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In recent years, an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process is conducted to the in situ alloying of TiAl alloys. As a newly invented method of TiAl alloy fabrication, the microcrack defect is a great concern. In the present research, two binary TiAl alloys containing different microcrack tendencies, Ti-45Al and Ti-48Al, are fabricated using the TW-PAAM method. To locate possible defects in the as-fabricated alloys, the non-destructive testing X-ray computed tomography is performed. According to the results, under the same TW-PAAM parameter set, microcracks are initiated throughout Ti-45Al while Ti-48Al stays integrated. Also, more α2-Ti3Al are generated in grain boundaries of Ti-45Al than Ti-48Al, thus lead to higher local misorientations and the intergranular microcracks. The clarification of the Al content-induced microcrack generation provides clear improvement direction of the innovative additive manufacturing technique.

Purpose The purpose of this article is the effect of doping minor Ni on the microstructure evolution of a Sn-xNi (x = 0, 0.05 and 0.1 wt.%)/Ni (Poly-crystal/Single-crystal abbreviated as PC Ni/SC Ni) solder joint during reflow and aging treatment. Results showed that the intermetallic compounds (IMCs) of the interfacial layer of Sn-xNi/PC Ni joints were Ni3Sn4 phase, while the IMCs of Sn-xNi/SC Ni joints were NiSn4 phase. After the reflow process and thermal aging of different joints, the growth behavior of interfacial layer was different due to the different mechanism of element diffusion of the two substrates. The PC Ni substrate mainly provided Ni atoms through grain boundary diffusion. The Ni3Sn4 phase of the Sn0.05Ni/PC Ni joint was finer, and the diffusion flux of Sn and Ni elements increased, so the Ni3Sn4 layer of this joint was the thickest. The SC Ni substrate mainly provided Ni atoms through the lattice diffusion. The Sn0.1Ni/SC Ni joint increases the number of Ni atoms at the interface due to the doping of 0.1Ni (wt.%) elements, so the joint had the thickest NiSn4 layer. Design/methodology/approach The effects of doping minor Ni on the microstructure evolution of an Sn-xNi (x = 0, 0.05 and 0.1 Wt.%)/Ni (Poly-crystal/Single-crystal abbreviated as PC Ni/SC Ni) solder joint during reflow and aging treatment was investigated in this study. Findings Results showed that the intermetallic compounds (IMCs) of the interfacial layer of Sn-xNi/PC Ni joints were Ni3Sn4 phase, while the IMCs of Sn-xNi/SC Ni joints were NiSn4 phase. After the reflow process and thermal aging of different joints, the growth behavior of the interfacial layer was different due to the different mechanisms of element diffusion of the two substrates. Originality/value In this study, the effect of doping Ni on the growth and formation mechanism of IMCs of the Sn-xNi/Ni (single-crystal) solder joints (x = 0, 0.05 and 0.1 Wt.%) was investigated.

We reported femtosecond laser-induced self-organized micro/nanostructures on Zr64.13Cu15.75Ni10.12Al10 amorphous surfaces. A set of process methods that induced sub-wavelength LIPSS (laser-induced periodic surface structures) structures and SWPSS (super-wavelength periodic structure) nearly twice the wavelength using 1030 nm wavelengths were observed. Using linear scanning can completely cover a layer of micro/nanostructures on the surface. Clear surface structures can be observed throughout the laser ablation area, and their cycles and structural fluctuations can be controlled by process parameters. The causes and structures of the two micro/nanostructures were discussed.

In this paper, the effect of vibration on microstructure and mechanical properties of CMT weld is studied. The results show that in the vibration laser welding joint, the penetration depth and residual height of the joint increase, and the weld pores are suppressed. Analysis of the microstructure of the welded joint by optical microscope and scanning electron microscope shows that vibration can refine the grains and reduce the width of the heat-affected zone, which is conducive to the optimization of the mechanical properties of the welded joint. The residual stress test results show that after vibration is applied, the residual stress of the welded joint is reduced, which is related to the grain refinement. In fatigue experiments, the morphology of the fatigue fracture after vibration is more regular and smooth, and the spacing between fatigue stripes of 1 mm is smaller than the spacing of fatigue stripes without vibration (2 mm). The fatigue strength of the sample with a vibration frequency of 1222 Hz can reach 84.61% of BM. The application of vibration can improve the fatigue performance of welded joints.

Online process monitoring and quality control has been a long-standing challenge for variable polarity plasma arc welding (VPPAW) due to the inherent instability and fluctuation of the keyhole molten pool. This work developed an innovative welder intelligence-enhanced deep random forest fusion (WI-DRFF) approach, aiming to describe the dynamics of front-side molten pool and accurately predict the weld penetration. Based on the human welder’s prior knowledge, we firstly proposed an image processing algorithm to extract the low-level handcrafted features, which could quantitatively describe the geometrical appearance of the keyhole. Afterwards, we constructed a convolutional neural network (CNN) to learn the high-level discriminative features of weld pool and interpret the physical characteristics of the deep features with visualization. Finally, we incorporated the handcrafted keyhole features and deep features to concatenate a multi-level feature vector for predicting the weld penetration based on random forest (RF) classifier. Extensive experiments demonstrate that our proposed approach yields a remarkable classification performance comparing with state-of-the-art machine learning algorithms even with limited training data. This approach is a new paradigm in the digitization and intelligence of welding process and can be exploited to provide a feedback in an adaptive quality control system.

The TaC/TiC/Stellite X-40 Co-based composite coatings with different contents were successfully cladded on a Cr-Zr-Cu alloy substrate. The microstructure, wear and corrosion resistance of the compo...

