper presents the first report of ultrasonic treatment (UST) assisted casting of independent 2D layered hexagonal boron nitride (hBN) reinforced aluminum matrix composites. A linear enhancement of nucleation density is obtained with treatment time from 0 to 90 s from dispersion of hBN platelets by UST. Outstanding grain refinement of 72% from 375.3 to 103.6 μm is obtained from UST for 90 s. The corresponding nanohardness is 709.2 MPa, a hardening of 36% due to symbiotic refinement and dispersion contributions. Anchoring and orientation of hBN platelets dictate that the elastic modulus improved up to 150% by ultrasonication for 90 s. The exfoliated 2D reinforcement layers reduce the coefficient of friction during sliding wear by 15% and loss of worn material by 46% in these composites. The mechanism of dispersion, improvement of properties, and quantitative correlations constitute a step of advancement towards understanding the role of 2D particle morphology and dimensionality on the ultrasonic casting of metal matrix composites.

Demanding environmental legislation and improve performance specifications requires increasing fatigue strength for the engine components that are made of lamellar graphite iron (LGI). Components design, metallurgy and casting conditions define the microstructure formation and mechanical properties. The graphite inclusions embedded in the metallic matrix acting as defects and have a detrimental effect on the fatigue strength of LGI. The cooling conditions determine the coarseness of the microstructure and also have, a great impact on the fatigue resistance. The experimental material was an LGI alloy produced with three different solidification times. A fully reversed fatigue test was performed, and various microstructure features were quantitatively estimated by utilizing the Gumbel's statistics of extremes. The stereological parameter of Hydraulic Diameter of the Inter-dendritic Phase and the graphite Feret size found to be the most suitable microstructure parameters to be correlated with the fatigue limit. The results also indicate the sizeable effect of the solidification time on the fatigue limit. Several other microstructure features that have been reported to influence the tensile strength were also found to be related to the fatigue limit. The obtained endurance ratio ranges from 0.25 to 0.30, a value that is in line with previous investigations.

Stir followed by squeeze casting techniques were used to produce A359 composites containing different weight percentage of (SiC+Si3N4) particles. Microstructures of the composites showed a homogeneous and even distribution of hybrid reinforcements within the matrix. Moreover, particles agglomerations, residual porosity, and other casting problems were not noticed. Interfacial reactions between the particles and the matrix were investigated using X-ray diffraction and energy dispersive X-ray analyses. The presence of particles in squeezed composites did not only increase the peak hardness of the composites, but also accelerated the aging kinetics. As compared with the A359 matrix alloy, a compression test of the hybrid composites exhibited a significant increase in the yield and the ultimate compressive strengths with a relative reduction in the failure strain. Finite element modeling of the composite compression showed that strain concentration near large SiC particles is the main reason for low ductility of the composite. The development of those lightweight hybrid composites with high mechanical properties has a high potential to be used for automotive and aerospace applications.

In the current work, three different solidification routes and a two-step heat treatment process were applied to a novel FeCrNiBSi alloy system to introduce a new candidate for advanced high-strength steels. The evolution of the microstructure after solidification, heat treatment, and tensile deformation was characterized using optical and electron microscopy techniques, as well as hardness and room temperature uniaxial tensile tests. The effects of the different solidification routes and heat treatment parameters on the deformation and fracture mechanisms of this steel are discussed. Grain refinement, precipitation hardening, and solid solution as a result of the fast casting cooling rate led to an increase in strength at improved ductility. This result can be explained partly by the less severe stress/strain partitioning at the matrix grain/M 2 B interfaces and better interface cohesion. Moreover, the stress/strain partitioning characteristics between the matrix grains and M 2 B led to a higher initial strain hardening rate. The fast casting cooling rate further promoted ductile fracture mechanisms, which is a result of increased cleavage fracture stress. The higher casting cooling rate and two-step heat treatment resulted in a strong increase in formability index, from 8 GPa% to 24 GPa%, at which the mechanical properties occupy the TRIP envelope. Heat treatment of the fast-cooling specimens led to a small reduction in yield and tensile strength and 22% total elongation percentage improvement (from 10% to 32%).

