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Electron beam melting (EBM) is a promising technology to manufacture various alloys with outstanding properties; however, the number of available alloys is limited. We propose in situ alloying to accelerate the development of advanced and novel alloys, based on thermophysical calculations and CALPHAD approach, during the EBM process. We demonstrate our concept through the design and fabrication of high entropy alloys (HEAs). Three CoCrFeNiMn-xTi (x=0.18, 0.50, 2.00, in molar %) HEAs are manufactured. EBM-built HEAs achieve a homogeneous distribution of elements while forming multiphase alloys resulted from the hot powder bed. The topological structures formed by secondary phases contribute to an increase in the hardness of EBM-built HEAs up to 900 HV1. Considering alloy design, a systematic analysis on CoCrFeNiMn-0.18Ti HEA elucidates the microstructural evolution in detail. These findings provide a deep understanding of in situ alloying and pave the way to develop new alloys specific to the EBM process.

A two-step double-side friction stir welding (DS-FSW) technique was proposed to join 20 mm thick Mg–Y–Nd magnesium alloy plates. FSW was first carried out on one side of the butt joint, and then th...

Hybrid laser-TIG welding of 10 mm thick SS400 low carbon steel to 304 austenitic stainless steel was performed in a butt joint configuration. The relationship between the heat sources’ location, metallurgical transmutations and mechanical properties of welds were investigated by simulation and experimental methods. Results showed that better weld quality was achieved when the heat sources were placed on the interface of welding materials as compared to having an offset to the stainless steel side. Weld quality in terms of microstructure, hardness distributions and tensile strength were strongly determined by the location of heat sources in hybrid laser-TIG welding due to the disparity of thermal behaviours developing in two welding materials. Tensile testing results show that the welded samples failed at the SS400 base metal with a ductile fracture mode.

Al0.1CoCrFeNi high-entropy alloys are firstly processed by cyclic dynamic torsion (DT) using Split-Hopkinson Torsional Bar. Gradient hierarchical microstructures produced by DT are investigated using electron backscattered diffraction, transmission electron microscopy, and nanoindentation tests. Based on the gradient torsional strain, significant grain refinement, remarkable work hardening, and hierarchical deformation substructures are introduced into the current DT-HEAs.

Achieving not only high mechanical strengths but also high ductility is recently established using an additive manufacturing technique called selective laser melting. In the present study, stainless steel 304L fully dense samples were successfully printed using the 3D systems – ProX 300 printing machine. The ductility and tensile yield strength were almost two and three times higher compared to those of ASTM cast’s alloy. Honey comb like nano-cellular structure with different orientation was observed in the fine grains (∼4 μm) due to fast cooling rate. In addition, the formation of martensite phase in random grains is also a contributor to the strengths. Furthermore, negative residual stresses in the build and horizontal directions were detected and assisted further increase in the tensile strength. Fractography revealed the ductile feature of plastic deformation and the crack openings at unmelted particles or pores.

Additive manufacturing (AM) enables the processing of heterogeneous materials with customised architectures to improve strength-ductility synergy. Herein, layerwise-heterostructured high-strength steels were processed by AM of two steel powders following the designed architectures. The fabricated high-strength steels with hierarchical heterogeneous characteristics at the layer, melt-pool and grain scales, exhibit good strength-ductility combination, reaching a strength of 1.32 GPa together with an elongation of 7.5%. The increased strength attributes to hetero-deformation induced strengthening. In-situ deformation monitoring reveals many unique deformation bands in heterostructure materials, which delays necking and improves ductility. The findings demonstrate a novel potential approach to circumvent material property trade-offs. Additively manufactured heterogeneous materials with controllable architectures achieved a good strength-ductility combination; the underlying strengthening mechanism and unique deformation behaviour are revealed.

The CoCrFeNiMn high entropy alloy (HEA) bulk alloy was successfully manufactured using laser aided additive manufacturing (LAAM). The microstructure and mechanical behaviors of the as-fabricated HEAs were investigated. The as-built HEAs exhibit directional solidification at regions close to the melt-pool boundaries, forming dendritic columnar grains and transiting to equiaxed grains further away from the boundaries. Compared with the conventionally cast HEAs, the CoCrFeNiMn fabricated using LAAM possesses significantly higher yield strength (518 MPa) and ultimate tensile strength (660 MPa). The strengthening effect is attributed to finer grains and could be explained quantitatively through grain boundary strengthening. The as-built HEA shows a simultaneous enhancement in yield strength and ductility with decreasing testing temperature. The improved low temperature tensile properties could be ascribed to the formation of deformation twins at low temperature, which results in a steady strain hardening behavior. This work demonstrates the potential of using LAAM technology to extend the application of HEAs with fabrication of larger and more complex parts with good mechanical properties.