Wire arc additive manufacturing (WAAM) is a novel manufacturing technique by which high strength metal components can be fabricated layer by layer using an electric arc as the heat source and metal wire as feedstock, and offers the potential to produce large dimensional structures at much higher build rate and minimum waste of raw material. In the present work, a cold metal transfer (CMT) based additive manufacturing was carried out and the effect of deposition rate on the microstructure and mechanical properties of WAAM Ti-6Al-4V components was investigated. The microstructure of WAAM components showed similar microstructural morphology in all deposition conditions. When the deposition rate increased from 1.63 to 2.23 kg/h, the ultimate tensile strength (UTS) decreased from 984.6 MPa to 899.2 MPa and the micro-hardness showed a scattered but clear decline trend.

The ordinary Weibull distribution function has been commonly accepted for empirical characterization of cleavage fracture toughness of nuclear reactor and containment pressure vessel steels. However, this method lacks a fundamental basis. This work adopts the standardized Weibull distribution function to analyze cleavage fracture toughness of ferritic steels measured from different sized fracture mechanics specimens at different temperatures to estimate the Weibull modulus. The toughness data of five different nuclear reactor and containment vessel steels are analyzed. The estimations obtained the Weibull modulus (m) in the range of 1.83 to 2.55 and strong temperature dependence of the threshold cleavage fracture toughness Kmin, as opposed to the constant values of mK = 4 and Kmin = 20 MPam1/2 given in ASTM E1921-19. The goodness of fit test by the one-sample Kolmogorov–Smirnov (K–S) test validated Weibull distribution function for describing the toughness distribution.

In order to estimate Weibull parameters in the Weibull statistical fracture theory as truly material properties independent of specimen geometry and loading mode, first the Weibull statistical fracture theory is transformed into the ordinary Weibull distribution function under certain approximation. Then the standardized format of ordinary Weibull distribution is introduced to enable Weibull modulus as the single parameter for estimation via the maximum likelihood method. The method of using standardized Weibull distribution for strength data synchronization and Weibull modulus estimation is validated by analyzing extensive strength data sets measured from uniaxial flexure, biaxial flexure and their combination, and from smooth and notched specimens. The technical path to estimate the scale parameter and threshold strength as material properties in the Weibull statistical fracture theory and effect of sample size on the estimation accuracy are also discussed.

A longitudinally varying magnetic field (MF) was applied during cold metal transfer (CMT) welding of Al alloys. The effect of MF on the welding process, macrostructure, microstructure and properties of weld beads was studied. The results showed that the application of MF led to arc expansion and rotation, the droplet transfer frequency decreased, the droplet volume became larger, and the droplets deflected. When the coil current was between 0 and 5 A, the weld beads porosity decreased from 4.47 to 0.26%. And when the coil current was increased to 8 A, the porosity of weld beads also increased to 2.84%. MF reduced the weld depth, wetting angle and weld reinforcement, and increased weld width. With the increase of coil current, the width of primary dendrite arms decreased from 7.75 to 5.01 μm. The area occupied by the Al–Si eutectic was enlarged, and the eutectic was refined. It was also found that the MF slightly increased the hardness of the weld beads, and the hardness distribution was more uniform.

The effects of different vibration frequencies on the microstructure, laves phases and microvoids of Inconel 718 weld joints were investigated in this research by using the laser welding combined with mechanical vibration. The results demonstrated that the application of vibration significantly suppressed the growth of columnar crystals, reduced the formation of carbides, and refined the microstructure. With the increase of the vibration frequency, the mechanical vibration stayed prominent. The molten pool was subjected to the pressure and vibration of the metal liquid, which expanded the depth and the width of the weld. Applying vibration could reduce the amount of Laves phase, and it is found that with the increase of vibration frequency, the morphology of Laves phase could change from granular to strips, as found during the experiment. The welds with 522 Hz vibration frequency had the smallest Laves phase size and the largest average microhardness value of 331 HV. At the vibration frequency of 1331 Hz, the amount of Laves phases was the least, and it was distributed in a miscellaneous manner in the form of granules and stripes. However, increasing the vibration frequency could not effectively reduce the number of microvoids in the weld.

Ni–Fe dissimilar butt joints (Inconel 718 and SUS316 sheets) were welded with an Ni-based filler metal using the cold metal transfer (CMT) process. The morphology of the joints was analyzed to elucidate the formation characteristics of the low-dilution-rate welding joints. The tensile strength of the joints was tested at different temperatures and with different macro morphologies to locate the weak area. The finite element method (FEM) was utilized to calculate the stress concentration of the Ni–Fe fusion line (FL) region at high temperatures. It was found that the stress was reduced and was not concentrated in the Ni–Fe FL region, if the WM reinforcement width was wider than 8 mm (original top gap of welding butt joint). The micro morphology and element distribution of the fracture surface and the interface cross section demonstrate that the interface of the Ni–Fe FL with insufficient diffusion is the region where the solidification crack occurs. Thus, this interface is the source of the FL crack and can substantially undermine the high temperature tensile properties of the joints. The metallurgical mechanism of solidification crack formation was also discussed by the phase diagram and solidus-liquidus temperature calculation. The surface of the solidification crack is Fe-based, indicating that it was formed during the melting and re-solidification of the SUS316 base metal surface. Furthermore, the formation temperature was higher than the initial solidification temperature of the welding metal.