Mechanical properties and failure characteristics of Mg97Y2Zn1 (at%) alloys, prepared by gravity casting and extrusion processes, are examined experimentally. High tensile and fatigue strengths are obtained for extruded Mg97Y2Zn1 alloys, with the values being approximately twice as high as those for cast Mg97Y2Zn1 alloys. Such high strengths are attributed to the high internal stress arising from the complicated microstructure and severe lattice strain in the α-Mg and long-period stacking order phases. On the other hand, a high fracture strain is obtained for cast Mg97Y2Zn1 alloy, which is affected not only by the lower internal stress, but also severe kink deformation. The failure characteristics, e.g., crack growth characteristics are examined via direct observation during tensile and fatigue tests at micro- and nano-scales. Meandering fatigue crack growth is obtained for the cast Mg97Y2Zn1 alloys. The mean roughness of the fracture surface for cast Mg97Y2Zn1 alloys is more than twice as high as that for commercially available AM60 and AZ91 cast alloys. A coarse fracture surface that was observed in the cast Mg97Y2Zn1 alloys would make severe roughness-induced crack closure, leading to high fatigue strength.

Twin roll casting (TRC) is recognised as an effective processing route for producing low cost Al sheet. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centre-line segregation which prevents its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed from the industrial point of view. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front which results in reduced sump depth and a refined, uniform, defect free microstructure. The results obtained show that the differences in microstructure and in the crystallographic texture have a significant impact on ductility (22% improvement) of the MC-TRC sheets compared to TRC sheets.

The progress on thermal mechanical fatigue (TMF) of Ni-based single crystal superalloys is summarized from an industrial perspective. The TMF behaviors of bare alloys at relatively high stress regime are well studied. However, the understanding of coated alloys is fairly limited, particularly at low stress region pertinent to application conditions. The potential influence of microstructure, chemistry, oxidation and bond coat on TMF life is discussed. Research directions for future studies are proposed.

Ductile Ni-resist cast iron (type D-5S) which is normally used in exhaust manifold in automotive industries mainly contains up to 29 wt% nickel, 4.9 wt% silicon and 1.7 wt% chromium. In this study, the microstructure and mechanical properties of as cast ductile Ni-resist cast iron have been investigated at room temperature. The microstructure consists of graphite nodules embedded in austenitic matrix and carbides. The FeNi 3 phase also found in the austenitic matrix. By adding inoculant, carbide formation tendency decreased and led to subsequent decrease of the hardness. Furthermore, tensile properties improved by inoculation exhibiting ultimate tensile strength and yield strength from the range of 273–303 MPa and 207–255 MPa for samples without inoculant to respectively 285–449 MPa and 260–334 MPa for inoculated samples.

The constitutive behaviour of the aluminium-copper casting alloy B206 has been investigated in both the as-cast and artificially aged states. For the as-cast material, a unified plastic flow stress model has been developed using experimental data acquired from compression tests performed using a Gleeble 3500 thermo-mechanical simulator. The unified model considers different constitutive models, each for a specific temperature interval, and the transition between these models at an intermediate temperature based on the material's strain rate sensitivity. For the artificially aged material, a linear-fit yield strength evolution model as a function of the heat treatment parameters has been developed using experimental data from tensile tests and hardness measurements. The yield strength model takes advantage of an independently developed microstructure model that specifically describes the precipitation kinetics of the material based on differential scanning calorimetry measurements. Evaluations of the developed models show good fits between the predicted strengths and data from experiments as well as the literature.

Additive manufacturing via selective laser melting can result in variable levels of internal porosity both between build plates and within components from the same build. In this investigation, sample porosity levels were compared to tensile properties for 176 samples spanning eight different build plates. Sample porosity was measured both by Archimedes density, which provided an estimation of overall porosity, and by observation of voids in the fracture surface, which provided an estimation of the porosity at the failure plane. The porosity observed at the fracture surface consistently demonstrated higher porosity than that suggested by Archimedes density. The porosity values obtained from both methods were compared against the mechanical results. Sample porosity appears to have some correlation to the ultimate tensile strength, yield strength, and modulus, but the strongest relationship is observed between porosity and ductility. Three different models were used to relate the fracture surface porosity to the ductility. The first method was a simple linear regression analysis, while the other two models have been used to relate porosity to ductility in cast alloys. It is shown that all three models fit the data well over the observed porosity ranges, suggesting that the models taken from casting theory can extend to additively manufactured metals. Finally, it is proposed that the non-destructive Archimedes method could be used to estimate an approximate sample ductility through the use of correlations realized here. Such a relationship could prove useful for design and for a deeper understanding of the impact of pores on tensile behavior.