The electrochemical corrosion behavior of laser aided additive manufactured (LAAMed) CoCrNi medium entropy alloys (MEAs) with different oxide inclusion level was investigated in 3.5 wt.% NaCl solution. Different from the 316 L stainless steel, no pitting appeared on the LAAMed CoCrNi within the test regime used in this study. The LAAMed sample with a higher oxide inclusion level demonstrated lower corrosion current density and higher charge transfer resistance, implying a more compact passive film. After holding in the solution at 0.2 VSCE for 30 mins, the passive films formed on the LAAMed CoCrNi MEAs have similar thickness and dominant constituent (i.e. Cr2O3 and Cr(OH)3). More oxides in the LAAMed CoCrNi MEA could facilitate the formation of a compact passive film, which leads to superior corrosion resistance.

Current heterostructured materials demonstrate the high potential to circumvent strength-ductility tradeoff but face challenges with unconfigurable hetero-zone distribution and mechanical anisotropy. This study explored laser aided additive manufacturing (LAAM) of non-laminar spatially heterostructured materials (SHM) with configurable architectures to combine superior properties from the constitutive AISI 420 stainless steel and C300 maraging steel to enhance the overall properties. It is proven that the hatch spacing (h) has a significant effect on the microstructural evolutions and mechanical properties of the SHMs as it affects the layer thickness and inter-dilution regions. The mechanical properties of the SHMs were evaluated at multi-scales. The sample with h of 1.5 mm possesses a high tensile strength of about 1.6 GPa along with a reasonable break elongation of 8.1%, showing a good strength-ductility combination. Micropillar compression tests were conducted to measure localised mechanical properties, which is critical to understand the strengthening mechanism. Additionally, the SHM substantiates a much higher strength than many lamellar and linear functionally graded materials reported in the literature. This can be explained by the rule-of-mixture effect and hetero-deformation induced strengthening (HDIS). Furthermore, in-situ deformation observation reveals multiple deformation bands in SHM, which delays necking and contributed to ductility in tandem with the transformation induced plasticity (TRIP) effect. The findings highlight a novel approach for the development of SHM with a tunable performance by using LAAM of multiple materials following the configurable architectures.

CrN/AlN nanolayered coatings with different Y doping levels were deposited to understand the effects of Y on the oxidation behaviour of the coatings at elevated temperatures. The coatings were deposited in a hybrid HIPIMS and magnetron sputtering deposition system, and their chemical composition and mechanical properties were characterized using EDS and nanoindentation tests respectively. The as-deposited coatings were annealed in air at 800 °C and 1100 °C for 1 h, and the evolution of coating microstructure were characterized by GI-XRD and FIB assisted cross-sectional SEM imaging. Different oxidation behaviours were observed at 800 °C and 1100 °C. At 800 °C, coating oxidation is dominated by the formation of Cr 2 O 3 , while at 1100 °C, the oxidation is characterized by the competing growth of Cr 2 O 3 and Al 2 O 3 scales. The Y content significantly influenced the oxide scale formation and voids distribution in the coatings, and this was attributed to the role of Y in changing the diffusion processes both within the coating and at the coating-air interface.

Through a diffusion controlled solidification process, transient liquid phase (TLP) bonding is widely exploited to repair or join materials in different industry areas for decades. However, a lack of modeling on the TLP process of multi-component systems makes design of the TLP process an expensive trial and error process. Here, by coupling with experiment, we developed a set of self-consistent kinetic model parameters, which are used to simulate the TLP process of AWS A5.8 BNi-9 (filler)/Inconel 718 (base metal) at 1090 °C such as the solidification kinetics, diffusion profile and the precipitated borides in the Diffusion Affected Zone (DAZ). Consistent with experiment results by X-ray diffraction (XRD) and scanning electron microscopy (SEM), our simulation demonstrate four types of boride precipitated in the DAZ: MB2, MB, M5B3 and M3B2, which consequently results in the non-parabolic solidification behaviors in the brazing system. Based on the calculations using the developed models, we predict that a post heat treatment at 1090 °C for 3500s will homogenize the boride phase distribution in the DAZ, thus increasing the strength of the brazing joint. The developed model parameters may be extended to the other Ni-based alloys.

