Investigation of In Situ and Conventional Post‐Weld Heat Treatments on Dual‐Laser‐Beam‐Welded γ‐TiAl‐Based Alloy

This paper describes a way to improve the microstructure and mechanical properties of welding seams by in situ and conventional post‐weld heat treatments for laser beam welding of the Ti–45Al–5Nb–0.2C–0.2B alloy. The seams are crack‐free with reduced longitudinal residual stress and higher elongation to fraction after post‐weld heat treatment. The welding zone consists of α₂ after welding, transforms to a massive γ during in situ post‐weld heat treatment, and finally forms a convoluted microstructure after conventional heating. The phase composition across the welding zone is discussed.     

XRD and TEM analysis of microstructure in the welding zone of 9Cr-1Mo-V-Nb heat-resisting steel

Under the condition of tungsten inert gas shielded welding (TIG) + shielded metal arc welding (SMAW) technology, the microstructure in the welding zone of 9Cr-1Mo-V-Nb (P91) heat-resisting steel is studied by means of X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The test results indicate that when the weld heat input (E) of TIG is 8.5 ∼ 11.7 kJ/cm and the weld heat input of SMAW is 13.3 ∼ 210 kJ/cm, the microstructure in the weld metal is composed of austenite and a little amount of δ ferrite. The substructure of austenite is crypto-crystal martensite, which included angle. There are some spot precipitates in the martensite base. TEM analysis indicates that the fine structure in the heat-affected zone is lath martensite. There are some carbides (lattice constant, 1.064 nm) at the boundary of grain as well as inside the grain, most of which are Cr₂₃C₆ and a little amount of (Fe, Me)₂₃C₆.     

Fatigue assessment of refill friction stir spot weld in AA 2024‐T3 similar joints

Refill friction stir spot welding is a solid‐state process technology that is suitable for welding lightweight materials in similar or dissimilar overlapped configuration. In this study, the fatigue behaviour of single overlapped spot joints of AA2024‐T3 was studied. To statistically analyse the fatigue data, a 2‐parameter Weibull distribution was deployed, considering several reliabilities (R_e = 0.99, R_e = 0.90, R_e = 0.5, R_e = 0.10). To obtain an optimized weld parameter according to the fatigue behaviour, 2 different weld conditions were studied, taking into account the effect of the hook formation. The microstructure analyses and microhardness profiles showed great similarity in both weld conditions. However, these conditions presented distinct interfacial hook profiles, in which the interfacial hook downward represented better fatigue life and infinite fatigue life at 15% of the maximum strength load. The fracture surfaces obtained from 3 different fracture modes were investigated by using scanning electron microscopy; the crack was tracked and described according to its fracture mechanisms from its initiation until the final failures. It was observed that the crack is initiated at hook profile.     

Comparative experimental study on the modification of microstructural and mechanical properties of friction stir welded and gas metal arc welded EN AW‐6061 O aluminum alloys by post‐weld heat treatment

In this paper, the effects of post‐weld heat treatment on modification of microstructures and mechanical properties of friction stir welded and gas metal arc welded AA6061‐O plates were compared with each other. Gas metal arc welding and friction stir welding were used as the applicable welding processes for AA6061‐O alloys. The applied post‐weld heat treatment consisted of solution heat treatment, followed by water quenching and finally artificial aging. The samples were classified as post‐weld heat treated and as‐welded joints. The microstructural evolution, tensile properties, hardness features and fracture surfaces of both as‐welded and post‐weld heat treated samples were reported. The results clearly showed that friction stir welding process demonstrated better and more consistent mechanical properties by comparison with the gas metal arc welding process. The weld region of as‐welded samples exhibited a higher hardness value of 80 HV0.1 compared to the base material. In addition, the feasibility of post‐weld heat treatment in order to enhance the mechanical properties and to obtain more homogeneous microstructure of 6061‐O aluminum alloys was evaluated.     