Co–Cr–Mo based alloy was widely used biomaterials for metallic implants fields. Recently, selective lase melting (SLM), one of the additive manufacturing technologies, is broadly studied because it can produce metallic implants with complex geometries and high degree of personalization. However, the possibility of mechanical failure must be avoided during the working period of SLM produced biomaterials. Therefore, in this paper we investigated the fatigue behaviors of the SLM Co–Cr–Mo–W alloy and their corresponding microstructure and fracture morphology. The sand mold casting fabricated (Cast) Co–Cr–Mo–W alloys with the same chemical composition as SLM specimens were also investigated and compared. The results showed that the SLM Co–Cr–Mo–W alloy exhibits higher fatigue life, tensile strength, and ductility than Cast Co–Cr–Mo–W alloy. The high fraction of Σ3 boundaries and precipitates in SLM Co–Cr–Mo–W alloy lead to secondary cracks and deflected fatigue crack path, which can alleviate the stress concentrations of the main fatigue cracks and decrease the fatigue crack growth rate. However, the dendritic structure of Cast Co–Cr–Mo–W alloy cause cleavage fracture mode and the large island-like precipitates can generate stress concentration, which result in lower fatigue life. We believed that the results of this work can provide a new perspective towards microstructure regulation for additive manufacturing technology and paved way for in-depth application of SLM fabricated biomedical alloy.

This paper compares key aspects of the manufacture and mechanical performance of porous Al, made by replication using salt beads, and structurally similar syntactic metal foams, made by replacing the salt with porous, expanded glass particles. Despite significant increases in stiffness, strength and energy absorbed, power law relationships between properties and relative density demonstrate that adding expanded glass particles is a less efficient route to increasing performance than increasing the metal fraction. That said, merit indices do indicate that 20% savings in mass are possible for stiff beams by substituting aluminium with syntactic metal foams. The manufacturing process for syntactic metal foams is shown to be simpler, driven by the lower thermal mass of the expanded glass particles and no requirement to dissolve a space holder from the structure. The balance of good performance and good manufacturability demonstrated herein, coupled with predicted low costs for raw material and manufacturing, highlights the scope for syntactic metal foams containing weak expanded glass particles to be researched and developed more widely.

This study quantitatively describes the mutual contributions of microporosity and loading conditions (i.e. strain rate and stress amplitude) on the fatigue properties of low-pressure die-cast A356 alloy, in terms of the defect susceptibility coefficient of fatigue life to microporosity variation under various loading conditions. A fatigue test is conducted in high-cycle axial fatigue mode with a variation of strain rate, applying different loading frequencies (0.3–30 Hz) in a stress ratio of R = - 1.0. Computational topography (CT) analysis and scanning-electron microscope (SEM) fractographic observations were performed to evaluate the size and distribution of micro-voids. The overall dependence of fatigue life on microporosity variation can be linearly described as an exponential form of the defect susceptibility coefficient of fatigue life to the microporosity variation, with the maximum fatigue life achievable in a defect-free condition. The contribution of microporosity to fatigue life becomes insensitive as the strain rate increases at given stress amplitudes, whereas it becomes more dominant with the increase of stress amplitude at a given strain rate. Even though the existence of micro-voids inside a material fundamentally influences major stress concentrations and the origins of fatigue-crack propagation, its practical contribution to fatigue life by the variation of strain rate and stress amplitude is intimately related to the stress concentration by position at the near-circumference of the specimen, rather than the nominal variation of load-carrying capacity by the size and area fraction of micro-voids.

A novel CoNiFeNb0.45 eutectic multi-principal element alloy (EMPEA) with a dual-phase structure is designed and synthesized by arc melting and suction casting. The compression strength of the CoNiFeNb0.45 EMPEA is 2.5 GPa at room temperature and high compression strength (1.2 GPa), furthermore, the excellent ductility with 40% fracture strain are maintained at 700 °C. The dislocation walls in the soft fcc phase provide the CoNiFeNb0.45 EMPEA with high ductility and work hardening, while the dislocations pined in the fcc/laves phase boundaries provide the alloy with high strength. The CoNiFeNb0.45 EMPEA also has excellent corrosion resistance in 3.5 wt% NaCl at room temperature as manifested by a corrosion rate of merely 0.003 mm/y and activation energy of corrosion of 53.6 kJ/mol.