An equiatomic CoCrFeNiMn high-entropy alloy (HEA) powder is produced by vacuum induction melting gas-atomization with argon gas. A detailed characterization reveals that gas atomized HEA powder is suitable for powder-bed-fusion additive manufacturing (PBFAM). Therefore, the powder with size ranging from 45 to 106 μm is applied in one of PBFAM, electron beam melting (EBM). The final EBM part is strongly affected by the feedstock. The entrapped gas pores result in a high porosity of EBM-built parts, requesting extra efforts to eliminate the pores for processing parameters optimization. After the optimization, the highly dense EBM-built parts with a hierarchical microstructure and a strong 〈100〉 texture are obtained. The hierarchical microstructure displays long columnar grains containing intragranular cellular structure (dendrites) along build direction. Deformation mechanism of the EBM-built CoCrFeNiMn HEA is dominated by dislocation with limited contribution from mechanical twinning. Experimental results reveal that the cellular structure (dendrites) does not contribute the strength without the pre-existing surrounding dislocation network. As a result, EBM-built CoCrFeNiMn HEA parts exhibit comparable tensile properties to their conventional cast counterparts. It is suggested that the EBM process and gas atomized HEA powder can be employed for fabricating single-phase HEA parts with complex geometry and good mechanical properties. Keywords: Powder characteristics, Pores, Mechanical twining, Texture, Electron beam melting

The CoCrNi medium entropy alloy with different amounts of oxides was fabricated by laser aided additive manufacturing (LAAM). The cryogenic tensile properties and microstructure evolution during tensile deformation were investigated. For the Sample B with higher oxide content (2.03 vol%), the yield strength (YS), ultimate tensile strength (UTS) and elongation (El) were all inferior to those of the Sample A with much fewer oxides (0.47 vol%). The lower YS of Sample B was mainly attributed to the lower initial dislocation density. The oxides contributed slightly to the increase in YS, while reducing the El significantly. The El of Sample B was comparable at 298 K and 143 K, owing to the compensation effect from twin boundaries (TBs). Higher YS and UTS were obtained at 143 K for both samples. With decreasing temperature from 298 K to 143 K, the YS and UTS of Sample A increased almost linearly, whereas the El decreased. Though a large amount of TBs were formed during the tensile deformation, they were unevenly distributed among the grains near the fractured location. Under higher stress at cryogenic temperature, the interaction of grain boundaries with massive TBs caused micro-voids to initiate more readily along the grain boundaries, resulting in premature failure.

The microstructure, mechanical properties and deformation mechanisms of the 304L stainless steel (SS) additively manufactured by selective laser melting (SLM) were systematically investigated. The SLM fabricated 304L SS contains two phases (face-centered-cubic γ-austenite and body-centered-cubic δ-ferrite) and exhibits a hierarchical microstructure with length scales spanning several orders of magnitude. The hierarchical microstructure includes the melt pools and slightly elongated columnar grains at the micron scale, cellular structures decorated with a high density of dislocations at the sub-micron scale and oxides at the nanoscale. Stacking faults formed due to the residual stress in addition to the low stacking fault energy of the 304L SS (19.2 mJ/m2) while massive annealing twins were generated arising from the combined effects of residual stress and intrinsic heat treatment. The as built 304L SS exhibits a significantly enhanced strength–ductility synergy compared to that of wrought and annealed counterparts. The enhanced yield strength stems from the hierarchically heterogeneous microstructure, while the outstanding tensile elongation is ascribed to the activation of multiple deformation mechanisms, involving the dislocation activities, the formation of stacking faults and mechanical twins, and the transformation-induced plasticity.

In this study, the CoCrNi medium entropy alloy (MEA) was successfully fabricated by laser aided additive manufacturing (LAAM), a directed energy deposition (DED) process. Results indicate that the ultimate tensile strength (UTS), yield strength (YS), and elongation after fracture (δF) are 873.5 MPa, 620.5 MPa, and 44.8 % respectively. Without any post heat treatment, the as-built state CoCrNi MEA exhibits an excellent combination of strength and ductility. The high YS is mainly attributed to the combination effects from lattice friction stress, boundary strengthening, and dislocation strengthening. Deformation twinning is activated during room temperature tensile deformation, inducing the steady work hardening behavior. Hence, fracture is postponed to higher deformation strain due to the synergistic effect of dislocation gliding and deformation twinning. LAAM is expected to be a versatile and efficient method to fabricate CoCrNi MEA and some other high-performance alloys.

