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Here, in order to develop the metallic materials with excellent relationship between strength and toughness for application, the FeCoCrNi + FeCoCrNiAl-laminated high-entropy alloys (HEAs) had been fabricated by laser melting deposition (LMD) additive manufacturing technique. The process parameter databases for FeCoCrNi and FeCoCrNiAl HEAs were built by orthogonal experiments and statistics method, which helped to select the optimised process parameters for the two HEAs. Then, the differences in the forming process, phase structure, and grain morphology of FeCoCrNi and FeCoCrNiAl HEAs were systematically investigated during the fabrication of the FeCoCrNi + FeCoCrNiAl-laminated HEAs. The results show that variations in surface tension, wettability, and other physical properties between the two HEAs led to significant differences in their LMD processes. Al not only influenced the fabrication process of both HEAs, but also promoted the phase transition from FCC (FeCoCrNi) to BCC (FeCoCrNiAl). In addition, Al also acted as a strong limiting factor to inhibit the effect of supercooling on the grain morphology, which transformed from coarse columnar grains to fine equiaxed grains. The columnar grains, with a preferred orientation in the FeCoCrNi-deposited wall, promoted the suppression of the inhibitive effects of the phase structure and strong limiting factor, led to the growth of the columnar grains in the FeCoCrNiAl-deposited wall, and helped achieve long-distance continuous growth of columnar grains across the interface in the subsequently deposited wall. The methodology used in this study could be meaningful for the exploration of process parameters of LMD, and the discoveries of grain characteristic help to understand the microstructure transition in laminated heterogeneous HEAs fabricated by laser additive manufacturing.

In this paper, Al/Ti joints are produced by resistance element welding (REW). A hole is made in the Al sheet and a welding element (Ti6Al4V rivet) is inserted into the hole followed by resistance welding to create a metallurgical bond between the Ti rivet and Ti sheet. The mechanical properties, fracture mode, microstructure, and hardness distribution of the joints are analyzed. The REW joints show higher tensile-shear load and energy absorption than present riveted joints. The maximum peak load and energy absorption of the REW joints improved with the increasing of rivet diameter. The failure mode for Al/Ti REW joints transits from interfacial to pullout mode, and then to fully pullout mode with increasing welding current. The nugget microstructure mainly consists of acicular α′ phase martensite. A discontinuous intermetallic compound layer is formed at the Ti/Al interface, and it consists of TiAl2, TiAl, or a mixture thereof.

FeCoCrNiMnAl high-entropy alloy cladding was fabricated on the surface of 4Cr5MoSiV1 substrate by laser cladding technology. The microstructure, hardness and wear resistance of the cladding layer w...

In this study, the high-strength pipe steel, i.e. X80, was heated at 260 °C for half an hour followed by air cooling for thermal simulation of pipe coating process. During the uniaxial loading of the specified tension samples, the migration behaviour of Luders bands and the re-distribution of stress after strain ageing heat treatment were investigated using digital image correlation. Digital image correlation was adopted to characterise the local axial and shear strain fields of samples that were extracted from different locations across the wall thickness of the high-strength pipe steel, and electron backscatter diffraction was used to compare the evolution of crystallographic texture among these locations. Other characterisation methods regarding microstructure and mechanical properties were conducted meanwhile, including optical microscopy, scanning electron microscopy and X-ray diffraction to accurately define the detailed performances that strain ageing, i.e. being heated to 260 °C for half an hour, has led to. The obtained results show that the Luders band-related microstructure, dislocation density, lattice constant and texture vary at the different locations across the wall thickness and influence the final mechanical properties of the selected high-strength pipe steel.

This review comprehensively covers the research accomplished in the field of laser welding of biomedical devices and implant materials. Laser welding technology in the recent past has been ...

Mg alloy/austenitic stainless steel (ASS) joints were produced by resistance element weld-bonding (REWB) and resistance spot weld-bonding (RSWB) techniques. Both joints consisted of two zones, namely, the adhesive zone and weld zone. The weld zone of the REWB joints was formed through a metallurgical reaction between the rivet and ASS. The microstructure of the fusion zone consisted of austenite and intercrystalline delta ferrite. The weld zone of the RSWB joints was formed via welding-brazing mechanism (involving molten Mg alloy and solid ASS), and the nugget was formed in the Mg alloy. The nugget microstructure consisted of fine columnar dendritic structure. Digital image correlation (DIC) analysis was used to study the deformation behavior and strain evolution in the loading direction during lap-shear test. The REWB and RSWB joints experienced lower degree of out-of-plane displacement, sustained higher peak load, and exhibited higher strain to failure than their conventional counterparts. However, the REWB joints exhibited superior lap-shear performance, with high energy absorption and favorable failure mode (pull out failure in the ASS sheet). Upon the failure of the adhesive zone at peak load, the load was sustained by the weld, and the final failure occurred via pullout mode in the ASS. The RSWB joints failed in a sudden manner, mainly by cohesive failure in the adhesive zone and Mg alloy nugget/ASS interface failure in the weld zone.