AlSi7Mg0.6 alloy is widely used in the automotive and aeronautical industries, and metal additive manufacturing (AM) is a breakthrough technology that motivates foundry companies to explore its potential in these industries; however, there is no deep knowledge of the mechanical properties and their relationship to microstructure in parts obtained by selective laser melting (SLM), as there is for parts obtained by casting. In this work, a comparison of the microstructure and mechanical properties of AlSi7Mg0.6 alloy obtained by SLM and investment casting processes was made. The mechanical properties of tensile specimens processed by both technologies were evaluated by uniaxial tensile tests and microhardness measurements in as-built/as-cast and heat-treated conditions with different build orientations in the case of SLM. An advanced characterization of the microstructure by field emission scanning electron microscopy (FESEM) and x-ray diffraction (XRD) analysis was also performed. After analyzing the microstructure and mechanical properties obtained with different heat treatments, the strengthening mechanisms of the two processes were identified. It is possible to obtain improved mechanical properties with SLM processing, exceeding the typical values required for aeronautical parts obtained in investment casting heat-treated (T6), and the ductility is satisfactory. Direct aging after SLM processing can effectively strengthen the AlSi7Mg0.6 alloy and is the more effective way to improve the as-built mechanical properties.

In the present work, attempts have been made to design and develop low-carbon, low-alloy, high strength carbide-free bainitic steels for application in heavy-haul rail tracks. The study starts with an analytical approach for understanding the effects of different alloying elements on the thermodynamic and kinetic aspects of bainite transformation and the resultant microstructure and mechanical properties. Based on the analysis, two steel compositions were designed for further investigation. The alloys were prepared by melting and casting and processed by hot-rolling and air cooling, which is the usual route for the industrial production of rail sections. Microstructures of the developed steels primarily consisted of plates of carbide-free bainitic ferrite, interspersed with fine films of retained austenite. These continuously cooled steels offer ultra-high strength levels with very good ductility which are far superior to existing pearlitic grade rail steels. Detailed characterization and quantification of microstructural constituents have been carried out and they are correlated with the tensile proprieties using semi-empirical formulations available in literature.

Additive manufacturing (AM) is an emerging technology capable of producing near net shape structures in a variety of materials directly from a computer model. Standard metallic alloys that were developed for cast or wrought processing have largely been adopted for AM feedstock. In many applications, these legacy alloys are quite acceptable. In the aluminum alloy family, however, there is a significant performance gap between the casting alloys currently being used in AM processes and the high strength/toughness capability available in certain wrought alloys. The precipitation hardenable alloys, most often used in high performance structures, present challenges for processing by AM. The near net shape nature of AM processes does not allow for mechanical work prior to the heat treatment that is often necessary to develop a uniform distribution of precipitates and give peak mechanical performance. This paper examines the aluminum (Al) alloy 2139, a composition that is strengthened by homogeneous precipitation of Ω (Al2Cu) plates and thus ideally suited for near net shape processes like AM. Transmission electron microscopy, microhardness, and tensile testing determined that, with proper processing conditions, Al 2139 can be additively manufactured and subsequently heat treated to strength levels comparable to those of peak aged wrought Al 2139.

Plastic deformation under fracture surface in non-combustible magnesium alloy was investigated using electron backscatter diffraction analysis after tensile tests of specimens having a fatigue pre-crack or shrinkage porosity, so that it revealed that the fracture mode of shrinkage porosity of the magnesium alloy can be treated as a crack.

The microstructure and mechanical properties of columnar-grained copper under casting and drawing conditions were investigated in the present work. It was found that the as-casted copper mainly consisted of continuous columnar grains with orientation preference along and exhibited excellent plastic deformation ability. Moreover, the texture of columnar-grained copper transferred from to during the drawing process. A typical microstructural morphology as multangular-cup feature of as-casted copper as well as drawn copper was observed on the fracture surfaces. With increasing the drawn strain, the texture evolution and the grains broken occurred, leading to a significant increase in the work hardening rate and tensile strength.