IN100 Ni-based superalloy fabricated by micro-laser aided additive manufacturing (micro-LAAM) was investigated in this study. After solution treatment and aging (STA) of the micro-LAAMed IN100 alloy, hierarchical γ′ phases were recognized and characterized, which contributed significantly to the high ultimate tensile strength (~1050 MPa) and acceptable ductility (5%) at 25 °C and 600 °C. The tensile fractured surfaces of the as-built and STAed IN100 were characterized by dimples and dimples/cleavages, respectively. The high cycle vibration fatigue (HCVF) behavior was preliminarily studied through simulating the service conditions of some cantilever structures in automobile and aerospace industries. Compared with the cast IN100, the micro-LAAMed IN100 superalloys (as-built or STAed) both exhibited inferior HCVF lives. The HCVF behavior was discussed and correlated with the microstructure characteristics, such as the preferred growth direction of the grains in micro-LAAMed IN100 and the massive interfaces existing in the final obtained material. In the present study, the hierarchical γ′ phases were beneficial to the static tensile property, whereas detrimental to the HCVF behavior of the final obtained IN100 to some extent.

An interstitial solute strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), was successfully additively manufactured via selective laser melting. The as-built iHEA exhibits a hierarchically heterogeneous microstructure with length scales across several orders of magnitude, which engenders an enhanced strength–ductility combination relative to those fabricated by conventional processing routes. The high yield strength mainly stemmed from the dislocation strengthening besides the friction stress and grain boundary strengthening. The joint activation of multiple deformation mechanisms involving dislocation slip, deformation twinning and phase transformation can maintain the steady work-hardening behavior at high stress levels, leading to a high ductility. A hierarchical interstitial high entropy alloy, Fe49.5Mn30Co10Cr10C0.5 (at.%), was successfully additively manufactured via selective laser melting. Its exceptional strength–ductility synergy surpasses that of the most SLM processed conventional alloys.

Laser aided additive manufacturing (LAAM) was used to prepare FeCrNiMnCo high entropy alloy (HEA) with pre-alloyed powders. The microstructure and properties of the HEA were investigated intensively, especially the cryogenic mechanical properties. The microstructure of the as deposited specimen was dense and occupied by cellular and dendrite grains. The tensile strength and yield strength increased with temperature decreasing from 25°C to -130 °C, yet the impact toughness decreased. The deformation mechanism of the HEA under low temperature was discussed accordingly.

In this study, an AZ31 magnesium alloy plate was processed by constrained groove pressing (CGP) under three deformation cycles at temperatures from 503 to 448 K. The process resulted in a homogeneous fine grain microstructure with an average grain size of 1.8 μm. The as-processed microstructure contained a high fraction of low-angle grain boundaries (LAGB) of subgrains and dislocation boundaries that remained in the structure due to incomplete dynamic recovery and recrystallization. The material's yield strength was found to have increased from 175 to 242 MPa and with a significant weakening of its initial basal texture. The microstructure stability of the CGP-processed material was further investigated by isothermal annealing at temperature from 473 to 623 K and for different time. Abnormal grain growth was observed at 623 K, and this was associated with an increased in nonbasal grains at the expense of basal grains. The effect of annealing temperature and time on the grain growth kinetics was interpreted by using the grain growth equation, Dn+D0n=kt, and Arrhenius equation, k=k0 exp (−(Q/RT)). The activation energy (Q) was estimated to be 27.8 kJ/mol which was significantly lower than the activation energy for lattice self-diffusion (QL = 135 kJ/mol) and grain boundary diffusion (Qgb = 92 kJ/mol) in pure magnesium. The result shows that grain growth is rapid but average grain size still remained smaller than the as-received material, especially at the shorter annealing time.

Layer by layer fabrication using micro-laser aided additive manufacturing (micro-LAAM) was successfully implemented on nickel-base superalloy IN100. It is known that IN100, a type of superalloy having high titanium and aluminum contents, has poor weldability due to weld liquation cracking in the heat-affected zone (HAZ) and strain age cracking. In this study, micro-LAAM process was optimized through a set of designed experiments to eliminate crack formation and reduce porosity. It was found that the crack-free deposition can be achieved owing to the fact that micro-LAAM process used in this study had very low heat input. Three distinct sizes of γ′ precipitates were observed on the post heat-treated samples. The volume fractions of γ to γ′ phases were found to be approximately 60–40%. Microstructure and chemical analysis results showed that γ′ phase was embedded within γ-Ni matrix while various carbides (MC, M 23 C 6 and M 6 C) were observed as precipitates at grain boundaries or within grains. Electron backscatter diffraction (EBSD) was used to compare grain morphologies and size distribution of three distinctly different regions on each layer. The achieved ultimate tensile strength and yield strength are much better than the minimum requirements specified in aerospace material specification 5397 for cast IN100.