Dissimilar lap joining of AZ31B Mg alloy to Cu-Ni coated Ti-6Al-4V was carried out by laser welding-brazing method. The effect of Cu and Ni contents on interfacial reaction and joint fracture load were analyzed. For the joint in which the Ni coating (15.36 µm) was thicker than the Cu coating (5.47 µm), thick intermetallic compound (IMC) composed of a mixture of light gray Al-Ni-Ti + Ti3Al + Ti2Ni phases mingled with dark gray Ti3Al was produced at the interface. In comparison, a mixed interfacial reaction layer consisted of Ti2Ni and Ti3Al was formed from the direct irradiation zone to the weld toe zone of the joint with comparable Ni and Cu coating thicknesses (10.78 µm Cu–9.30 µm Ni). In this case, the thickness of the mixed layer was below the critical thickness of 10 µm. For the joint in which the Cu coating is much higher than the Ni coating thickness (17.12 µm Cu–4.23 µm Ni), Ti3Al and Ti2Ni mixed reaction layer was produced at the brazed interface of direct irradiation zone, whereas, only Ti3Al phase was formed at the middle zone. At the weld toe zone, Ti2Cu uneven interfacial reaction layer was evolved. With increasing Cu and decreasing Ni coating thicknesses, the fracture load first increased and then slightly decreased, the maximum tensile-shear fracture load attained 2020 N for joints with comparable Cu and Ni coating thicknesses. This is twofold higher than that of uncoated joint. The tensile-shear investigation showed that the joint would fracture at the fusion zone when the coating thickness of Ni was comparable or higher than Cu. In contrast, interfacial failure was observed when the thickness of Cu was much higher than the Ni. For the joint with interfacial failure mode, tear ridge was observed from the fracture surface, whereas, the fusion zone fracture surfaces was noted to display a typical dimple feature. The authors would like to acknowledge the support provided by University of Malaya (Project No. GPF073A-2018 ), National Natural Science Foundation of China (Project No. 51504074 and 51875129 ) and Key Research & Development Program in Shandong Province (Grant No. 2017CXGC0811 and 2017GGX30147 ). Scopus

Recently, there has been increased need for alternative lubrication techniques in grinding of advanced engineering ceramics. Literatures have indicated that the minimum quantity lubrication (MQL) method is a viable option for lubrication during grinding operations. The MQL technique was found to be non-hazardous, sustainable, eco-friendly, inexpensive and possess superior tribology than the conventional flood cooling technique. This work entails an experimental investigation on grinding of silicon nitride (Si3N4) ceramic using MQL with vegetable-oil-based silicon dioxide (SiO2) nanofluid. The grinding experiments were conducted using different process based on Taguchi L16 experimental design. The acquired results of tangential, normal grinding forces, and surface quality were analysed using Taguchi SN ratio analysis. Subsequently, an ANFIS prediction model was developed to analyse the effects of the process parameters on the forces and surface quality. The developed ANFIS prediction model was found to be highly accurate in predicting the performances of the grinding operations. The results obtained show that, by increasing the nanofluid concentration, the grinding forces decreases significantly, with resultant improvement in the surface quality. The 2 w/w percentage was found to be the optimum nanofluid concentration whereas the air pressure of 0.8 MPa gave better performance. Lastly, an optimum parameter setting for grinding the engineering ceramic was obtained.