In order to avoid decomposition of solid lubricants during laser processing and improve the lubrication stability of laser-clad self-lubricating coating, a novel multilayer structural WS2/NbC/Stellite 6 metal matrix self-lubricating coating (MMSC) with laser texturing layer (LTL), lubricant layer (LL) and laser-clad sealing layer (LSL) was fabricated on Cr12MoV steel via laser surface texturing (LST) and laser surface cladding (LSC) two-step process. The microstructure, phase and tribological properties of the multilayer structural MMCC were investigated by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy, as well as dry sliding wear test in wide-temperature (room temperature, 200 °C, 400 °C, and 800 °C). Results show that the multilayer structure partially inhibited the decomposition of the lubricant so that WS2 particles retained in the coating and NbC promoted the formation of a new lubricating phase CrS in decomposed WS2. The multilayer structural coating prepared in two steps had lower friction coefficient (0.226) and wears volume (31.916 mm3) at room temperature (RT). Moreover, the friction coefficient of the multilayer WS2/NbC/Co-based self-lubricating coating was still low (0.306) at 800 °C, only. Adhesive wear was identified as the main wear mechanism and the discrepancies of tribological properties were found to be mainly dependent of the kind of lubricating films generated at the wear surface.

“Weibull statistics” for strength distribution analysis refers to either the ordinary Weibull distribution function or the Weibull statistical fracture theory. The ordinary Weibull distribution function is an empirical distribution function on an equal footing with other type of classical empirical distributions such as normal and log-normal distributions for fitting the statistical data of various random variables nonexclusive to materials strength. It has no explicit physical meaning and cannot be used for size scaling and prediction of strength. The Weibull statistical fracture theory is a weakest-link statistical fracture model for a solid with the strength distribution of an elemental volume being described by the ordinary Weibull distribution function. It has the capability of size scaling and prediction of strength for specimens with different geometries and different loading modes. The three-parameter Weibull statistical fracture theory in uniaxial flexure of prismatic beams is reformulated and validated by both numerical and real strength experiments of different ceramics.

Vibration-assisted laser welding experiment of 5052 aluminum alloys was carried out to focus on the influences of vibration parameters on microstructure and fatigue fracture. The experiment found that the vibration process homogenized the microstructure and promoted the formation of fine equiaxed grains significantly. In addition, vibration made the hardness uniform by reducing the area of the fusion zone. Experimental results of double welds showed that residual stress reduced by 58 to 77% under the action of micro-vibration. Vibration frequency and vibration acceleration had significant effects on the longitudinal and transverse residual stress, respectively. Under the same fatigue load condition, the average fatigue limits of the base metal (BM) and the welded joints were 160 and 120 MPa, respectively. Under the conditions of 107 cycles, the maximum fatigue strength of the welded joints has reached 74.95% fatigue strength of the BM. Vibration-assisted laser welding could obviously improve the fatigue performance and increase the fatigue life of the welded joints, which was a potential way to improve the quality of welded joints.

The mechanical properties of 2060 and 2099 T-joints welded by double-sided laser beam welding were analyzed in this experiment. Various mechanical tests were performed to determine the transverse tensile properties of the region perpendicular to the weld, the tensile properties of the skin and stringer, the compressive properties of the region parallel to the weld seam, and the fatigue properties perpendicular to the weld seam. The T-joint was found to consist of the base metal, partial melting zone, equiaxed zone (EQZ), columnar crystal zone, and cellular dendritic zone from the base metal to the center of the weld seam. The EQZ was proved to be the weak area during mechanical properties tests of the T-joint. Moreover, the welding penetration had a great influence on the transverse tensile properties and weak areas were located in the EQZ near the lower fusion line. The EQZ near the upper fusion line was the weak area in the process of compression. In addition, severe porosities and cracks greatly influenced the mechanical properties of the joint. The fatigue property was closely related to the deformation angle; the fatigue source was located at the weld toe, and the crack propagation was along the base metal.

Cold metal transfer plus pulse (C + P) arc was applied in the additive manufacturing of 4043 Al alloy parts. Parameters in the manufacturing of the parts were investigated. The properties and microstructure of the parts were also characterized. Experimental results showed that welding at a speed of 8 mm/s and a wire feeding speed of 4.0 m/min was suitable to manufacture thin-walled parts, and the reciprocating scanning method could be adopted to manufacture thick-walled parts. The thin-walled parts of the C + P mode had fewer pores than those of the cold metal transfer (CMT) mode. The thin- and thick-walled parts of the C + P mode showed maximum tensile strengths of 172 and 178 MPa, respectively. Hardness decreased at the interface and in the coarse dendrite and increased in the refined grain area.

The 5% Fe-based amorphous reinforced AlCoCrFeNi high-entropy alloy (HEA) specimens were prepared by selective laser melting (SLM) technique. The mixed of Fe-based amorphous reduces the grain diameter and eliminates the presence of texture. Meanwhile, the anisotropy of the specimen was reduced. The addition of Fe-based amorphous causes the precipitation of FCC phase in the body-centered cubic (BCC) matrix, and the face-centered cubic (FCC) phase is uniformly distributed at the grain boundaries. The presence of FCC phase significantly reduces the internal stress of the specimen. The elements in the amorphous alloy solidly dissolve into the BCC matrix during the printing process, further strengthening the BCC matrix. The residual amorphous and nanocrystalline phases also result in a significant improvement in the performance of the specimen.

Variable polarity cold metal transfer (VPCMT) is a newly developed welding process and has drawn extensive interests because of its potential in further reducing the heat input, offering greater gap bridging ability, and improving the deposition rate. Using 6061 aluminum alloy as an example, this paper systematically investigated the influences of the key parameter in VPCMT, i.e., “positive/negative electrodes ratio (EP/EN Balance),” on weld formation, microstructure, and mechanical properties. It was found that, with the increase of the EP/EN Balance value, the duty cycle of the negative phase was reduced but the peak current and maximum temperature of the weld pool were both increased resulting in a higher heat input. The positive/negative electrode ratio had little impact on the grain misorientation angle in the fusion zone. Hardness of the joint was higher with larger positive duty cycles because of the precipitate phase and small grain size. An increased negative duty cycle resulted in lower heat input leading to insufficient melting of the material and deterioration of the mechanical strength of the welds.