The present article investigates the age hardening behavior of thixoformed A356–5TiB 2 in-situ composite prepared by a salt-metal reaction. The non-dendritic feedstock for thixoforming was prepared by a cooling slope casting technique. The results show that the thixoformed A356–5TiB 2 composite attains the peak hardness in only four hours. The detailed microstructural analysis of thixoformed composite shows that enhanced ageing can be attributed to high dislocation density generated in α-Al matrix due to thermal mismatch and the thixoforming process itself. Furthermore, it was found that thixoforming not only increases the tensile strength of thixoformed composite in peak-aged condition but also increases the ductility by ~7.4%.

To better understand the effects of aging on mechanical properties of cast Al–Si–Cu-based aluminum alloy (ADC12), artificial aging was carried out under a range of conditions after water quenching. Two casting methods were employed: gravity casting (GC) and heated-mold continuous casting (HMC). The Vickers hardness of GC samples was higher overall than that of HMC samples both before and after aging. This was due to the high solubility of the alloy elements in the α-Al matrix and to the presence of complicated dislocation walls in the GC samples. The hardness of GC and HMC samples aged at 433, 448 and 463 K increased similarly with increasing aging time, and an aging peak occurred in the age-hardening curve between 10 and 15 h. In contrast to the hardness results, the ultimate tensile strength (UTS) for the as-cast GC sample was about 30% lower than that for the as-cast HMC sample, because of large brittle eutectic Si and Fe structures. The highest UTS was obtained for the HMC sample aged at 448 K for 13 h, and was about 20% and 10% higher, respectively, than those of the as-cast HMC sample and the GC sample aged at 448 K for 13 h. The high UTS for the HMC sample was due to precipitation of θ′ (Al2Cu) metastable phases as well as the formation of tiny microstructures. With over-aging at 463 and 493 K for 100 h, the tensile strength decreased for both GC and HMC samples, although the fracture strain increased only for HMC samples (by more than 30%). Such high ductility in ADC12 alloy has not been seen in previous studies. In contrast, no clear improvement in material ductility was detected in GC samples. The tensile strength was directly related to fatigue properties, although no significant effect of ductility on fatigue strength was found.

The mechanical properties of Composite Metal Foams (CMFs) under low speed loading conditions have been considered in a number of studies. This paper aims to extend the current knowledge by investigating the compressive behavior of CMF under higher loading rates. Hopkinson bar experiment was conducted on samples processed through powder metallurgy and casting techniques. The effect of loading rate, sample geometry and sphere size on the mechanical properties and energy absorption capacity was studied. The obtained results reveal that increasing the loading rate improves the strength of CMF especially at strain levels below 30%. This strengthening due to high strain rate loading is mostly attributed to the strain rate sensitivity of the parent metals and the pressurization of the entrapped air inside the spheres.

Three steels (G22, G32 and SCOR) containing 0.15–0.19% phosphorous and different amounts of Cr, Cu and Ni were made in laboratory and characterized for evaluating mechanical and textural properties. The values of Charpy V notch impact (CVN) energy, YS, UTS and %EL of G22 and SCOR steels conformed to that of S235JR, S275JR and S335JR structural grades of EN10025-2:2004 specification. SCOR steel exhibited the highest CVN energy although it had the highest pearlite content (15.16%) and carbon equivalent (0.34). While CVN energy was suppressed significantly with increasing phosphorus concentration beyond 0.15%, it was also decreased on increasing Cr and decreasing Ni content as in the case of G32 steel. However, all specimens revealed dimpled fracture mode after tensile testing. In the case of G22 steel, Φ2 45° section of ODF obtained using EBSD revealed a weak but nearly uniform γ fiber texture (( )||ND), an indicative of good forming quality of hot rolled structurals. Hot tensile and compression test was performed for evaluating casting and hot rolling problems. The reduction in area (RA) values obtained through hot tensile test in temperature range of 850–1100 °C were found to be greater than 60% for G22 steel. In contrary to this, susceptibility to hot cracking of G32 steel was attributed to its minimum RA value (37%) at 900 °C. The flow stress characteristics of G22 steel at different temperatures were comparable to that of other commercial grade structural steels.