Purpose Optimisation of grinding processes involves enhancing the surface quality and reducing the cost of manufacturing through reduction of power consumptions. Recent research works have indicated the minimum quantity lubrication (MQL) system is used to achieve near dry machining of alloys and hard materials. This study aims to provide an experimental analysis of the grinding process during machining of aluminium alloy (Al6061-T6). MQL nanofluid was used as the lubricant for the grinding operations. The lubricant was formed by suspending silicon dioxide nanoparticles in canola vegetable oil. The effect of input parameters (i.e. nanoparticle concentration, depth of cut, air pressure and feed rate) on the grinding forces and surface quality was studied. Adaptive neuro-fuzzy inference system (ANFIS) prediction modelling was used to predict the specific normal force, specific tangential force and surface quality, the ANFIS models were found to have prediction accuracies of 97.4, 96.6 and 98.5 per cent, respectively. Further study shows that both the specific grinding forces and surface roughness are inversely proportional to the nanofluid concentration. Also, the depth of cut and table feed rate were found to have a directly proportional relationship with both the grinding forces and surface roughness. Moreover, higher MQL air pressure was found to offer better delivery of the atomised nanofluid into the grinding region. Design/methodology/approach Grinding experiments were performed using MQL nanofluid as the lubricant. The lubricant was formed by suspending silicon dioxide nanoparticles in canola vegetable oil. The effect of input parameters (i.e. nanoparticle concentration, depth of cut, air pressure and feed rate) on the grinding forces and surface quality has been studied. Findings The grinding process parameters were optimised using Taguchi S/N ratio analysis, whereas the prediction of the response parameters was done using ANFIS modelling technique. The developed ANFIS models for predicting the specific normal force, specific tangential force and surface quality were found to have prediction accuracies of 97.4, 96.6 and 98.5 per cent, respectively. Further findings show that both the specific grinding forces and surface roughness are inversely proportional to the percentage of nanoparticle concentration in the lubricant. Also, the depth of cut and table feed rate were found to exhibit a direct proportional relationship with both the grinding forces and surface roughness, while high MQL air pressure was observed to offer more efficient delivery of the atomised nanofluid into the grinding region. Practical implications The work can applied into manufacturing industries to prevent unnecessary trials and material wastages. Originality/value The purpose of this study is to develop an artificial intelligent model for predicting the outcomes of MQL grinding of the aluminium alloy material using ANFIS modelling technique.

Different types of aluminum alloy 5052 (AA5052) resistance spot clinching joints with different local stiffness values were designed in this study. The values of the polar moment of inertia in the cross-section of the fusion zones were calculated. Digital image correlation (DIC) technique was used to study the local rotation and in-plane strain distribution during the lap-shear test of half-cut samples. The results show that the joint with higher local stiffness (polar moment of inertia) value experiences the lowest joining zone rotation during the lap-shear test. This leads to higher proportion of normal component of strain/stress at the periphery of the joining zone. According to Tresca or von Mises failure criterion, for a spot joint, high proportion of normal stress at the load-bearing site enables the global joint to sustain high peak load during tensile test. This mechanism indicates that, compared with conventional resistance spot welding process, the resistance spot clinching technique can improve the mechanical properties of AA5052 joints by increasing the local stiffness.

AZ31B and Ti-6Al-4V were lap welded using laser welding-brazing technique with and without Ni, Cu, and Cu-Ni coatings under similar welding conditions. The effect of the coatings on the appearance, microstructural evolution, and mechanical properties of the joint was studied. For the uncoated joint, only Ti3Al phase formed along the entire brazed interface. In comparison, Ti3Al phase was observed at direct irradiation and middle regions, whereas, Ti2Cu phase was produced at the weld toe region of Cu-added joint. For Ni- and Cu-Ni-coated joints, Ti2Ni mingled with Ti3Al interfacial reaction products evolved from the direct irradiation region to the weld toe region. Interestingly, the thickness of Cu-Ni joint mixed reaction layer was less than the critical thickness of 10 μm. The Cu-Ni-coated joint sustained the highest tensile shear fracture load of 2014 N (which was about 100% higher than that of the uncoated joint), followed by the Ni- and Cu-added joints with 1795 N and 1680 N, respectively. The fracture mode of the uncoated and Cu-added joints was interfacial failure, whereas, FZ fracture with typical dimple feature was observed for both Ni and Cu-Ni joints.

Nigeria a nation adequate blessed with large deposit of non-ferrous metallic ores which are yet to be fully harnessed, characterized, and beneficiated due to lack of sufficient studies and technology. The non-ferrous metallic ores found in Nigeria include lead-zinc ore, tin ore (Cassiterite), niobium ore, uranium ore, and precious metals such as (Gold and Silver). This article focuses on trends in previous works on characterization and beneficiation of some non-ferrous metallic ores deposits, showing the mineral, elemental contents, and the amount present in a particular ore. Knowledge gap in the field of study was identified to enable further research on non-ferrous metallic ore characterization and beneficiation of yet to be characterized and beneficiated non-ferrous metallic ore which include; columbite, cassiterite, tantalite, chalcopyrite, gold, niobium ore, etc. Also, in-depth and further studies be done on lead-zinc ore, which is the most characterized and beneficiated non-ferrous metallic ore in Nigeria.