There are mainly two kinds of laser used in welding of materials, continuous laser beam welding (CLBW) and pulsed laser beam welding (PLBW). The two kinds of laser welding have each welding characteristics. Compared with CLBW, the heat input of PLBW can be controlled lower, and has been widely used in the connection of thin plate materials. Since the pulse shaping can be edited to realize the preheating and slow cooling of each welding process, pulsed laser has been studied to suppress hot cracks and other defects in the field of aluminum alloy, magnesium alloy and other materials welding. This article describes the application of pulsed laser in alloy welding. The main welding problems and prevention methods were summarized. By discussing the index had been proposed to predict the sensitivity of solidification cracks, the formation and growth processes of hot cracks in pulsed laser welding was more comprehensively analyzed. Finally, the outlook for the future trends of pulsed laser welding is presented.

With the development of the aerospace and automotive industries, high heat exchange efficiency is a challenge facing the development of various industries. Pure copper has excellent mechanical and physical properties, especially high thermal conductivity and electrical conductivity. These excellent properties make pure copper the material of choice for the manufacture of heat exchangers and other electrical components. However, the traditional processing method is difficult to achieve the production of pure copper complex parts, so the production of pure copper parts through additive manufacturing has become a problem that must be overcome in industrial development. In this article, we not only reviewed the current status of research on the structural design and preparation of complex pure copper parts by researchers using selective laser melting (SLM), selective electron beam melting (SEBM) and binder jetting (BJ) in recent years, but also reviewed the forming, physical properties and mechanical aspects of pure copper parts prepared by different additive manufacturing methods. Finally, the development trend of additive manufacturing of pure copper parts is also prospected.

Phase stability, elasticity, hardness and relevant electronic structures of M nB m (M = Ni, Cr, Mo, W, n = 23,5,3,1, m = 6,3,2,1) binary borides have been investigated by first-principles calculati...

Cold metal transfer welding/brazing of AA5754 aluminum alloy and Q235 low carbon steel was performed in a lap joint configuration using an AlSi12 filler wire. The effect of heat input on intermetallic compounds (IMC) formations and mechanical properties has been investigated. At a low heat input of 157 J/mm, a layer of IMC, with a composition of Al7.2Fe1.8Si was generated at the fusion zone/steel interface. Low interfacial energy and good interfacial bonding between fusion zone and Al7.2Fe1.8Si led increased joint strength. An additional Fe(Al,Si)3 IMC formed at the interface in conjunction to Al7.2Fe1.8Si at a high heat input of 201 J/mm. The joint strength was significantly decreased and attributed to a high interfacial energy and poor interfacial bonding between Al7.2Fe1.8Si and Fe(Al,Si)3. The results showed that the interfacial energy at interphase boundaries had remarkable effect on the joint strength.

The strong effects of Zn addition on the reaction in Sn/Cu systems are widely known. Nevertheless, the micro-mechanism for Zn accumulation behavior, phase evolution and void formation in Sn-xZn/Cu systems remain unclear. In this work, the effect of Zn addition on the interface behavior is investigated. Structures of Sn-xZn/Cu are prepared by the reaction between Sn-xZn solders (x = 0, 0.2, 0.5 and 0.8 wt.%) and the electroplated Cu (EP Cu) joints. During the thermal aging of Sn/EP Cu joints, many voids formed inside the Cu3Sn phase, especially close to the Cu3Sn/EP Cu interface. Whereas the voids were greatly suppressed with Zn addition. During the thermal aging, Zn gradually accumulated near the interface of IMCs/EP Cu. As thermal aging time to168 h and Zn content to 0.8 wt.%, Zn participated in the interface reaction, three layers with different contrasts, (Cu, Zn)6Sn5, Cu6(Sn, Zn)5 and (Cu,Sn)3Zn were formed. Theoretical analysis shows that Zn atom accumulated near the interface of IMCs/EP Cu due to lower diffusion energy barriers (Edf) of Zn than Cu atom. The preferential occupation of Zn atoms in Cu6(Zn,Sn)5 and (Cu,Zn)6Sn5 are Sn3 (4e) and Cu3 (4a) site because of the lowest lattice strain. The diffusion path interstice size for Cu atom via the IMCs layer gradually shrink with the increment of charge transfer in Zn-rich layers, corresponding the Edf of Cu atom gradually rise. As a result, the fluxes of Cu and Sn via the IMCs layer were balanced out, which reduced the Kirkendall voids formation. Keywords: Sn/Cu solder joint, Zinc, Accumulation behavior, Phase evolution, Kirkendall void

Cobalt-based self-lubricating coatings with the addition of NbC and h-BN were prepared by means of laser cladding on a die steel (Cr12MoV) substrates, aiming at their implementation in metal forming applications involving demanding friction reduction and anti-wear. The effect of different amounts of h-BN with constant content NbC (30 wt%) on the microstructure of the laser-clad Stellite 6 alloy was probed through measures of XRD, SEM, EDS and EBSD. Microhardness, wear properties, and antifriction mechanism of the self-lubricating coatings were also discussed as well. The results proved that with an increase in h-BN content, in addition to NbC phase with polygonal shape observed in the h-BN-free Stellite 6/NbC coating, Nb(C, B) interstitial compounds having multiple grain orientations were formed in inter equiaxed crystal regions. Following the microstructural characterization, the tribological behavior and micro-hardness were analyzed. The results exhibit considerably better anti-friction properties of Stellite 6/NbC/h-BN coating compared to subtract and its excellent tribological performance could be reflected by data of friction test. The lubrication effect of h-BN is proved by the reduction in coefficient of friction. Meantime, the addition of NbC helps in improving micro-hardness and anti-wear. The NbC solidification-growth was detailly discussed in the paper.