In this work the heat treatment response of SSM processed A356 and A357 casting alloys was analysed. The development of a new T6 heat treatment, with solution times less than 30 mm has been allowed. In this new short heat treatment, the alloy showed better mechanical properties compared to the same alloy which heat treated in standard conditions (solution time between 6 h and 8 h). This new heat treatment, carried out at the solution temperature of 540 degrees C, enabled the complete magnesium dissolution, while minimizing the grain growth of the alpha-phase and the eutectic silicon. Although this experiment was carried out on A356 and A357 aluminium alloys, conformed by Sub-Liquidus-Casting process (SLC), the results may be applied to the components produced with other semi-solid technologies or others Al-Si hardenable alloys that form coherent magnesium inter-metallic precipitates. (C) 2014 Elsevier B.V. All rights reserved.

In the present investigation, magnesium strips were produced by twin roll casting (TRC) and melt conditioned twin roll casting (MC-TRC) processes. Detailed optical microscopy studies were carried out on as-cast and homogenized TRC and MC-TRC strips. The results showed uniform, fine and equiaxed grain structure was observed for MC-TRC samples in as-cast condition. Whereas, coarse columnar grains with centreline segregation were observed in the case of as-cast TRC samples. The solidification mechanisms for TRC and MC-TRC have been found completely divergent. The homogenized TRC and MC-TRC samples were subjected to tensile test at elevated temperature (250-400 °C). At 250 °C, MC-TRC sample showed significant improvement in strength and ductility. However, at higher temperatures the tensile properties were almost comparable, despite of TRC samples having larger grains compared to MC-TRC samples. The mechanism of deformation has been explained by detailed fractures surface and sub-surface analysis carried out by scanning electron and optical microscopy. Homogenized MC-TRC samples were formed (hot stamping) into engineering component without any trace of crack on its surface. Whereas, TRC samples cracked in several places during hot stamping process. EPSRC – LiME, UK and Towards Affordable, Closed-Loop Recyclable Future Low Carbon Vehicle Structures – TARF-LCV(EP/I038616/1), Department of Mechanical Engineering, Imperial College London, UK, Mr. Steve Cook, Mr. Peter Lloyd, Mr. Graham Mitchell and Mr. Carmelo and BCAST, Brunel University London.

The present work includes the effect of addition of graphite for improving the properties of Al6061–TiB2 composite using the high-energy stir casting method. The characterization was performed with X-ray diffraction, energy dispersive spectrum and scanning electron microscope. The thermal and mechanical behaviors such as thermo gravimetric analysis/differential thermal analysis, hardness, tensile strength using acoustic emission and fatigue behaviors were investigated. The composite of composition Al6061–20% TiB2 with 2% graphite shows the greatest improvement in mechanical behavior. An Acoustic Emission system was employed in all the tests to monitor the acoustic energy release during the whole deformation process and some useful conclusions were drawn.

High-Mn steels exhibit both high tensile strength and good ductility and have attracted much attention as a promising candidate for next-generation automotive steel. However, there are still problems in conventional production, such as severe slab crack and pronounced element segregation, which restrict their widespread applications. An alternative way to produce this alloy is the near net shape method under a sub-rapid solidification process. We studied the microstructure and mechanical properties of Fe–18Mn–(0.14, 0.50)C alloy thin strip produced by injecting casting at a sub-rapid solidification rate, in which the cooling rate was about 5×10 3 K/s. The results demonstrated that the formation of cementite particles was suppressed, but apart from the γ-phase, the e-martensite and α′-martensite also formed in the Fe–18Mn–C thin strips. The lath width of e-martensite is lower than that of sample produced by conventional method. The mechanical properties of Fe–18Mn–0.50C surpass the Fe–18Mn–0.60C alloy steel processed by hot rolled and heat treated. This paper discusses the relationship between the microstructure and the mechanical properties of the strips.

A standard Aluminum–Silicon–Magnesium cast alloy (A357 foundry alloy without Beryllium) modified with different weight percentages of Scandium (Sc), has been studied to evaluate the effects of Sc contents on microstructure and strength. Study has been conducted under optimized parameters of melting, casting and heat treatment. Characterization techniques like optical microscopy, SEM, TEM and tensile testing were employed to analyze the microstructure and mechanical properties. Results obtained in this research indicate that with the increase of Sc contents up to 0.4 wt%, grain size is decreased by 80% while ultimate tensile strength and hardness are increased by 28% and 19% respectively. Moreover along with the increase in strength, elongation to failure is also increased up to 165%. This is quite interesting behavior because usually strength and ductility have inverse relationship.