In recent years, there is an increased in used of titanium alloys for some parts of mass-produced automobiles and aerospace. However, titanium alloys are characterized by difficult machinability, high melting temperature, high strength, low thermal conductivity, and high reactivity to oxygen, which overshadowed conventional manufacturing processes. To this end, there is a pressing need for more efficient technologies for the manufacture of low-cost titanium structures. Over the years, several joining techniques have been considered for fabrication of titanium alloys. Nevertheless, laser beam welding presents a viable option for welding of titanium due its versatility, high specific heat input, and flexibility. To date, under optimum processing conditions, the strength of the laser-welded titanium alloys can be close to the original material; however, there are still some processing problems such as lower elongation and corrosion resistance coupled with inferior fatigue properties. In this document, the laser beam welding of similar and dissimilar titanium alloys is reviewed, focusing on the influence of the processing parameters, microstructure-property relationship, metallurgical defects, and possible remedies.

In the present research, the double FeCoCrNiCux medium-entropy alloy coatings were manufactured on Cr12MoV die steel by laser cladding technology. The cross-section, microstructure, phase, micro-hardness, and tribological behaviour of the coatings were investigated systemically. The dilution effect of the substrate had the weaker effect on the second layer compared to the first layer. The double FeCoCrNiCux cladding layers only had a single face-centred cubic phase led to the lower micro-hardness and wear resistance. The different atomic radius of the alloying elements in the coatings generated a large lattice dislocation. The face-centred cubic structure of coatings had a higher hardness than that of the traditional face-centred cubic structure. The Cu element is easy to segregate at the grain boundaries to form the Cu-rich solid solution.

The influence of laser parameters and alloying elements on the dilution rate of FeCoCrNi coating was investigated. It was found that the alloying elements have more significant effect on the dilution rate than the laser cladding parameters. Optical microscopy, scanning electron microscopy, X-ray diffraction analysis, micro-hardness test, wear test, electrochemical corrosion test, and high-temperature oxidation test were conducted to study the influence of dilution rate on the microstructure and properties of the FexCoCrNi coatings. The results have shown that the transfer of Fe element from the substrate could make the mixed entropy of the Fe0CoCrNi coating close to the theoretical mixed entropy of the FeCoCrNi high-entropy alloy. Excess Fe element in the original powder increased the dilution rate, leading to a reduction in the mixed entropy of the coating. However, it did not lead to the generation of new phases in the coating. The atomic radius differences between the elements could lead to large lattice distortion in the FCC crystal lattice. The level of lattice distortion and micro-hardness of the FexCoCrNi coatings increased with increasing Fe element content. Furthermore, the excess Fe elements weakened the self-passivation ability and deteriorated the high-temperature oxidation resistance of the coating. Keywords: Dilution rate, FexCoCrNi, FCC crystal lattice, Lattice dislocation, Self-passivation ability, High-temperature oxidation

With exponentially increasing copper alloys consumption globally, the automatization, high efficiency, and quality improvement of the copper joining process become inevitable in modern industrial production. In this context, laser welding presents a particular suitability for joining of copper alloys because of its high precision, high-energy concentration, and rapid processing potentials. Studies have shown that laser welding of copper has two principal challenges. Namely, the high laser power required due to the low laser beam absorptivity and high thermal conductivity of copper, and the low process stability. Interestingly, with the recent laser technology improvements, the tremendous laser power required will eventually be solved. Although some achievements were obtained in previous studies, the problem of spattering and high porosity associated with low process stability still exist. In this review, the effect of the most significant laser welding processing parameters on the joint’s performances were systematically examined; the microstructure, mechanical, and electrical properties and metallurgical defects of copper laser welds were discussed. Furthermore, laser welding of copper to other metals particularly copper to steel and copper to aluminum dissimilar alloys in terms of structure and properties relationships were also discussed.