Three Ni-Mo-Fe-Si quaternary metal silicide alloy coatings were fabricated on the pure nickel plate by laser cladding. The mixed powders of different compositions of Ni, Mo, Fe, and Si elements were employed as the cladding materials. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electron backscattering diffraction were adopted to investigate the microstructure. The wear properties of three coatings were evaluated under large contact load at room-temperature, dry-sliding wear-test conditions. Results indicated that the phases of three coatings were Ni3Si2/Ni2Si/Mo2(Ni, Fe)3Si as matrix phases and Mo2(Ni, Fe)3Si/Mo6Ni5Si2 as primary phases, Ni3Si2/γ-(Ni, Fe)+Ni31Si12/γ-(Ni, Fe) as matrix phases and Mo2(Ni, Fe)3Si/Mo6Ni5Si2 as primary phases, and γ-(Ni, Fe) as the matrix phase and Mo2(Ni, Fe)3Si as the primary phase for Coatings 1, 2, and 3, respectively. For quaternary metal silicide alloys, ideal configurational entropy of Ni-Mo-Fe-Si alloys increased as the composition tends to equimolar ratio. High entropy will reduce the kinds of phases and be easy to form phases with simple structure in Ni-Mo-Fe-Si coatings. The solubility of Fe element in Mo2(Ni, Fe)3Si phase is high because of the its replacement of Ni in this complicate phase. The average micro-hardness for Coatings 1, 2, and 3 is 987 HV, 977 HV, and 734 HV, respectively. Three coatings exhibited outstanding wear resistance due to a combination of excellent strength and toughness.

TiN reinforced Ni matrix composite coating with in-situ synthesized TiS lubricant phase were fabricated on Ti-6Al-4V alloy by laser cladding with NiCrBSi, TiN and WS2 powder mixtures. The phase compositions, microstructure, microhardness and tribological behaviors are investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) as well as a multi-functional Tribometer. Effect of WS2 contents on the microstructure, microhardness and tribological properties were studied. And the corresponding wear mechanism was discussed. Results indicate that the hardness of the coating (about 900HV0.2) is significantly improved compared with substrate (370HV0.2). Coefficients of friction and wear rates of these coatings have been significantly reduced. Besides, laser-clad coating using the mixed powders of 50%Ni-30%TiN-20%WS2 has good anti-wearing and friction reducing effects, and the main wear mechanisms of that are abrasive wear and micro-plowing.

The reliability of flip-chip Sn/Cu solder joint strongly depends on Kirkendall voids, since it affects the mechanical properties of the solder joints. In this paper, the formation of Kirkendall voids (KVs) at the interface of Sn/Cu joints was investigated by changing various Cu substrates, namely, high-purity Cu (HP Cu), electroplated Cu (EP Cu), and vacuumed-sputtered Cu (VS Cu). It is found that the KVs did not appear at the Sn/HP Cu interface even after aging at 180 °C for 720 h. A large number of voids were observed at the Cu3Sn/Cu interface in Sn/EP Cu and Sn/VS Cu joints during a short aging time. The size of voids formed in the Sn/EP Cu was larger relative to that in Sn/VS Cu. The formation of different voids at the Sn/Cu joints can be attributed to three factors, i.e., diffusion paths for Cu atoms increased from Sn/HP Cu, Sn/EP Cu, and Sn/VS Cu, different impurities (S, Cl, et al.) in Cu substrates, and different stress states at the Cu3Sn/Cu interface.

Laser melting deposition (LMD) is a promising way for the fabrication of complex reduced activation ferritic/martensitic (RAFM) steel components which provides an exceptional opportunity to improve the existing designs and move toward fabricating fine features and complex geometries with higher efficiencies. It is well known that if steel members are welded together, defects will be concentrated in the welded joint. We can control the scanning path of the laser to achieve a complex component molding, avoiding welding. Considering the LMD technology, the results of each layer of powder molding will affect the fineness of the final processed products, such as laser power, scanning speed, defocus and so on, which are all important factors affecting the products. We control the size of powder particle size as a variable to conduct a study on RAFM steel supplemented by LMD technology for the first time, and its microstructure and grain size were analyzed. In terms of mechanical properties, its hardness, Charpy impact, tensile strength and elongation were measured. The results show that compared with other processing processes including Selective Laser Melting (SLM), rolling, welding joints, the tensile strength of RAFM steel manufactured by laser coaxial powder feeding is up to 1057.75 MPa, and there are no pores and almost no defects when using small particle size powder. The microstructure of LMD RAFM steel contains a large amount of lath martensite and δ-ferrite, and some precipitates such as Mx, whose average grain size decreases with the increase of powder particle size (5–150 μm). In this study, the laser additive process was monitored by high-speed photography, and the average grain size of LMD RAFM steel was further analyzed, and a reasonable explanation of its mechanical properties was given.

The remote Laser spiral welding was utilized to zero-gap lap welding of galvanized steel. The relationship between pores in the weld zone and process parameters were investigated. The combination of high keyhole velocity and small spiral spacing was found to be essential for eliminating porosity in the weld area. As the spiral pitch decreases from 0.7 mm to 0.3 mm, the porosity decreases rapidly from approximately 20% to 0%. When the spiral spacing of 0.3 mm, the porosity reaches an acceptable quality level of less than 1%. The porosity tends to decrease as the keyhole velocity increases from 3 m min−1 to 9 m min−1. At the spiral pitch of 0.3 mm and keyhole velocity of 9 m min−1, the porosity in the weld area is almost completely suppressed. The peak shear strength of the spiral weld without porosity was 8819.8 N and the peak shear strength of the spiral weld with porosity was 5104.3 N. The tensile shear performance was increased by 72.8%. The low porosity caused by the spiral laser beam improves the tensile shear strength of the oscillating weld.