There are increasing demands for polymer based microfluidic devices as polymer devices can be mass produced using the injection molding process for disposable analytical applications. However, fabrication of precision injection molds with micro features down to tens of micro meters is a challenging task. The most efficient manufacturing method for this type of high precision micro mold is the tool-based micro machining. In this paper, studies have been conducted to develop metal injection mould manufacturing technologies for polymer microfluidic device fabrication. An injection mould with a micro feature of 29m wall thickness, 20m height and 9.8mm length has been successfully achieved on brass with very precise dimensional accuracy and surface quality for molding process of polymer microfluidic devices. Polymer microfluidic chips with micro channels of 30m width have been successfully produced using the machined metal injection mould.

Feasibility study on producing components with embedded channel by powder injection moulding (PIM) is conducted in this paper. The base plate with open channel was produced by co-injection moulding of polymeric material and ceramic feedstock. The polymeric material acted as the sacrificial material to fill the channel feature. A cover plate was then over-moulded onto the base plate to form green assembly, which was then debinded and sintered. The embedded channel was created when the sacrificial material was removed during the debinding and sintering stage. This process eliminates the need for additional joining process required by conventional manufacturing process which could cause misalignment and leakage under harsh working environment such as high pressure and high temperature. Joining was found between the cover plate and the base plate in over-moulding stage. Temperature and time during solvent debinding played important roles in order to obtain defect free component. Low temperature and short time in solvent debinding resulted in insufficient wax loss which led to various types of defects. A fair good sintered component was obtained under optimal debinding condition at 50 °C for 48 hours. The embedded channel was observed clearly by x-ray inspection without blockage by foreign particles. The shape of the channel was well retained and perfect bonding was formed in the central area of the two plates.

Zinc-based solutions have been used for many decades to protect steel against corrosion. Having cathodic protection properties on top of the barrier protection, zinc has become an effective way of protecting steel. Hot dip galvanized techniques have a deep rooted history under this category although they have some disadvantages in terms of application process and productivity which limit their usage in modern applications. Zinc coatings and zinc-rich paints have come to the market aiming to overcome these disadvantages. However, the suitability of these coatings for various atmospheric and service conditions, vulnerability of their protection properties, especially cathodic protection characteristics, due to the influence of external factors like surface preparation and the application of top coats have posed some challenges. Proper methodology for corrosion evaluation and validation is a mandatory step towards solving these issues. The aim of this publication is to address the need of a corrosion characterization methodology to assess zinc-based coatings and also to discuss protection and failure mechanisms of zinc coatings based on selected case studies. A systematic methodology for evaluation and validation of corrosion protection properties of zinc coatings is introduced in this work. This methodology is based on scientific principles and international standards using standard and advanced techniques to assess both barrier and galvanic protection properties. Cyclic corrosion test has been used as the accelerated exposure and samples were withdrawn periodically for progressive evaluation. Barrier and galvanic protection assessment is based on protection rating, mass loss and subsequent corrosion rate measurements, electrochemical properties together with advanced electrochemical scanning techniques to investigate the anodic and cathodic activities. Another part of this work is to discuss failures of zinc coatings based on a selected case where zinc coatings were used as primers together with top coats. Corrosion products of zinc have been detected around zinc particles clusters and also at the substrate / coating interface at the early stage of accelerated exposure. This led to a compromise of the galvanic protection properties of the substrate by zinc and eventually caused the overall protection to fail. Top coat incompatibility and/or improper application were suspected to have led to this failure. Possible causes such as saponification of the zinc primer due to the drying of the binder in the primer coat without proper curing and the high porosity of top coat which can be detrimental to the zinc primer were analyzed in detail. Corrosion mechanisms were scrutinized based on electrochemical principles supported by scanning electron microscopy studies together with micro-Raman spectroscopy. The causes of coating incompatibilities and improper coating application practices and their influence on protection properties will be explained from a corrosion mechanistic point of view. Details will be explained in the presentation.