Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni_50.8Ti_49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2_2 0 0 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈1 0 0〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M_s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state.     

Narrow-gap tungsten inert gas welding of 78-mm-thick Ti–6Al–4V alloy

Narrow-gap tungsten inert gas welding with in situ protective system is applied to 78-mm-thick Ti–6Al–4V alloy. The martensite basketweave microstructure is formed in fusion zone (FZ), and heat-affected zone (HAZ) near FZ consists of acicular martensite and a small portion of primary alpha phase. The HAZ near base metal (BM) consists of primary alpha and transformed beta phase. Microhardness values of FZ and HAZ are relatively higher, and peak values appear at HAZ near FZ. The mechanical heterogeneity is dependent on variation of beta phase and martensite, which in turn depend on specific thermal cycle(s). The average tensile strength reaches 783.3?MPa and is 91.2% of BM, which appears in the middle of the weld with orthogonal martensites.     

Quenching microstructure and properties of 300M ultra-high strength steel electron beam welded joints

The 300M steel was welded by electron beam welding (EBW) with optimized welding parameters in the annealed state. As-welded, for comparison, and as-quenched (oil quenching at 870 °C × 1 h and tempering at 315 °C × 2 h) welded joints were investigated in this paper. The microstructure and fracture morphology were analyzed using scanning electron microscopy (SEM) and optical microscope. X-ray energy spectrum analysis was used to determine chemical composition of phases formed at the joint. The microhardness and tensile strength were evaluated. Results indicate that the weld metal microstructures of the as-welded joint are lower bainite, retained austenite and pro-eutectoid ferrite; the heat affected zone microstructure is sorbite with undissolved particles. The microstructure of as-quenched joint is tempered martensite. The tensile strength of the joints after quenching reached 1900 MPa.     

Microstructure evolution in heat affected zone of T4003 ferritic stainless steel

In this work, microstructural characteristics and development within the heat affected zone (HAZ) of T4003 ferritic stainless steel (FSS) welded joint were investigated combining experimental measurement with finite element simulation of welding temperature field. The results indicate that the HAZ was characterized with heterogeneous microstructure due to the extensive peak temperature range which could be divided into three sub-zones named as HAZ1, HAZ2 and HAZ3. The HAZ1 (the region next to weld zone boundary) experienced peak temperatures of 1300–1500 °C during welding process. This region presented almost fully δ ferrite microstructure with irregular grain, which was attributed to the high element diffusion rate and the absence of elevated-temperature austenite. The HAZ2 (center region of HAZ) suffered the peak temperatures of 1150–1300 °C. It presented martensite + δ ferrite dual microstructure with limited grain growth due to the formation of γ phase at grain boundaries. The HAZ3 (the region closed to the base metal) was undergone the peak temperatures of 830–1150 °C and was characterized with both martensite and ferrite structure.     

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

The purpose of this study was to evaluate microstructural and mechanical change of DP780 steel after tungsten inert gas (TIG) welding and the influence of notch locations on the fatigue crack growth (FCG) behavior. The tempering of martensite in the sub-critical heat affected zone (HAZ) resulted in a lower hardness (~ 220 HV) compared to the base material (~ 270 HV), failure was found to originate in the soft HAZ during tensile test. The fusion zone (FZ) consisted of martensite and some acicular ferrite. The joint showed a superior tensile strength with a joint efficiency of 94.6%. The crack growth path of HAZ gradually deviated towards BM due to the asymmetrical plastic zone at the crack tip. The FCG rate of the crack transverse to the weld was fluctuant. The Paris model can describe the FCG rate of homogeneous material rather well, but it cannot precisely represent the FCG rate of heterogeneous material. The fatigue fracture surface showed that the stable expanding region was mainly characterized by typical fatigue striations in conjunction with secondary cracks; the rapid expanding region contained quasi-cleavage morphology and dimples. However, ductile fracture mechanism predominated with an increasing stress intensity factor range (ΔK). The final unstable failure fractograph was subtotal dimples.     

Microstructure and mechanical properties of a new Al–Zn–Mg–Cu alloy joints welded by laser beam

A new type of Al–Zn–Mg–Cu alloy sheets with T6 temper were welded by laser beam welding (LBW). Microstructure characteristics and mechanical properties of the joints were evaluated. Results show that grains in the heat affected zone (HAZ) exhibit an elongated shape which is almost same as the base metal (BM). A non-dendritic equiaxed grain zone (EQZ) appears along the fusion line in the fusion zone (FZ), and grains here do not appear to nucleate epitaxially from the HAZ substrate. The FZ is mainly made up of dendritic equiaxed grains whose boundaries are decorated with continuous particles, identified as the T (AlZnMgCu) phase. Obvious softening occurs in FZ and HAZ, which mainly due to the changes of nanometric precipitates. The precipitates in BM are mainly η′, while plenty of GPI zones exist in FZ and HAZ adjacent to FZ, in the HAZ farther away from FZ, η phase appears. The minimum microhardness of the joint is always obtained in FZ at different times after welding. The ultimate tensile strength of the joint is 471.1 MPa which is 69.7% of that of the BM. Samples of the tensile tests always fracture at the FZ.     