Iron-based amorphous alloys have received extensive attention due to their high hardness, elastic modulus/limit and wear/corrosion resistance. In this research, an attempt has been made to develop an amorphous coating of Fe-Cr-Mo-C-B-Y metallic glass coating on the steel substrate through laser surface treatment. During the test, various process parameters are used to determine the position of the amorphous phase. After coating, the microstructure and phase distribution of the coating were analyzed by scanning electron microscope, X-ray diffraction and transmission electron microscope. Mechanical properties of the coating were analyzed by using microhardness testing, abrasion resistance and nanoindentation methods. The results show that the coating thickness varies directly with the incident laser power and interaction time. The microstructure of the coating can be divided into three layers: the first layer (columnar crystals), the second layer (the crystalline phase filled with the unit structure) and the third layer (the unit structure consists of a crystalline phase and an amorphous phase). As the heat input of laser cladding decreases, the volume fraction of the amorphous phase increases, and the average microhardness and nanohardness increase.

The effects of minor Cu (0.7%), Ag (3.5%) and Ni (0.1%) additions in Sn solder on the interfacial transfer and phase evolution were clarified in Sn/Cu solder joints. (Cu,M)6Sn5 and (Cu,M)3Sn (M = Cu, Ni) layers were observed at the Sn/Cu interface during thermal aging at 150 °C. The additions of Cu, Ag and Ni alloy elements were found to change the growth of the Cn–Sn intermetallic compounds (IMCs). The results showed that Cu, Ag and Ni additions were able to suppress the growth of (Cu,M)3Sn (M = Cu, Ni) layers, but promoted the (Cu,M)6Sn5 growth. This was because the reactions on the (Cu,M)6Sn5 side were promoted by the refined IMCs grain size, whose growth was at the expense of (Cu,M)3Sn. In addition, the interfacial transfer rate (thickness ratio, y) of (Cu,M)3Sn/(Cu,M)6Sn5 versus the thermal aging time (t) was found to be close to parabola relationship, the y increased gradually with the alloy elements from Ni, Cu to Ag. The phase evolution (η′-Cu6Sn5, η-Cu6Sn5, e-Cu3Sn, Ag3Sn, η′-(Cu,Ni)6Sn5, η-(Cu,Ni)6Sn5 and e-(Cu,Ni)3Sn) at the interface between Cu and Sn solder doped with minor Cu, Ag and Ni was also elucidated.

TiN and WS2 + hBN reinforced Ni-based alloy self-lubricating composite coatings were fabricated on TC4 alloy by laser cladding using TiN, NiCrBSi, WS2, and hBN powder mixtures. Energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffractometry (XRD), and optical microscopy (OM) were adopted to investigate the microstructure. The wear behaviors of the self-lubricating composite coatings were evaluated under large contact load in room temperature, dry-sliding wear-test conditions. Results indicated that the phases of the coatings mainly include &gamma, Ni, TiN, TiNi, TiW, WS2, and TiS mixtures. The average microhardness of the composite coating is 2.3&ndash, 2.7 times that of the TC4 matrix. Laser cladding TiN/WS2 + hBN/NiCrBSi self-lubricating composite coatings revealed a higher wear resistance and lower friction coefficient than those of the TC4 alloy substrate. The friction coefficient (COF) of the coatings was oscillating around approximately 0.3458 due to the addition of self-lubricant WS2 + hBN and hard reinforcement TiN. The wear behaviors testing showed that the wear resistance of the as-received TC4 was significantly improved by a laser cladding TiN/WS2 + hBN/NiCrBSi self-lubricating composite coating.

A crack-free Fe-Cr-Mo-C-Y-B amorphous coatings has been prepared on the surface of Q235 steel by the method of magnetic-field-assisted laser cladding. The uniformity of the distribution of elements in the coating is increased by alternating the magnetic field, which increases the plastic toughness of the coating. The mechanical behaviour of indentation points after nanoindentation of the coated sample was observed by transmission electron microscopy.

Fe-based bulk metallic glass (BMG)(FeCrMoWMnSiBC) was produced by selective laser melting (SLM) successfully in this study. The best parameters were determined through extensive experiments. The relative density (95%) and amorphous rate (95.47%) samples were obtained by this parameter. The analysis of the microstructure reveals that the crystalline phases in the heat affected zone (HAZ) are mainly α-Fe and M23(CB)6 phases, and co-exist with the amorphous phases. The Heat treatment is employed to study the crystallization behavior of amorphous phases. The α-Fe phase, as the primary phase, grows into a submicron crystal phase under the action of multiple thermal cycles. Nanoindentation test results show that the hardness of the amorphous phase is higher than that of the nano-grain region, and the hardness of the nanocrystalline region is higher than that of the submicron-grain region. The free volume content is different and the amorphous phase is not uniform due to the complex thermal cycle. The maximum hardness occurs in the amorphous phase with 22.6 GPa.