With the growing demand for vehicle weight reduction and the increased application of multimaterial design, it is imperative to address the challenges of welding dissimilar light alloys. This paper presents a review on laser welding of Al/Mg, Al/Ti, and Mg/Ti alloys, with focus on the techniques used to suppress the formation of brittle intermetallic compounds (IMCs) and improve joining mechanism. For Al/Mg joints, studies have shown that the use of structural adhesives, the use of interlayers such as Ce, Ni, Ti, and Fe foils and Zn-Al filler metals, and hybrid adhesive-interlayers could suppress the formation of brittle IMCs and improve joint strength. The formation of brittle IMCs during laser welding of Al/Ti alloys could be minimized by offsetting the laser beam at an appropriate distance towards either the Al or Ti alloy, the use of split-beam laser with appropriate joint design, the use of high welding speed, improving the laser energy distribution, and the use of V-groove with Al-Si filler metals. For Mg/Ti alloys, offsetting the laser beam at an appropriate distance towards the Mg alloy, the use of AZ series Mg alloy-based filler wires, and coating the Ti alloy with Ni were found to facilitate the joint formation and improve the joint strength.

Ceramic particles can be used as reinforcements to improve the mechanical properties of high entropy alloy (HEA) coatings. However, brittle and hard ceramic particles cause high dislocation density and defects in the coating, which are the potential sites for crack initiation. In this study, in order to improve the crack resistance of CoCrFeMnNi + x(TiC) composite coating (CC), CoCrFeMnNi cladding layer was pre-coated on the substrate before fabricating the composite cladding layer. The results showed that the CoCrFeMnNi transition cladding layer with good plasticity could effectively suppresses crack propagation in the composite cladding layer under compression, and no cracks were found in the substrate. This paper provides a new idea for enhancing the properties of CC.

In this paper, the laser melting deposition (LMD) technology was applied to fabricate FeCoCrNi high entropy alloy (HEA), and the microstructure of the deposited HEA (FeCoCrNi-LMD) was systematically studied. The width of the columnar grains was found to increase gradually from the bottom to the top of the material. The top layer consisted of equiaxed grains. Tensile stress and compressive stress were applied to the material along and perpendicular to the direction of columnar grain growth, respectively. Electron Backscatter Diffraction (EBSD) and transmission electron microscopy (TEM) were used to observe the changes in the microstructure and properties of the material under the applied stresses. It was found that the phase structure of the material did not change when the stress was applied, but the maximum polar density increased. After stretching, the Schmid factor decreased, resulting in strain hardening. Dislocations moved during the plastic deformation and their number increased, resulting in many small angle grain boundaries and subgrain boundaries. The dislocations concentrated at the grain boundaries, leading stress concentration at the grain boundaries. In addition, due to the differences in the distribution of elements between the grains and the grain boundaries, the properties of grain boundaries changed. Cracks were produced easily along the grain boundaries during the tensile test. Under the compressive stress (rolling), for the same amount of rolling deformation, as the width of the columnar grains increased from the bottom to the top of the FeCoCrNi-LMD, the size of the columnar grains increased. As a result, the columnar grains were inclined at an angle to the rolling direction, and the rolled specimen was bent.

An AZ31 Mg alloy and 316L austenitic stainless steel (ASS) were joined via resistance spot welding (RSW) and resistance element welding (REW). The RSW joints were found to be produced through welding-brazing mode, in which the Mg alloy melted and spread on the solid steel, forming a nugget only in the Mg alloy. The microstructure of the nugget consisted of columnar dendritic structure, indicating that columnar-to-equiaxed transition was interrupted. Shrinkage porosity and cracking were also observed in the RSW nugget. In contrast, a two-zone nugget was formed during REW, consisting of a peripheral nugget on the ASS side and the main nugget. Compared with the RSW joints, the REW joints were obtained at lower welding currents, and they exhibited superior mechanical performance, with 63% higher peak load and 9 times higher energy absorption. Irrespective of the welding current, the RSW joints failed in interfacial failure mode, while the failure mode of the REW joints transited from interfacial to pull out with increased welding current.

To use carbon nanotube fibers (CNT) extensively in a wide range of electrical and electronic applications, an essential key step is to produce a low-resistance, high-strength and reliable connection between the CNT fibers and other live parts in the circuit. In this study, meniscus-confined electrochemical deposition (ECD) process with silver was proven to be a practical way of joining CNT fibers together head-to-head. The whole ECD process was stable. The shape of the joints was found to depend on the shape of the tips of the CNT fibers. The deposited silver exhibited a dense and uniform microstructure and it was tightly bound to the CNT fibers, with a distinct interface between them. In the ECD process, the original morphology of the CNT network was maintained. The lowest electrical resistance of the CNT fibers joints was measured to be 8.72[Formula: see text][Formula: see text], which is 45% lower than that of the original CNT fibers. The deposited joint sustained a fracture load of 7.5cN with an elongation of 0.4%.

