Microstructure and Its Influence on the Mechanical Properties of Ni–28W–6Cr-Based Alloy-Welded Joints by GTAW

Microstructure and mechanical properties of Ni–28W–6Cr alloy-welded joints produced by gas tungsten arc welding were investigated in this work.Results showed that original fine-grain base metal near fusion line totally transformed into coarse heat affected zone after welding.Carbides with different shapes were found in the weld metal and base metal, which all were determined as M6C carbides.In comparison with carbides in base metal, M6C carbides in weld metal are rich in Si and Cr but deficient in W.Moreover, M6C carbides are extremely scarce and hard to be detected in weld metal.Mechanical tests show that the hardness value of weld metal is only about 60% of base metal; the yield strength and tensile strength of welded joint are much lower than those of base metal due to the absence of carbides in weld metal.     

Joint performance of CO2 laser beam welding 5083,H321 aluminum alloy

Laser beam welding of aluminum alloys is expected to offer good mechanical properties of welded joints. In this experimental work reported, CO2 laser beam autogenoas welding and wire feed welding are conducted on 4 mm thick 5083- H321 aluminum alloy sheets at different welding variables. The mechanical properties and microstructure characteristics of the welds are evaluated through tensile tests, micro-hardness tests, optical microscopy and scanning electron microscopy （SEM）. Experimental results indicate that both the tensile strength and hardness of laser beam welds are affected by the constitution of filler material, except the yield strength. The soften region of laser beam welds is not in the heat-affected zone （ HAZ ）. The tensile fracture of laser beam welded specimens takes place in the weld zone and close to the weld boundary because of different filler materials. Some pores are found on the fracture face, including hydrogen porosities and blow holes, but these pores have no influence on the tensile strength of laser beam welds. Tensile strength values of laser beam welds with filler wire are up to 345.57 MPa, 93% of base material values, and yield strengths of laser beam welds are equivalent to those of base metal （264. 50 MPa）.     

Effect of the heat input on microstructure and properties of submerged arc welded joint of 08Cr19MnNi3Cu2N stainless steel

SAW308L submerged arc welding wire and SJ601A submerged arc welding flux were selected to weld the 12 mm 08Cr19MnNi3Cu2N low nickel and high nitrogen austenitic stainless steel plates with three different welding heat input, and microstructure,tensile properties, microhardness and corrosion properties of the welded joints were studied. The results show that no defects are found in the three groups of welded joints, and the welded joints have better performance.The tensile strength of 08Cr19MnNi3Cu2N stainless steel welded joints with different heat input is slightly lower than that of the base metal,and fracture occurs in the weld zone, and the hardness of the weld zone is lower than that of the base metal. The weld microstructure of stainless steel welded joints with different heat input is composed of austenite + δ ferrite, and ferrite is uniformly distributed in austenite. With the increase of the welding heat input, the ferrite content in the weld zone decrease gradually, the grain size in the thermal affected zone increase gradually, and the impact toughness reduce.     

Microstructure and mechanical properties of the joint of 6061 aluminum alloy by plasma-MIG hybrid welding

Plasma-MIG (metal inert gas arc welding) hybrid welding of 6061 aluminum alloy with 6 mm thickness using ER5356 welding wire was carried out.The microstructures and mechanical properties of the welded joint were investigated by optical microscopy,X-ray diffraction (XRD),energy dispersive spectroscopy (EDS),tensile test,hardness test and scanning electron microscope (SEM) were used to judge the type of tensile fracture.The results showed that the tensile strength of welded joint was 142 MPa which was 53.6％ of the strength of the base metal.The welding seam zone was characterized by dendritic structure.In the fusion zone,the columnar grains existed at one side of the welding seam.The fibrous organization was found in the base metal,and also in the heat affected zone (HAZ) where the recrystallization occurred.The HAZ was the weakest position of the welded joint due to the coarsening of Mg2Si phase.The type of tensile fracture was ductile fracture.     

Comparison of 2A12 aluminium alloy joint by ultrasonic assisted friction stir welding and friction stir welding

Ultrasonic assisted friction stir welding （UAFSW） is a recent modification of conventional friction stir welding, which adds ultrasonic energy directly into the friction stir welding area by the pin. In this study, 2A12 aluminum alloy was welded by this process and conventional, respectively. The tensile tests, microstructure and fracture surface of FSW joint and UAFSW joint were analyzed. The research results show that the surface forming texture of ultrasonic assisted friction stir welding joint, compared with conventional, is finer and more uniform, showing metallic matte color. The grains are much finer in weld nugget zone, thermo-mechanically affected zone and heat-affected zone; S-phase particles size is much smaller and distribution is more homogeneous in the matrix. The tensile strength of UAFSW joint is 94. 13% of base metal, and the elongation is 11.77%. The tensile strength of FSW joint is 83.15% of base metal, and the elongation is 8.81%. The tests results reveal that ultrasonic vibration can improve the tensile strength and the elongation of welded joints.     