Influences of Friction Stir Welding and Post‐Weld Heat Treatment on Al–Cu–Li Alloy

In this paper, the influences of friction stir welding (FSW) and post‐weld heat treatment (PWHT) on the microstructures and tensile properties of Al–Cu–Li alloy are investigated. After FSW, strengthen loss occurred in the welding area. Remarkable softening occurs in the thermo‐mechanically affected zone (TMAZ) resulting from dissolution of Al₃Li (δ′) phases. Recrystallization and precipitation of ultra‐fine δ′ phases take place in the nugget zone (NZ) that lightens the softening degree of this zone. A noteworthy enhancement in the hardness and tensile strength of the joint is achieved after T8 re­aging treatment (3% ? pre‐deformation, 30 h at 152 °C). However, re‐solution treatment coupled with re‐aging treatment leads to ductility deterioration in the joint because coplanar slip of coarse Al₃Li phases induces severe stress concentration during plastic deformation.

     

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.     

The effect of repeated repair welding on mechanical and corrosion properties of stainless steel 316L

The purpose of this study is to evaluate changes in the mechanical, micro structural and the corrosion properties of stainless steel 316L under repeated repair welding. The welding and the repair welding were conducted by shielded metal arc welding (SMAW). The SMAW welding process was performed using E316L filler metals. Specimen of the base metal and different conditions of shielded metal arc welding repairs were studied by looking in the micro structural changes, the chemical composition of the phases, the grain size (in the heat affected zone) and the effect on the mechanical and corrosion properties. The microstructure was investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The chemical composition of the phases was determined using energy dispersive spectrometry (EDS). The corrosion behavior in 1 M H₂SO₄ + 3.5% NaCl solution was evaluated using a potentiodynamic polarization method. Tensile tests, Charpy-V impact resistance and Brinell hardness tests were conducted. Hardness of the heat affected zone decreased as the number of repairs increased. Generally an increase in the yield strength (YS) and the ultimate tensile strength (UTS) occurred with welding. After the first repair, a gradual decrease in YS and UTS occurred but the values of YS and UTS were not less than values of the base metal. Significant reduction in Charpy-V impact resistance with the number of weld repairs were observed when the notch location was in the HAZ. The HAZ of welding repair specimen is more sensitive to pitting corrosion. The sensitivity of HAZ to pitting corrosion was increased by increasing the number of welding repair.     

Conventional and in situ tensile test of friction stir welded steel – Optimization of processing parameters

In the present investigation, steel plates were joined at different tool traversing speed by friction stir welding keeping other parameters same. Microstructural characterization was carried out with optical and scanning electron microscopes. At weld nugget pearlite and bainite were present within ferrite matrix. Thermo‐mechanically and heat affected zones microstructure consisted of pearlite and ferrite. Second phase area fraction and matrix grain size at different regions were varied depending on welding parameters. Weld nugget exhibited substantial improvement in microhardness with respect to base metal. In this respect heat affected zone revealed minimum microhardness and was below base metal value. Tensile tests were carried out on standard and miniature specimens in scanning electron microscope. Highest joint efficiency to the tune of ～82 % and ～120 % of that of base metal obtained for standard and miniature specimens, respectively machined from weld fabricated at lowest welding speed. With increment in welding speed assembly strength was reduced for both types of specimens. Standard specimens failed from heat affected zone and miniature specimens failed through centre of weld nugget. Apart from matrix grain size and second phase area fraction, precipitation of microalloyed carbide / carbonitride was responsible for altering the joint strength.     

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.     

Identification of the pin offset effect on the friction stir welding (FSW) via Taguchi – Grey relational analysis: A Case study for AA 7075 – AA 6013 alloys

The aim of this work is to present a case study relating to the dissimilar friction stir welding (FSW) ability of AA 7075‐T651 and AA 6013‐T6 by applying pin offset technique. An orthogonal array L18 was conducted to perform the overlapped weld seams using three different values of pin offset, welding speed and tool rotational speed along with two different pin profiles determine the impact of welding parameters on the tensile properties of friction stir welded joints. The nugget zone for each of overlapped weld seams exhibited a complex structure and also, the pin offset and profile also were found to have a great impact on the microstructural evolution of the nugget zone. The ultimate tensile strength, elongation at the rapture and bending strength of welded joints were measured in the ranges of 194–215 MPa, 1.79–3.34 % and 203–352 MPa. From the Taguchi based Grey relational analysis, the optimum welding condition was determined for the welded joint performed using a single fluted pin profile with the zero pin offset, tool rotational speed of 630 min^?1 and welding speed of 63 mm/min. Microstructural and macro‐structural observations revealed that welded joints exhibiting lower tensile strength are consistent of various types of defects (e. g. cracks, tunnels and cavities). The fracture location of welded joints was found to be on the heat affected zone and between the heat affected zone and AA 6013‐base metal. The tool and pin wear was not observed during the welding applications     

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75 μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti₂Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300 MPa.     