Experiments were carried out on 4 mm thick 304 stainless steel to evaluate the keyhole behaviour in keyhole mode Tungsten Inert Gas (K-TIG). Based on the vision system, keyhole exit dimensional and positional parameters were measured from the keyhole exit image sequence. K-TIG had very narrow parameters window to get stable keyhole process in the constant parameters keyhole welding process. “One pulse one open keyhole” K-TIG welding process was achieved with square waveform pulse welding current. Stable welding process and acceptable quality welds were obtained with a relatively lower average welding current. Pulse keyhole process is feasible to improve the process window to achieve stable welding process to obtain qualified weld in K-TIG.

Jet electrochemical machining is a promising shaping method that has the potential to replace traditional sinking ECM in the industry owing to its flexibility and stability. However, its machining quality is impaired by the occurrence of a unique phenomenon (i.e., the anode cannot be uniformly dissolved with high nozzle travel rate). This paper aims to analyze this unique phenomenon from the viewpoint of anodic behavior and to select the optimum travel rate parameter window. The anodic behavior of TB6 titanium alloy in sodium chloride solution was investigated by linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry. A time-dependent mechanism of pitting/large localized corrosion/polishing for the anodic interface structure was developed. The full electricity quantity of the equivalent oxide layer capacitor, Q , was proposed as a criterion for the initiation of uniform dissolution state. Subsequently, a novel model that incorporates the parameter Q was developed to account for the effect of anodic behavior. This model is able to predict the machined profile as well as the localized corrosion range. Finally, the effect of nozzle travel rate on jet ECM performance was investigated, and a nozzle travel rate of 10 to 25 μm s −1 was selected as the optimum parameter window in the present work.

A fiber laser welding-brazing procedure has been developed for joining AZ31B magnesium alloy to Cu-Ni coated Ti-6Al-4V titanium sheet using AZ92D filler wire. The effect of the interlayer arrangements (AZ31B/Ni-Cu/Ti-6Al-4V and AZ31B/Cu-Ni/Ti-6Al-4V) on appearance, interfacial reaction and mechanical properties were investigated at different heat input. It was found that the feasibility of this process depends strongly on the pre-existing Cu-Ni layer on the Ti surface that promotes wetting of the AZ92 filler. Within the range of 1200-1600 W, defect free joints in both interlayer arrangements. Depending on the interlayer arrangements chosen, different reactions layers formed inside the joint region. Nevertheless, at optimum heat input (1400 W), Ti2Ni mingled with Ti3Al interfacial reaction products was produced along the fusion zone (FZ)-Ti brazed interface in both interlayer arrangements. The tensile-shear fracture load of the joints produced at the optimum laser power reached a maximum value of 2016.5 N for AZ31B/Ni-Cu/Ti-6Al-4V and 2014.6 N for AZ31B/Cu-Ni/Ti-6Al-4V, representing an efficiency of 71% compared to AZ31B alloy. Under suitable heat input, the joints failed at the fusion zone of the AZ31B base metal. In contrast, incomplete brazing or large volume of intermetallics at the brazed interface resulted in interfacial failure at lower/higher heat input. This work was financially supported by University of Malaya (Grant No. GPF073A-2018 ), National Natural Science Foundation of China (Grant. No. 51504074 and 51875129 ) and Key Research & Development Program in Shandong Province (Grant No. 2017CXGC0811 and 2017GGX30147 ). Scopus

The FeCoCrNiMnAlx (x = 0.5, 0.75) high-entropy alloy coatings were produced by laser cladding method, and the influence of phase transition was studied. The density and distribution of dislocation, the evolutions of phase and microstructure were investigated by using EBSD, XRD and TEM. Results presented that the higher content of Al element not only refined the grain size of the cladding layer, but also promoted the transition from FCC phase to BCC phase. All the phases in the FeCoCrNiMnAlx (x = 0.5, 0.75) cladding layers developed serious lattice distortions. The anti-plastic deformation ability of the FeCoCrNiMnAl0.75 cladding layer was effectively improved due to the accumulation of a large number of dislocations in the soft FCC phase. The yield strength and tensile strength of the FeCoCrNiMnAl0.75 cladding layers were improved significantly under the joint action of fine-grain strengthening, lattice distortion enhancement, dislocation strengthening and solid-solution strengthening.