CO2 laser welding of 5A06 aluminum alloy plates with different thicknesses

CO2 laser beam welding of aluminum alloys with different thicknesses was carded out. The influences of laser power and travel speed on the weld width were analyzed. The mechanical characteristics of tailor-welded blanks （TWB） with unequal thickness were evaluated using tensile tests, and the fracture appearance was inspected after tensile tests. The microstructure of welded joints was analyzed by SEM. The results indicate that this alloy can be laser welded with full penetration. All the tensile specimens fracture on the base metal, far from the weld in the transverse direction. The tensile strength and yield strength of TWB are 89% and 91.2% compared with the base metal. The percentage of the thinner plate in the specimen has an important effect on the transverse elongation. The transverse elongation of TWB approaches that of the base metal when the thinner plate has a large percentage in TWB specimen. The weld microstructure shows extra-fine grains. Dendrite exists around weld fusion line and the equiaxed grains in the weld.     

STUDY ON DYNAMIC J-INTEGRAL OF MECHANICAL HETEROGENEOUS WELDED JOINT

Welded joint is a mechanical heterogeneous body, and mechanical heterogeneity has great effect on dynamic fracture behaviour of welded joints. In the present investigation, dynamic response curve and dynamic J-integral of practical undermatched welded joint and whole base and whole weld three-point-bend (TPB) models containing longitudinal crack are computed. Dynamic J-integral is evaluated using virtual crack extension (VCE) method and the computation is performed using MARC finite element code. Because of the effect of inertia,dynamic load response curve of computed model waves periodically. Dynamic J-integral evaluated by VCE method is path independent. The effect of inertia has little influence on dynamic J-integral curve. The value of dynamic J-integral of undermatched welded joint is lower than that of whole base metal and higher than that of whole weld metal. The results establish the foundation of safety evaluation for dynamic loaded welded structures.     

Effect of Laser Welding Process Parameters on Microstructure and Mechanical Properties on Butt Joint of New Hot-Rolled Nano-scale Precipitation-Strengthened Steel

A 4 kW fiber laser was chosen to weld the new hot-rolled nano-scale precipitation-strengthened steel with a thickness of 4.5 mm. The effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated. Results showed that increasing laser welding power may cause faster downward flow of molten metal to produce greater root humping. With the welding speed increasing, the average welding seam （WS） width decreased, and the average WS and heat-affected zone （HAZ） hardness increased. The microstructures of WS, fusion line, and coarse grain heat-affected zone were lath martensite, but the growth direction of the original austenite grain boundaries was significantly different. The microstructures of fine grain heat-affected zone were ferrite and martensite, and the microstructure of mixed grain heat- affected zone contained ferrite, massive M/A island, and a small amount of martensite. The micro-hardness values of WS, HAZ, and base metal （BM） were 358, 302, and 265 HV, respectively. The butt joint fracture at the BM far from the WS and the welded joint tensile strength are observed to follow proportional relationship with hardness.     

Microstructures and properties of welded joint of TiNi shape memory alloy and stainless steel

The fracture characteristics of the joint were analyzed by means of scanning electron microscope(SEM).Microstructures of the joint were examined by means of optical microscope, SEM and an image analyzer. The results show that the tensile strength of the inhomogeneous joint of TiNi shape memory alloy and stainless steel is lower than that of the homogeneous joint and a plastic field appears in the heat affected zone on the side of TiNi shape memory alloy. Because TiNi shape memory alloy and stainless steel melted, a brittle as-cast structure was formed in the weld. The tensile strength and the shape memory effect of the inhomogeneous joint are strongly influenced by the changes of composition and structure of the joint. Measures should be taken to reduce the base metal melting and prevent the weld metal from the invasion by O for improving the properties of the TiNi shape memory alloy and stainless steel inhomogeneous joint.     