Microstructure characterization in the weld metals of HQ130 + QJ63 high strength steels

Microstructural characterization of the weld metals of HQ130 + QJ63 high strength steels, welded under 80% Ar + 20% CO₂ gas shielded metal arc welding and different weld heat inputs, was carried out by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The relative contents of acicular ferrite (AF) and pro-eutectic ferrites (PF) in the weld metals were evaluated by means of XQF-2000 micro-image analyser. The experimental results indicate that there is acicular ferrite in the grain and some pro-eutectic ferrite on the boundary of original austenite grains when the weld heat input is small (E = 9.6 kJ/cm), but the main microstructure is ferrite side plate (FSP) when the heat input is larger (E = 22.3 kJ/cm). So the weld heat input should be strictly controlled in the range 10 ∼ 20 kJ/cm and then the content of pro-eutectic ferrite is limited to < 25%. Thus weld metals of HQ130 + QJ63 high strength steels with high toughness and excellent resistance to cracking can be ensured.     

Effect of an electron beam surface treatment on the microstructure and mechanical properties of SAF 2205 joints produced with electron beam welding

The aim of present investigation is to study the effect of an electron beam surface treatment on 2205 duplex stainless steel joints produced by the same electron beam process. Heat treatment of the joints is necessary, to re-establish the ferrite/austenite balance, which is extremely disturbed during the electron beam welding process. For assessing the effectiveness of the electron beam surface treatment, similar weld joints were subjected to conventional furnace heat treatment at 1050 °C, and used as reference. Metallographic techniques and mechanical testing were used to assess the microstructure and corresponding mechanical properties of the as-welded and heat treated specimens. The metallographic results showed that an austenite/ferrite ratio approaching the one produced by a typical furnace post-weld heat treatment can also be achieved by heating locally the weld surface with an electron beam. At the same time the tensile properties show considerable improvement, approaching those obtained by means of a conventional furnace heat treatment, whereas the impact strength is also improved compared to the as welded specimen, but remains clearly lower than the one of the furnace heat treated weld.     

Microstructure characteristics of thick aluminum alloy plate joints welded by fiber laser

It's difficult to weld high strength thick plate since the groove is huge when using traditional arc welding, and the weld tends to be softened and large deformation could occur after multi-layer welding. All of these can affect the industrial application of high strength thick plate wielding. In this case, developing advanced welding technology and welding material is necessary to optimize the microstructure and performance of the welds. Fiber laser has many advantages such as good monochrome and high quality laser beam. In order to decrease the heat damage to the base metal from the welding heat source, low heat input is employed for welding thick plate. Fiber laser is applied in the welding of 20 mm thick Al–Zn–Mg–Cu alloy with super narrow gap filler wire. The microstructure comparison of Al–Mg–Mn alloy and Al–Mg–Mn–Zr–Er alloy welded joints reveals that a huge amount of fine equiaxed grains is formed in the weld zone of Zr and Er micro-alloying Al–Mg–Mn alloy welding wire and a great number of precipitation strengthening phases are precipitated in the weld zone after the heat treatment of welded joints in the entirety.     

Multisheet structure of Inconel 718 superalloy processed by LBW/SPF technology and its microstructure

Abstract

Multisheet structure of Inconel 718 superalloy will be widely used in vehicles as heat resisting and heat shielding structure due to its lightweight, high strength and stiffness. Multisheet structure of Inconel 718 superalloy was processed by laser beam welding and superplastic forming (LBW/SPF) technology in the present paper. Multisheet structure of Inconel718 superalloy processed by LBW/SPF technology exhibits good configuration and uniform thickness distribution. Laser beam welding parameters for multisheet structure were as follows: pulse frequency was 32 Hz; pulse duration 3 ms; peak power per pulse 4500 W; welding speed 180 mm min^–1; SPF parameters were as follows: temperature T_f=965°C; forming pressure P _f=4·2 MPa; forming time t _f=130 min. Microstructure of multisheet structure was studied carefully. Microstructure in weld fusion zone was constituted of austenite dendritics and Laves phase precipitated in interdendritics. After SPF process, austenite dendritics in the weld fusion became coarser and most of Laves phases were dissolved and turned into δ precipitated phase but a few of Laves phases were still reserved. And Nb concentration in dendritics increased to 5·42% compared to 2·82% under as welded condition. Weld metal hardness increased from 331·63 under as welded condition to 391·74 under post-SPF condition which was closed to the base material hardness of post-SPF. Grain size of base material grew slightly and an amount of precipitated phase appeared in the base material undergoing SPF process. The tensile test results of base material show that tensile strength increased obviously and the ductility decreased slightly after SPF process. Therefore, LBW/SPF technology is an appropriate forming technique for multisheet structure of Inconel 718 superalloy.     