The two-phase FeCoCrNiMnAl high entropy alloy cladding layer was produced by laser cladding method, and its strengthening mechanism was studied. Using XRD, EDS, TEM and EBSD analyses, the phase and microstructural evolutions, dislocation density and distribution, and degree of lattice distortion were investigated. Nano-indentation and tensile tests were employed to study the hardness and tensile properties of the cladding layer, respectively. The results show that Al element, due to its large atomic radius, promoted the formation of BCC solid solution in the cladding layer and significantly refined the grains. Diffraction peak shift, change lattice constant change, interplanar spacing and crystal plane included angle calculations indicate that all the phases in the FeCoCrNiMn and FeCoCrNiMnAl cladding layers experienced serious lattice distortions. Dislocations in the FeCoCrNiMnAl coating mainly accumulated and moved in the softer FCC phase, increasing the material's resistance to plastic deformation. The combined effects of grain size, lattice distortion, dislocation density and dual-phase interaction significantly increased the plastic deformation resistance (i.e., tensile strength, yield strength and microhardness) of the FeCoCrNiMnAl coating compared to the FeCoCrNiMn coating, but also reduced the elastoplasticity.

This paper presents a review on the resistance spot welding (RSW) of Al/Al alloys, Al alloys/steel, Al/Mg alloys, and Al/Ti alloys, with focus on structure, properties, and performance relationships. It also includes weld bonding, effect of welding parameters on joint quality, main metallurgical defects in Al spot welds, and electrode degradation. The high contact resistance, induced by the presence of oxide layer on the surface of Al alloys, and the need for application of high welding current during RSW of Al alloys result in rapid electrode tip wear and inconsistency in weld quality. Studies have shown that cleaning the oxide layer, sliding of a few microns between sheets, enhancing the electrode force, and the application of a low-current pre-heating can significantly reduce the contact resistance and improve joint quality. For Al/steel dissimilar RSW, the technique of resistance element welding, the use of optimized electrode morphology, the technique of RSW with cover plates, and the use of interlayers such as Al-Mg, AlSi12, and AlCu28 alloys were found to suppress the formation of brittle intermetallic compounds (IMC) and improve the joint quality. The employment of pure Ni foil, Au-coated Ni foil, Sn-coated steel, and Zn-coated steel interlayers was also found to restrict the formation of brittle IMCs during RSW of Al/Mg alloys. Furthermore, the techniques of RSW with cover plates and RSW under the influence of electromagnetic stirring effect were found to improve the weldability of Al/Ti dissimilar alloys.

This paper presents a review on resistance spot welding of magnesium alloys, with emphasis on the relationship between microstructure, properties, and performance, under quasi-static and dynamic loading conditions. It also compares the resistance spot welding of magnesium-to-aluminum alloys and the various techniques used to suppress the formation of brittle intermetallic compounds. Resistance spot welding of magnesium-to-steel, weld bonding, the effects of process parameters on joint quality, and the main metallurgical defects in resistance spot welding of magnesium alloys are also deliberated. Studies have shown that the pre-existence of coarse second phase particles in the base metal, the addition of particles, such as titanium powder, and welding under the influence of electromagnetic stirring effect can promote columnar-to-equiaxed transition, microstructure refinement, and improvement in mechanical properties of magnesium alloys resistance spot welds. For magnesium-to-aluminum alloys spot welds, the use of interlayers, such as pure nickel, gold-coated nickel foil, and zinc-coated steel, was found to suppress the formation of brittle intermetallic compounds and thus significantly improve the joint strength.

Since its inception, laser beam welding as a high-quality fusion joining process has ascertained itself as an established and state of art technology exhibiting tremendous growth in a broad range of industries. This article provides a current state of understanding and detailed review of laser welding of titanium (Ti) alloys with corresponding dissimilar counterparts including steel, aluminium, magnesium, nickel, niobium, copper, etc. Particular emphasis is placed on the influence of critical processing parameters on the metallurgical features, tensile strength, hardness variation, percentage elongation and residual stress. Process modifications to improve dissimilar laser weldability by virtue of techniques such as laser offsetting, split beam, welding-brazing, hybrid welding and materials modifications by means of the introduction of single or multiple interlayers, fillers and pre-cut grooves are exploited. Detailed and comprehensive investigations on the phenomena governing the formation and distribution of the intermetallic phase, material flow mechanisms, their relations with laser parameters and their corresponding impact on the microstructural, geometrical and mechanical aspects of the welds are thoroughly examined. The critical issues related to the evolution of defects and the corresponding remedial measures applied are explored and the characteristics of fracture features reported in the literature are summarised in thematic tables. The purpose of this review is tantamount to emphasise the benefits and the growing trend of laser welding of Ti alloys in the academic sector to better exploit the process in the industry so that the applications are explored to a greater extent.