Laser induced micro-nano texture formation on the surface of solid materials has become a research hotspot in the field of material surface modification. However, due to its extremely easy crystallization characteristics, the study of laser-induced surface modification of bulk amorphous alloy lags behind. In this study, femtosecond laser was used to induce one-dimensional LIPSS (Laser-induced periodic surface structures), SWPSS (Super-wavelength periodic surface structure), a mixed state structure of LIPSS + SWPSS and two-dimensional self-organized microholes structure on surface of the Zr55Cu16Ni15Ti10Al4 bulk amorphous alloy. The numerical difference between LIPSS, SWPSS and self-organized microholes structure in different dimensions such as period, groove width, profile height, etc. were compared, and the roughness of different micro-nano textures and the size of surface undulations were evaluated. The effects of different process parameters on the formation and transformation of micro-nano texture on the surface of zirconium bulk amorphous alloys were investigated. The mechanism of the transformation from LIPSS to SWPSS and finally to two-dimensional self-organized microholes structure was analyzed in detail. Finally, Escherichia coli was cultured on the amorphous surface of Zr-based metallic glasses with different structure. To examine the effect of micro-nano texture on the antibacterial properties of Zr-based metallic glasses surfaces. It is found that the self-organized microholes structure has the best effect in reducing the adhesion of bacteria on the surface of the material.

The hot cracking sensitivity dramatically increases because of the longitudinal macro-segregation of craters and the transverse macro-segregation of welding center in pre-layers. Statistical analysis of the total area, average size and Nb concentration of interdendritic low-melting point Laves phase, the Nb concentration of the γ phase in the dendritic core, and the arm spacing of different sub-regions was used to explore the effects of macro- and micro-segregation of Nb. Solidus and liquidus curves were used to determine the temperature at which solid and liquid phases coexist in different sub-regions, which were combined with Scheil equation curves to identify Nb sources and interlayer transfer, so as to develop a model for the molten pool of subsequent layers. This model explains the effect of transversal and longitudinal macro-segregation in the pre-layer on the Nb concentration of the melt pool of subsequent layers, the dynamic balance of the solidification interface and the relative area of Laves phase in the macro-segregated sub-region of the solidified metal.

The macro-structure, microstructure and mechanical properties of 4043 aluminium-alloy parts manufactured by variable polarity cold metal transfer welding (CMT + Advance) have been investigated. The...

A vacuum brazing process using BNi7+3% Cu composite solders to braze Cr18-Ni8 steel was carried out. The joint can be divided into three zones: Isothermally Solidified Zone (ISZ), Athermally Solidified Zone (ASZ) and Diffusion Affected Zone (DAZ). The characteristics of microstructure were observed by scanning electron microscope (SEM). The element distribution in the Cr18-Ni8 steel brazed joint and the content of elements in the organizations were measured by energy dispersive spectrum (EDS). The brazed joints are mainly composed of FeNi3 phases, Ni solid solutions, Ni3P eutectic structures and Cr-Ni-P compounds. The effects of different factors on the microstructure and properties were studied. The growth rate of solid solution increased first and then decreased with the rise of temperature. The hardness of diffusion zone increased first and then decreased when the brazing temperature went up. Cu played an important role to increase the hardness of brazing seam. Prolonging holding time or reducing the brazing gap width was beneficial for the formation of the solid solution. The hardness increased with the extension of holding time. The brazing gap had little effect on hardness. The fracture mode of the joint was ductile fracture. The joints had higher strength when the brazing temperature increased.

Nano-Ni encapsulated h-BN/Ni-based alloy (Ni60) self-lubricating composite coatings on a medium carbon steel were fabricated by laser cladding using two types of lasers: a 5 kW continuous wave (CW) CO2 laser and a 400 W pulsed Nd:YAG laser, respectively. A high-energy ball milling method was adopted to clad nano-Ni onto nano-h-BN with an aim to enhance the compatibility between the h-BN and the metal matrix during laser cladding processing. The microstructure, phase structure and wear properties of the self-lubricating composite coatings were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), as well as dry sliding wear testing. The research indicated that laser cladding of the self-lubricating composite coatings demonstrates sound cladding layers free of cracks and porosities. It was found that a reaction between h-BN and Ni-based alloy occurred, which generated hard phase CrB and Ni3B leading to the increasing of the microhardness of the coatings by CO2 laser cladding, while laser molten pool suppressed h-BN floating up to upper regions of coating for lower temperature and rapid solidification by YAG laser cladding. The high energy ball milling of nano-Ni onto nano-h-BN significantly improved the interfacial compatibility between h-BN and Ni60 matrix. The friction coefficient of the laser-clad Ni60/nano-Ni encapsulated h-BN coating was reduced obviously.

The dynamic behavior of keyhole plays a critical role in determining the weld penetration and process stability during pulsed laser welding of thin-sheet aluminum alloy. This study aimed to explore the influence mechanism of the keyhole instability on the penetration depth under different pulsed laser parameters. To this end, we firstly applied a high-speed camera to directly observe the oscillation characteristic of inner keyhole through a transparent glass, and its fluctuation period was nearly coincided with one pulse duration. Based on maximum deviation ratio, the pulse frequency has a more obvious effect on the fluctuation in keyhole depth comparing with laser power and focus distance. Along with the increase of laser power and frequency, the incident laser intensity also increased, which leads to an increasing of keyhole depth and decreasing of inclination angle. The weld penetration depth is determined primarily by the variation of the keyhole depth and inclination angle. The findings of this paper will provide theoretical basis for achieving the desired weld penetration and improving weld quality regarding keyhole formation.

The investigation of rock damage behaviour is an important requirement for ensuring stability control and safety prediction in rock engineering. In this study, based on the linear energy dissipation law and energy dissipation coefficient, a theoretical method is introduced for characterising the damage of intact rocks under uniaxial compression conditions. The existing uniaxial compression test data of 14 kinds of rock materials (including 6 types of granite , 4 types of sandstone , 2 types of marble, 1 type of slate, and 1 type of limestone) were used to verify the effectiveness of the damage characterization method. The results indicate that the damage evolution process of rocks under uniaxial compression can be fully expressed by the proposed theoretical damage characterisation method, and the damage variable exhibits a quadratic functional relationship with the loading stress level. Moreover, with the linear energy storage law serving as a theoretical basis, the peak dissipated strain energy in a rock damage expression under uniaxial compression can be accurately calculated. The new theoretical damage characterisation method overcomes the shortcomings of conventional damage characterisation methods (e.g. estimating the peak dissipated strain energy through a hypothesis), and provides a new means for analysing rock damage from an energy viewpoint.