High cycle fatigue behavior of casting Mg 96.34 Gd 2.5 Zn 1 Zr 0.16 alloy was investigated for its improvement by heat-treatment. After solution treatment (T4, 10 h@773 K) or solution treatment plus artificial aging (T6, 10 h@773 K+128 h@473 K), fatigue strength of this alloy was found to be enhanced. The T6-treated alloy achieved the highest fatigue strength, 130 MPa, being 25 MPa and 18 MPa greater than those of the as-cast and T4-treated alloy, respectively. The average fatigue life of the heat-treated alloys is longer than that of the as-cast alloy a given stress amplitude. For the distribution of fatigue life, a fatigue life gap spanned from 10 5 to 10 7 can be observed in the as-cast and T4-treated alloy. Such a gap is absent after the alloy received artificial aging. The mechanism for the high cycle fatigue behavior of the casting alloy after heat-treatment was also discussed.

The isothermal creep resistance of an overaged AlSi12CuMgNi alloy was studied at 300 °C as a function of solution heat treatment time at 480 °C. The microstructure of the alloy in as cast condition is formed by a three dimensional network of Si lamellae and intermetallic compounds embedded in an age-hardenable α-Al matrix. The solution heat treatment provokes the partial dissolution of Mg-rich intermetallic compounds and the spheroidisation of eutectic Si. These microstructural changes reduce the creep resistance of the alloy with increasing solution heat treatment time. The determination of the load transfer loss during solution heat treatment and an analogy with a modified shear-lag model for metal matrix composites indicate that the largest reduction in creep resistance occurs in the first 20 min–1 h of solution heat treatment.

The effect of hot extrusion process on the microstructure and tensile properties of aluminum matrix composite containing 15, 20, 25 and 30 wt% Mg 2 Si phase has been studied in this investigation. Microstructural examinations were assessed by the use of optical microscope and scanning electron microscopy. The results revealed that hot extrusion breaks primary Mg 2 Si phase and changes its morphology from dendritic to regular shape. It was also found that hot extrusion reduces the size of primary particles as well as casting defects such as porosity. Further examination showed that hot extrusion has strong effect on breaking eutectic Mg 2 Si network. Remarkable result of this study was concerned with significant improvement in ductility by hot extrusion. Fractographic examinations revealed that hot extrusion encourages ductile mode of fracture by introducing homogenous and fine dimples on the fracture surface of the aluminum matrix composites.

This article presents the tension–compression high cycle fatigue behavior of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy produced by semi-continuous casting at ambient temperature. The relationship between stress amplitude and cycles to failure is established, which indicates that fatigue strength of this alloy is approximately 105±8 MPa. Fracture surface of specimens were examined using a scanning electron microscope, indicating that the fatigue cracks all initiate from the oxides located at the surface. Different from other cast Mg alloys, there exist two kinds of unique fatigue morphologies at the fatigue propagation region, which consists of fine steps. Meanwhile, there is a fatigue life gap between 105 and 107 cycles on the S–N curve, which probably demonstrates that the growth rate of the fatigue cracks of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy is relatively large, and once the fatigue cracks form, the samples could fails in less than 105 cycles.

In this investigation, in-situ precipitation of vanadium carbides was employed to reinforce Fe–13Mn and Fe–13Mn–3W alloys by means of conventional melting and casting route. Microstructures were characterized by optical and scanning electron microscopy techniques. Mechanical properties of the materials were determined by hardness, impact toughness and tension tests. It was observed that tungsten improved the strength of the matrix and the reinforcements as well as tensile properties and work hardening rate of the VC-reinforced composite. Ball mill abrasion test was utilized to simulate impact–abrasion wear condition using two types of abrasive minerals. The results showed that the degree of benefit to be gained by the use of in-situ VC-reinforced composite materials depends strongly on crush strength of the abrasives. It was found that the studied particle-reinforced composite materials were only advantageous when the abrasives were relatively soft, providing low-stress abrasion condition.