Investigation on microstructure and properties of dissimilar joint between SA553 and SUS304 made by laser welding with filler wire

The dissimilar joints between SA553 and SUS304 were produced by CO₂ laser welding with the ERNiMo-8 and ER308L filler wire. After welding parameters were optimized, qualified weld formations were made. Investigation on the microstructure showed that there were dual phases (martensite and austenite) in the ER308L weld, but only austenite in the ERNiMo-8 weld. For both joints, not only the microstructure gradient, but also the element gradient was observed near interfaces between weld metals and base metals. The Charpy impact and tensile test at room (25 °C) and low temperature (− 196 °C) was implemented. The cryogenic impact energy of the ER308L weldment was 51 J, lower than the value (84 J) of the ERNiMo-8 weldment. The corresponding cryogenic tensile strength of the two weldments was 1070 MPa and 960 MPa, respectively. The cryogenic tensile properties of both weldments were rather higher than requirements in the relevant standards. The ERNiMo-8 weldment showed relatively better comprehensive performance when the cryogenic toughness was considered.     

Research on fatigue behavior of electron beam welding joint of 06Cr19Ni10 austenitic stainless steel sheet

06Cr19Ni10 austenitic stainless steel sheet square butt joints without filler metal addition were fabricated using EBW (electron beam welding) processes. Fatigue properties, tensile properties and microstructure of the welded joints were studied. It was found that the yield strength, tensile strength, elongation and Vickers hardness of EBW joint can reach the level of base material performance. Fusion zones consist of coarse columnar dendritic grains, which are perpendicular to the weld pool boundary. The hardness of heat affected zone is lower than weld centreline. The reason is that the grain of heat affected zone under the action of electron beam heat source happened recovery and recrystallization. The present work suggests that S–N curve slope of welding joint should be revised to m = 10 under the condition of high stress levels, and S–N curve slope is still m = 3.0 under the condition of low stress levels. Fatigue cracks did not extend along the minimum thickness of the section. On the contrary, fatigue cracks extended along the maximum height of the section.     

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO₂ laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α + β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482 °C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α + β microstructures in the basket-weave form.     

Effect of welding parameters on microstructure and mechanical properties of AA6061-T6 butt welded joints by stationary shoulder friction stir welding

Stationary shoulder friction stir welding (SSFSW) butt welded joints were fabricated successfully for AA6061-T6 sheets with 5.0 mm thickness. The welding experiments were performed using 750–1500 rpm tool rotation speeds and 100–300 mm/min welding speeds. The effects of welding parameters on microstructure and mechanical properties for the obtained welds were discussed and analyzed in detail. It is verified that the defect-free SSFSW welds with fine and smooth surface were obtained for all the selected welding parameters, and the weld transverse sections are obviously different from that of conventional FSW joint. The SSFSW nugget zone (NZ) has “bowl-like” shapes with fairly narrow thermal mechanically affected zone (TMAZ) and heat affected zone (HAZ) and the microstructures of weld region are rather symmetrical and homogeneous. The 750–1500 rpm rotation speeds apparently increase the widths of NZ, TMAZ and HAZ, while the influences of 100–300 mm/min welding speeds on their widths are weak. The softening regions with the average hardness equivalent 60% of the base metal are produced on both advancing side and retreating side. The tensile properties of AA6061-T6 SSFSW joints are almost unaffected by the 750–1500 rpm rotation speeds for given 100 mm/min, while the changing of welding speed from 100–300 mm/min for given 1500 rpm obviously increased the tensile strength of the joint and the maximum value for welding parameter 1500 rpm and 300 mm/min reached 77.3% of the base metal strength. The tensile fracture sites always locate in HAZ either on the advancing side or retreating side of the joints.     