Study of the Laser-MIG Hybrid Welded Al-Mg Alloy

The technology of laser-MIG hybrid welding is hotspot in welding researched field at present.It can improve the velocity of the welding,reduce the distortion of the welding,optimize the structure of the welding joint and etc..The 5052 aluminum alloy of the 10mm thick was welded by the laser-MIG hybrid welding.The structure,the alloy elements profile and the mechanical property of the welded joint are researched by the optical microscope,SEM,sclerometer and etc..The results showed:The medium thick Al alloy was welded in high speed by the laser-MIG hybrid welding.The appearance of the welding joint is well.The weld joint and the weld interface are fine.The intenerate region in the welding joint is small.The tensile strength in welding joint has achieved 94.4% of that in base metal.     

Effect of heat treatment on microstructure and properties of single crystal copper cold-welded joints

With the means of electron backscattered diffraction (EBSD), mechanical properties test and digital eddy current metal conductivity, the single crystal copper cold- welded joint was tested and analyzed, the structure change of cold- welded joint and the effect of heat treating on the structure and property of cold- welded joint were discussed. The results show that: The deformation area of the single crystal copper cold welded joint is broken, the crystalline grain at the interface of the joint is refined, and the single crystal structure is still maintained in the base metal area. The hardness of the deformation area increases greatly, the conductivity of the joint does not change much, and the tensile strength of the joint reaches about 70% of that of the base metal. At the interface of the heat treating joint, the single crystal structure of the deformation area and the base metal area are destroyed, and the grains grow up at the interface and the orientation is different. The hardness of the joint interface is much lower than that of the non-heat treating joint, the electrical conductivity of the joint is good, and the tensile strength of the joint is higher than that of the base metal.     

Characteristics of 5A90 Aluminum-lithium Alloy by YAG-MIG Hybrid Welding

The characteristics of 5A90 Aluminum-Lithium alloy by YAG laser and MIG arc hybrid welding (Hybrid welding) were studied.Compared with the laser beam welding (LBW),the hybrid welding could not only improve the weld appearance significantly,but also have better engineering compatibility.The obvious microstructure characteristics of joint by the hybrid welding are fine-grained layer near fusion-line and the equiaxed grain in most area of welded seam.The subgrains of the equiaxed grains,located in the weld center,tend to coarse from bottom to top of weld joint.The microhardness of welded seam by the hybrid welding (83.57HV0.2) is lower than that by LBW (95.65HV0.2),but the uniformity of the former is better than that of the latter.The ultimate strength and the elongations after fracture of the joint by the hybrid welding are lower than that by LBW.The tensile fracture always occurs in HAZ or weld centerline,and the fractography presents mixture rupture.Therefore,if the combined mechanical properties of joint by the hybrid welding meet the operation requirements,it should be improved by reasonable artificial aging or heat treatment after welding,and it also should develop a better filler wire matched with the base metal.     

Microstructural zones and tensile characteristics of friction stir welded joint of TC4 titanium alloy

TC4 titanium alloy was friction stir welded using a W-Re pin tool,and the defect-free weld was produced with proper welding parameters.The joint consists of stir zone,heat affected zone and base material.The stir zone is characterized by equiaxed dynamically recrystallized α phases and transformed β phases with fine α+β lamellar microstructure.The microstructure of the heat-affected zone is similar to that of the base material,but there is an increase in the volume fraction of β.Transverse tensile strength of the joint is 92% that of the base material,and the joint is fractured in the stir zone and the fracture surface possesses typical plastic fracture characteristics.The stir zone is the weakest part of the joint,through which the tensile characteristics of the TC4 joint can be explained.     

The comparison of multi-layer narrow-gap laser and arc welding of S32101 duplex stainless steel

Multi-layer narrow-gap welding of thick S32101 duplex stainless steel was conducted using laser welding with beam wobble process. The phase transition, grain size, phase proportion and crystal texture of welded joint were also studied and compared with gas metal arc welding process. The microhardness and tensile strength were measured and fracture surface was analyzed to evaluate the mechanical properties of welded joints. The results showed that beam wobble technology improved the misalignment ...     

Friction stir welding of AZ31 magnesium alloy

Friction stir welding (FSW) is an new solid-phase joining technology which has more advantages over fusion welding methods in welding of aluminum and other non-ferrous metals. The effects of welding parameters on mechanical properties and microstructure during friction stir welding of AZ31 magnesium alloy were studied in this paper. Microstructures and mechanical properties of the joints were investigated by means of optical microscopy, scanning electric microscopy ( SEM ) , micro-hardness analysis, and tensile test. Experimental results show that the magnesium alloy can be successfully welded by FSW method, and the ultimate tensile strength (UTS) of FSW joint reaches up to 90 percent of base metal. The microstructures of welded joints exhibit the variation from dynamically recrystallized fine grains to greatly deformed grains. Hardness in nugget zone was found lower than the base metal but not too obvious.     