The FeCoCrNiMnAlx high-entropy alloy (HEA) coatings were produced on 4Cr5MoSiV steel by laser cladding, which can improve the high temperature oxidation resistance and wear resistance of the substrate steel in its service environment. The effects of Al addition on the microstructure and properties of the coatings were systematically investigated. The addition of Al element promoted the transition of FCC to FCC + BCC (B2) phases in the cladding layer and also refined the grains. Thereby, with the Al addition from 0 to 0.75, the micro-hardness and room-temperature wear weight loss of the HEA coatings increased and decreased, respectively, i.e. from 224.4 HV0.5 and to 344.2 HV0.5 and from 6.0 mg to 1.1 mg, compared with those of the base metal (227.1 HV0.5 and 10.5 mg). In the high-temperature oxidation process, fine and dense α-Al2O3 was preferentially generated because of its lowest absolute Gibbs free energy compared to the other oxides. The generation of α-Fe2O3 and Cr2O3 also promoted the heterogeneous nucleation of α-Al2O3. The high-temperature oxidation weight gain and wear weight loss of the FeCoCrNiMnAl0.75 cladding layer was approximately 58% and 17% of those of the base metal, respectively, and the wear mechanism was mainly abrasive wear.

In order to study the strengthening mechanisms in the multi-phase (FCC + BCC) high-entropy alloy cladding layer, FeCoCrNiAl1.0 coating was fabricated by laser cladding technology. For comparison, single phase (FCC) FeCoCrNiAl0.3 coating, pure copper, and pure iron were also considered. XRD analysis, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscope (TEM) were used to investigate the evolution of phases and microstructures in the materials. The hardness and tensile property of the materials were investigated using micro-hardness and small punch tests. It was found that Al element could promote the formation of BCC solid solutions. The solidification pattern of coatings changed from columnar to equiaxed crystals. The FeCoCrNiAl1.0 high-entropy alloy cladding layer (FCC + BCC) exhibited the best mechanical properties. Furthermore, the strengthening mechanisms in the multi-phase high-entropy alloy cladding layer were analyzed systematically. The rapid solidification in the laser cladding process resulted in the formation of fine grains that strengthened the material. In addition, the FCC and BCC lattice structures contain different slip directions and types of dislocation. The lattice distortions of the two crystal structures were determined by calculating the interplanar spacings and the interplanar angles. Under external load, the FCC and BCC phases in the multi-phase high-entropy alloy cladding layer experienced different degrees of displacements, which also played a role in strengthening the material.

The Mg alloy and two austenitic stainless steel sheets were joined together by a metallurgical bond across the rivet and two austenitic stainless steel sheets. More heat was generated at the austenitic stainless/austenitic stainless interface than at the rivet/ austenitic stainless steel interface, leading to larger nugget size at the austenitic stainless steel /austenitic stainless steel interface at all welding currents. Thus, the nugget size at the austenitic stainless steel /austenitic stainless steel interface mainly influenced the transition from interfacial to pullout failure modes. The fusion zone microstructure consisted of ferrite and austenite. The microstructure in the edges of the nugget (both in the rivet and ASS) consisted of fine columnar dendritic grains. Owing to variation of temperature gradient and solidification growth rate, the grains morphology changed from columnar dendritic to equiaxed dendritic in the nugget center. The fine grains resulted in high fusion zone hardness. Digital image correlation analysis revealed that the joints could experience joining zone rotation/out-of-plane displacement during lap-shear tests, which reduced the magnitude of strain sustained by the joints in the loading direction. The joint configuration that did not undergo joining zone rotation and failed via pullout failure in the austenitic stainless steel sheet exhibited superior lap-shear performance.

FeCoCrNiAl +x(TiC) composite coatings were fabricated by laser cladding technology. Four different types of ceramic particles were observed in the cladding layers, namely, small-sized, flower-shaped, fishbone-shaped, and aggregate particles. The concentration of C element in the cladding layer, the duration of growth of the ceramic particles, and the hysteresis diffusion effect are the major factors that determine the morphology of the ceramic particles. The small-sized, flower-shaped, and fishbone-shaped particles were well bonded to the metal matrix, making crack formation difficult. On the other hand, the hard and brittle, large-sized particles were weakly bonded to the metal matrix. They broke away easily from the cladding layer during wear test to form more ploughing grooves, thereby deteriorating the wear resistance of the cladding layers.