Annealing temperature effects on superduplex stainless steel UNS S32750 welded joints. II: pitting corrosion resistance evaluation

This study deals with the effect of the annealing temperature on the pitting corrosion resistance of UNS S32750 submerged-arc welded joints. In a companion article (Part I), the influence of post-weld annealing temperature on microstructure evolution and chemical composition of austenite and ferrite was analyzed; this study can thus be considered directly connected with the previous one. The pitting corrosion resistance of the heat-treated welded joints was evaluated by using both electrochemical measurements and ASTM G48 standard gravimetric tests; examinations of initiation sites of pitting attack were carried out in order to correlate the experimental data obtained in this study with the predicted pitting corrosion behavior obtained by using the results described in Part I. Generally, the post-weld annealing treatment enhances the pitting corrosion resistance of UNS S32750 welded joints. By using PREN analysis of single phases, a correlation between the chemical composition evolution of ferrite and austenite and the experimental pitting behavior of the welded joints was found, in relation to welding and post-welding heat treatment temperature. In particular, an exponential relationship between PREN of weaker phase and pitting potential in 3.5% NaCl solution at 80 °C for the weld metal was obtained. The most favorable annealing temperature for the analyzed welded joints was found to be 1100 °C.     

Effect of welding parameters on microstructure and mechanical properties of AA7075‐T6 friction stir welded joints

The effects of advancing speed and rotational speed on the microstructure and the mechanical properties of friction stir welded 7075‐T6 aluminium alloy sheets were studied. The fatigue strength of sound joints was measured and compared to tensile testing results. Macrographs and microhardness maps were carried out to reveal the microstructure transformations. Fractographic observations were made to identify the failure mechanisms. The effects of welding parameters on the fatigue strength are discussed in terms of welding pitch k (mm/rev) and heat input (J/mm). At a high welding pitch, crack initiation at the root of the circular grooves left by the tool on the weld surface is the most detrimental failure mechanism. As the size and the depth of the grooves are related to the welding pitch, the fatigue strength increases when the welding pitch is reduced. However, when the heat input is excessive, the failure is caused by sub‐surface defects produced after abnormal stirring and/or by softening of the heat‐affected zone. Lateral lips on the weld surface edges also have an effect on the fatigue strength for intermediate welding pitch values.     

Study on laser welding of AA1100-16 vol.% B4C metal–matrix composites

Laser welding of AA1100-16 vol.% B₄C metal–matrix composites was explored in the study. It was found that most B₄C particles were decomposed and that needle-like AlB₂ and Al₃BC phases were substantially formed during the welding process without filler. Consequently, a joint efficiency of 63% (UTS) was obtained. The addition of Ti with 150 μm thick foil increased the joint efficiency to 75% due to the decrease of needle-like phase formations. On the other hand, the addition of Ti with filler wire did not show significant tensile property improvement due to the Ti segregation and microstructure inhomogeneity in the weld zone. The fracture surfaces of laser welded joints were investigated to understand the fracture mechanisms.     

Simulation on welding thermal effect of AZ61 magnesium alloy based on three-dimensional modeling of vacuum electron beam welding heat source

The deep-penetration thermal effect of keyhole and surface thermal effect of high-temperature metal vapor by the direct-acting mechanism during vacuum electron beam welding were analyzed. According to the thermal effect, a composite source model working for magnesium alloy welding was developed. This model was composed of Gaussian surface source and conical heat source. By the welding experiments on AZ61 magnesium alloy, it can be obtained those two key factors, which were welding heat input and focus coil current (I_f), affected the keyhole thermal effect and weld shape significantly. In order to simulate the shape of keyhole and weld, the varying of focus conditions in model were realized by the power coefficient of composite welding heat source. It can be seen that there was a good consistency between the calculation results and experimental results.     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.