Monocrystalline silicon is highly promising for applications as semiconductor materials in electronic devices. However, the intrinsic brittleness of Si as well as the relatively low conductivity has restricted its wide application. In this paper, the optimized electronic and mechanical properties of monocrystalline silicon doped with trace content of N or P (SiMx, X = 0, 0.2%, 0.46%, 1.5%, M = N, P) were investigated by first-principles calculations. The results show that the doping of N or P elements can improve the mechanical and electronic properties of monocrystalline silicon, and with the increase of doping concentration, the improving effect becomes more obvious. In addition, the doping of N has a stronger effect on Si than that of P. The doping N, P atoms can form polar covalent bonds to enhance the covalency between atoms, improve the mechanical of Si. Electronic structure analysis shows that the doping of N or P atom makes the hybridization of s-orbital and p-orbital stronger, thus narrows the band gap and improves the conductivity of Si. Furthermore, as the concentration of doping elements increases, Si goes from indirect to direct semiconductor, which is beneficial to improve the recombination of excited electron-hole pairs due to there is no momentum difference between the two positions in k space.

The KF-2-WC-CaF2 Ni-based self-lubrication coatings were synthesized on the surface of 6061 aluminum alloy with preset powder by laser cladding / friction stir processing (FSP). OM, SEM, EDS and XRD were used to characterize the phase constituents, microstructure and properties of the coatings, and the micro-hardness distribution and wear properties were also analyzed. The results showed that smooth and good metallurgical bonding zone is formed between the coating and the substrate. The microstructure of the one-stepped coating is dendrite, cell crystal, columnar crystal and equiaxed crystal. The main phase constituents of the two-stepped coating consist of α-Ni, α-Al, AlNi3, Al3Ni2, WC and CaF2. And the micro-hardness is significantly higher than that by direct laser cladding. With the increase of distance between coating and stirring tool, mechanical effect gradually weakened, micro-hardness curve and friction coefficient showed a gradient feature along the thickness direction.

In recent years, the twin wire arc additive manufacturing technique has been proved feasible of fabricating titanium aluminides with low cost. To address the concerns on the low thermal efficiency induced by the previous gas tungsten arc welding-based process, in the present research the twin-wire plasma arc additive manufacturing system is developed using the compressed plasma arc power. Subsequently, the newly developed system is used to fabricate the binary Ti–45Al titanium aluminide to verify the feasibility. And the analysis on microstructure and mechanical properties is performed afterwards. Generally, the titanium aluminide with target composition and low texture level has been successfully fabricated using the compressed plasma arc system. Compared to other titanium aluminides fabricated using the previous system, the layer band microstructure is considerably weakened. However, the intergranular cracks in the as-fabricated deposition indicate that higher interpass temperature is certainly required for the new technique. The improvement of the microstructural homogeneity achieved by the new system provides valuable reference for weakening the microstructural and mechanical anisotropies in the wire arc additively manufactured titanium aluminides.

It is difficult to collect the crack propagation signal under general continuous welding condition due to other signal interference of molten pool. In order to study the effect of residual stress on crack propagation, acoustic emission technology was successfully applied to monitor welding process according to the characteristics of pulsed laser welding. Crack free welding is achieved by reducing the pulse interval to limited the crack size of single pulse welding spot. The welding process was monitored synchronously by high speed photography and acoustic emission, the evidence of crack propagation after solidification of weld is successfully captured.

In this work, the effects of electroplated Cu (EP Cu) and Cu addition (0.7%) in Sn solder on the intermetallic compounds (IMCs) growth and void formation were clarified by comparison with solder joints comprising of high purity Cu (HP Cu) substrate and pure Sn solder. After aging processes, a new IMC, Cu3Sn, was formed at the interface, in addition to Cu6Sn5 formed in the as-soldered joints. The EP Cu and Cu addition (0.7%) both had limited effects on the total IMCs thickness. However, the effects varied on the growth behaviors of different IMCs. Comparing to the void-free interface between Sn and HP Cu, a large number of voids were observed at the Cu3Sn/Cu interface in Sn/EP Cu joints. The formation of these voids may be induced by the impurities and fine grain, which were introduced during the electroplating process. The addition of Cu suppressed the inter-diffusion of Cu and Sn at the interface. Consequently, the growth of the Cu3Sn layer and formation of voids were suppressed.

The name of coauthor Peilei Zhang was misspelled as “Peilie Zhang” in this article as originally published.

Solidification crack was a common welding defect in welding of aluminum alloy. When solidification shrinkage and heat shrinkage of the vulnerable zone and its surrounding solids were hindered, it will cause grain boundary cracking owing to insufficient supply of liquid metal. Therefore, cracks generally appear in the middle part of aluminum alloy welding spot at the late stage of solidification. Compared with continuous laser, pulsed laser could meet more precise heat input control and was widely used in aluminum alloy sheet welding. In this research, by designing different pulse shaping to improve the solidification condition of the 5083 aluminum alloy pulsed laser welding spot, the numerical simulation was used for analyzing the solidification rate and temperature gradient of molten pool. Application of ramp-down pulse decreased crack cross-sectional area from 420475 µm2 to 0, and step-down pulse could also effectively control the crack radius. The results showed that effect of suppressing solidification crack on the horizontal direction of the welding spot was obvious to the vertical direction. Using ramp-down or step-down pulse shaping also reduced the volume of the liquid molten pool at the end of the laser process, which decreased the shrinkage stress at the late stage of solidification.