船用TA5钛合金激光焊接接头组织和性能研究

采用IPG光纤激光器对8 mm厚的TA5钛合金进行激光自熔焊接,并对焊接接头的微观组织和力学性能进行分析。结果表明,激光焊接接头表面成形连续、均匀、无飞溅,内部无气孔和裂纹等缺陷。母材组织为细小均匀的等轴α相;焊缝区组织主要由粗大的β柱状晶粒、大量的针状马氏体α'以及少量的板条马氏体组成;热影响区组织主要由等轴α相、少量的针状马氏体α'和少量的残余β组成;在熔合线的边界,柱状晶粒与等轴晶粒联生结晶、外延生长,保证了焊接接头的稳定连接。焊接接头各区域的显微硬度差异较大,最高硬度出现在熔合线附近,焊缝区和热影响区的显微硬度明显高于母材的。对拉伸断裂部位进行观察,拉伸断裂发生在远离焊缝的母材处,这说明激光焊接接头的抗拉强度与母材等强或者略高于母材的,这与大量针状马氏体形成的网篮组织有直接的关系。      

Mechanical properties of electron beam welded dissimilar joints of TC17 and Ti60 alloys

Microstructure, hardness, tensile and high cycle fatigue (HCF) properties of the welded dissimilar joints of Ti60 and TC17 titanium alloys had been investigated in this study. A significant microstructural change was observed to occur after welding, with rod-like α and β phases in the fusion zone (FZ), equiaxed α phases, fine α laths and β phases in the heat-affected zone (HAZ) of TC17 side and acicular martensite α' phases+“ghost” α phases in the HAZ of Ti60 side. The microhardness across the joints exhibited an inhomogeneous distribution with the highest hardness of ～404 HV in FZ and the lowest hardness of ～304 HV in base material (BM) of Ti60. All the joints tested in tension fractured at BM of Ti60 side. Fatigue limits of the joints at 10⁷ cycles were 425?MPa at room temperature and 380?MPa at 400?°C, respectively. Welding micropores were found to be the main source of fatigue crack initiation.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

Study on cold metal transfer welding–brazing of titanium to copper

3 mm Pure titanium TA2 was joined to 3 mm pure copper T2 by Cold Metal Transfer (CMT) welding–brazing process in the form of butt joint with a 1.2 mm diameter ERCuNiAl copper wire. The welding–brazing joint between Ti and Cu base metals is composed of Cu–Cu welding joint and Cu–Ti brazing joint. Cu–Cu welding joint can be formed between the Cu weld metal and the Cu groove surface, and the Cu–Ti brazing interface can be formed between Cu weld metal and Ti groove surface. The microstructure and the intermetallic compounds distribution were observed and analyzed in details. Interfacial reaction layers of brazing joint were composed of Ti₂Cu, TiCu and AlCu₂Ti. Furthermore, crystallization behavior of welding joint and bonding mechanism of brazing interfacial reaction were also discussed. The effects of wire feed speed and groove angle on the joint features and mechanical properties of the joints were investigated. Three different fracture modes were observed: at the Cu interface, the Ti interface, and the Cu heat affected zone (HAZ). The joints fractured at the Cu HAZ had higher tensile load than the others. The lower tensile load fractured at the Cu interface or Ti interface was attributed to the weaker bonding degree at the Cu interface or Ti interface.     

Laser,tungsten inert gas,and metal active gas welding of DP780 steel: Comparison of hardness,tensile properties and fatigue resistance

The microstructural characteristics, tensile properties and low-cycle fatigue properties of a dual-phase steel (DP780) were investigated following its joining by three methods: laser welding, tungsten inert gas (TIG) welding, and metal active gas (MAG) welding. Through this, it was found that the size of the welded zone increases with greater heat input (MAG > TIG > laser), whereas the hardness of the weld metal (WM) and heat-affected zone (HAZ) increases with cooling rate (laser > TIG > MAG). Consequently, laser- and TIG-welded steels exhibit higher yield strength than the base metal due to a substantially harder WM. In contrast, the strength of MAG-welded steel is reduced by a broad and soft WM and HAZ. The fatigue life of laser-and TIG-welded steel was similar, with both being greater than that of MAG-welded steel; however, the fatigue resistance of all welds was inferior to that of the non-welded base metal. Finally, crack initiation sites were found to differ depending on the microstructural characteristics of the welded zone, as well as the tensile and cyclic loading.     

Fracture toughness (J1C) of electron beam welded AA2219 alloy

AA2219 (Al–6%Cu) was butt welded in T87 temper (solution heat-treated, cold worked and precipitation hardened) and T6 temper (solution heat-treated and precipitation hardened) using electron beam welding (EBW). Variables studied were base metal temper condition and mode of EBW. Mechanical properties of the weld joint and fracture toughness at fusion zone (FZ) and heat-affected zone (HAZ) were evaluated and compared with those of the base metal. Results showed that EB welds have higher joint efficiency and fracture toughness than that of gas tungsten arc welding (GTAW). Fracture toughness of T6 base metal was found to be higher than its T87 counterpart. When welded, FZ and HAZ in T87 showed higher fracture toughness than that of T6; HAZ was the toughest. Pulsed current (PC) EB weld showed marginal reduction in toughness compared to constant current (CC) weld. Toughness variation is analyzed with the help of tensile test, Charpy impact test and scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy

The heat-affected zone (HAZ) of electron beam welded (EBW) joint normally undergoes a unique heat-treating process consisting of rapid temperature rising and dropping stages, resulting in temperature-gradient in HAZ as a function of the distance to fusion zone (FZ). In the current work, microstructure, elements distribution and crystallographic orientation of three parts (near base material (BM) zone, mid-HAZ and near-FZ) in the HAZ of Ti-6Al-4V alloy were systematically investigated. The microstructure observation revealed that the microstructural variation from near-BM to near-FZ included the reduction of primary α (α_p) grains, the increase of transformed β structure (β_t) and the formation of various α structures. The rim-α, dendritic α and abnormal secondary α (α_s) colonies formed in the mid-HAZ, while the “ghost” structures grew in the near-FZ respectively. The electron probe microanalyzer (EPMA) and electron back-scattered diffraction (EBSD) technologies were employed to evaluate the elements diffusion and texture evolution during the unique thermal process of welding. The formation of the various α structures in the HAZ were discussed based on the EPMA and EBSD results. Finally, the nanoindentation hardness of “ghost” structures was presented and compared with nearby β_t regions.     

Texture and microstructure characterization in laser additive manufactured Ti–6Al–2Zr–2Sn–3Mo–1.5Cr–2Nb titanium alloy

Laser additive manufacturing (LAM) is a novel manufacturing technique in which metal components can be fabricated layer by layer. In this study, a recently developed damage tolerance titanium alloy TC21 (Ti–6Al–2Zr–2Sn–3Mo–1.5Cr–2Nb) was deposited by LAM process. Texture and microstructure characterization have been investigated by XRD, SEM and EBSD. Prior β grains texture analysis indicates that the (100) poles concentrate in build direction with a texture intensity about 18.7. During cooling down from β phase field, the β to α phase transformation follows the Burger orientation relationship and a pronounced variant selection occurred. Besides, morphology and scale of α phase are quite different along the build direction due to different thermal history. Very fine rib-like α phase with the length less than 2 μm and acicular martensite α' can be obtained at the bottom and the top of the sample, respectively. In the middle position, distribution and morphology of α phase is quite uneven and the precipitation sequence of α phase is α_GB → α_WGB → α_WM → α_S. The reasons by which they formed are discussed.     

Influence of cooling rate on microstructure of Ti–Nb alloy for orthopedic implants

Beta titanium alloys form one of the most versatile group of materials with respect to processing, microstructure and mechanical properties, mainly in applications as biomaterials. Development of new Ti-based alloys for implant application involves more biocompatible metallic alloying elements, such as Nb, Ta, Zr and Mo. Heat treatment of Ti alloys plays an important role in determining microstructure. The aim of this work is the analysis of microstructure and phases formed during water quenching of β Ti–20Nb alloy through different cooling rates. Ti–20Nb alloy was swaged at 780–860 °C and then machined as a cylinder. Cylindrical sample was treated within the β field and then water quenched from the bottom imposing different cooling rates through the sample. Samples from different regions (cooling rates) were characterized by using X-ray diffractometry (XRD), scanning (SEM) and transmission electron microscopy (TEM), and Vickers microhardness. XRD results showed the increase of β/α phases peak intensity ratio increase with decreasing of cooling rate. As the distance from the bottom (water source) of Ti–20Nb sample decreases, the imposed cooling rate increases, the volume of α martensite acicular phase increases and the size decreases with diminishing of α phase quantity. The lowest elastic modulus E = 74 GPa was found for water quenched sample under a cooling rate of 160 K/s.     

Microstructure and mechanical properties of laser welded S960 high strength steel

S960 steel is an advanced low carbon and low alloy ultra-high strength steel (with a minimum yield strength of 960 MPa) developed by Tata Steel. At present, there is a scarcity of data for laser welding of such a material. In this study, 8 mm thick hot rolled and quenched S960 high strength low alloy (HSLA) steel plates were welded using a 16 kW fibre laser system. The microstructure, microhardness, and tensile properties were characterised, Charpy impact testing and three-point bending testing were carried out, and fracture surfaces were investigated. Preliminary results suggest that the laser welding process can produce single-pass welds which are free of macroscopic defects. The microstructures in the fusion zone and heat affected zone were predominately martensite and some self-tempered martensite, with grain size variation in different sub-zones. The tensile properties of the laser welded joint matched those obtained for the base material, with failure occurring in the base material away from the weld. While the welded joint performed well when subjected to bending, the impact toughness was reduced when compared with that of the base material.