Texture and mechanical properties of metal inert gas welded 6082-T651 aluminum alloy joints

Metal inert gas(MIG) welding was conducted with 12 mm thick 6082-T651 aluminum alloy plate to investigate the microstructure and mechanical properties of welded joint. The microstructure and element distribution of weld seam were characterized by electron backscattered diffraction(EBSD) and electron probe microanalysis(EPMA). The weld seam has typical cube texture({001}100) characteristics. The closer to the center of weld seam, the weaker the texture feature, the higher the proportion of high-angle grain boundaries. The average tensile strength of joint was 232 MPa which is up to 72% of 6082 aluminum alloy base metal, and the bending angle for the root bend test sample reached 90° without cracks. The lack of strengthening phase and the existence of welding pores and inclusions in the weld seam caused the degradation of mechanical properties of resultant joint. The microhardness increased from the weld center to the base metal,but the overaging zone caused by welding thermal cycle was softening part of the joint, which had lower hardness than the weld seam.     

Study on the J-Integral and Failure Assessment Line of Electron Beam Welded Thick TC4-DT Alloy Joints

The effects of strength mis-matching on J-integral and failure assessment line (FAL) of electron beam welded thick TC4-DT titanium alloy joints have been investigated.Elastic and elastic-plastic finite elements (FE) calculations on centre crack tensile (CCT) specimens were made,and the condition of 20% strength mis-matching was considered.The results indicate that the limit load of the joint can be substituted by that of the base metal.The error of substitution is less than 5%.The failure assessment lines for both over-matching and under-matching joints are increasingly close to that of the base metal,with the ratio of weld width to specimen width (H/W) decreasing.Moreover,the failure assessment lines of the joints can be devised using the stress-strain curves of the base metal,which will be convenient for the practical application of engineering structures     

Fatigue properties of friction stir welded butt joint and base metal of MB8 magnesium alloy

The fatigue properties of friction stir welded(FSW) butt joint and base metal of MB8 magnesium alloy were investigated.The comparative fatigue tests were carried out using EHF-EM200K2-070-1A fatigue testing machine for both FSW butt joint and base metal specimens.The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063 LA type.The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased.The conditional fatigue limit(2 × 10~6) of base metal and welded butt joint is about77.44 MPa and 49.91 MPa,respectively.The conditional fatigue limit(2 × 10~6) of the welded butt joint is 64.45%of that of base metal.The main reasons are that the welding can lead to stress concentration in the flash area,tensile welding residual stress in the welded joint(The residual stress value was 30.5 MPa),as well as the grain size is not uniform in the heat-affected zone.The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface,the fracture type of the FSW butt joint belongs to a brittle fracture.     

Double-sided gas tungsten arc welding process on TC4 titanium alloy

TC4 titanium alloy was welded by double-sided gas tungsten arc welding（GTAW） process in comparison with conventional GTAW process, the microstructure and mechanical performance of weld were also studied. The results indicate that double-sided GTAW is superior over regular single-sided GTAW on the aspects of increasing penetration, reducing welding deformation and improving welding efficiency. Good weld joint was obtained, which can reach 96.14% tensile strength and 70.85% elongation percentage of the base metal. The grains in heat-affected zone（HAZ） are thin and equiaxed and the degree of grain coarsening increases as one moves to the weld center line, and the interior of grains are α and α′ structures. The coarse columned and equiaxed grains, which interlace martensitic structures α′ and acicular α structures, are observed in weld zone. The fracture mode is ductile fracture.     

Joint Characteristics and Corrosion Properties of Bypass-Current Double-Sided Arc-Welded Aluminum 6061 Alloy with Al–Si Filler Metal

Sheets of aluminum 6061 alloy were welded using bypass-current double-sided arc welding with Al–Si filler wire to investigate the effect of Al–Si intermetallic compounds on the microstructure, microhardness and corrosion behavior of weld joint. Experimental results indicated that the Al_(4.5)FeSi phase in the topside of the weld joint was finer than that in the backside and newly formed phase of Al_(0.5)Fe_3Si_(0.5)was observed in the backside. The formation of reinforcing phases of Al–Fe–Si in the weld improved the microhardness of the weld by about 18%. The corrosion resistance of the weld zone was greater than that of the base metal, while the corrosion current displayed opposite, and the corrosion resistance of the weld region was better than that of the base metal.