Effect of Post Weld Heat Treatment on Cryogenic Mechanical Properties of Electron Beam Welded Cast Inconel 718

Electron beam welding of pre-aged cast 718 produced a slender Laves phase, while welds from solutionized cast 718 exhibited a blocky Laves phase with an initial stage of δ precipitation and formation of γ′+γ″ strengtheners after post weld heat treatment (PWHT). The presence of primary strengthener and coarse Laves particles in PWHT weld may cause reduction of the micro-plastic zone ahead of a crack, leading to a significant decrease in Charpy impact toughness at 77 K (?196 °C).     

Inertia Friction Welding of Dissimilar Superalloys Mar-M247 and LSHR

The solid state inertia friction welding (IFW) process was used for the first time to join two dissimilar Ni-based superalloys, LSHR, a powder metallurgy alloy, and Mar-M247, a directionally solidified alloy. Extensive studies of the microstructure, phase composition, re-distribution of the alloying elements between the welded alloys, microhardness, and welding defects were conducted at different distances from the weld interface, and the results were correlated with the loading and friction conditions during IFW. Possible reasons leading to the formation of the welding defects were discussed and directions for the further improvement of the quality of the IFW of these two dissimilar alloys were outlined.     

Effects of Initial Temper Condition and Postweld Heat Treatment on the Properties of Dissimilar Friction-Stir-Welded Joints between AA7075 and AA6061 Aluminum Alloys

In this study, dissimilar AA7075-O/6061-O and AA7075-T6/6061-T6 butt joints were produced by friction stir welding (FSW), and postweld heat treatment (PWHT) was applied to the joints obtained. The effects of initial temper condition and PWHT on the microstructure and mechanical properties of the dissimilar joints were thus investigated. It was demonstrated that sound dissimilar joints can be produced for both temper conditions. A hardness increase in the joint area (i.e., strength overmatching) was obtained in the joints produced in the O-temper condition, whereas a hardness loss was observed in the joint area of the joints obtained in the T6 temper condition. It was also well demonstrated that PWHT could be used in order to improve the joint properties for both O and T6 joints provided that the joint is defect-free prior to subsequent heat treatment.     

2A97铝锂合金搅拌摩擦焊焊缝的微观组织特征

对厚度为25 mm的T851态2A97铝锂合金进行搅拌摩擦焊焊接,利用显微硬度、金相显微镜(OM)和透射电镜(TEM)等对焊缝的显微硬度和微观组织进行研究.结果表明:接头基材硬度最高,热影响区和热机影响区硬度降低,焊缝中心硬度又升高,硬度最低位置在热影响区.焊核区发生动态再结晶,形成细小等轴的晶粒;焊核区S′相全部溶解,T1相儿乎全部溶解,在随后的冷却和时效过程中,焊核区析出GP区和细小弥散的δ′相;热影响区的T1相部分溶解,S′相全部溶解,析出θ"相、δ′相和δ′/β′的复合相.     

Characterization of a friction-stir-welded aluminum alloy 6013

The aluminum alloy 6013 was friction-stir welded in the T4 and the T6 temper, and the microstructure and mechanical properties were studied after welding and after applying a postweld heat treatment (PWHT) to the T4 condition. Optical microscopy (OM), transmission electron microscopy (TEM), and texture measurements revealed that the elongated pancake microstructure of the base material (BM) was transformed into a dynamically recrystallized microstructure of considerably smaller grain size in the weld nugget. Strengthening precipitates, present before welding in the T6 state, were dissolved during welding in the nugget, while an overaged state with much larger precipitate size was established in the heat-affected zone (HAZ). Microhardness measurements and tensile tests showed that the HAZ is the weakest region of the weld. The welded sheet exhibited reduced strength and ductility as compared to the BM. A PWHT restored some of the strength to the as-welded condition.     

焊接速度对机器人搅拌摩擦焊AA7B04铝合金接头组织和力学性能的影响

针对熔化焊在焊接AA7B04铝合金时易在焊缝中出现孔洞等缺陷,且接头性能下降明显、焊后变形大,以及采用铆接等机械连接方式会增加连接件的重量等问题,采用集成了搅拌摩擦焊末端执行器的KUKA Titan机器人对2 mm厚AA7B04高强铝合金进行了焊接,在转速为800 r·min^-1的条件下,研究了焊度对焊接过程中搅拌头3个方向的受力F_x、F_y和F_z的影响.研究发现,F_z受焊速的影响显著,随焊速的增加而降低.利用光学显微镜、透射电子显微镜、拉伸试验、三点弯曲试验和硬度测试等方法,研究了不同焊速下AA7B04铝合金接头的微观组织和力学性能.结果表明：当焊速为100 mm·min^-1时,接头的抗拉强度最高为447 MPa,可达母材的80%,且所有接头的正弯和背弯180°均无裂纹;接头横截面的硬度分布呈W型,硬度最低点出现在热力影响区和焊核区的交界处,焊速不同会导致不同的焊接热循环,且随着焊速的增加接头的硬度随之增加;焊核区组织发生了动态再结晶,生成了细小的等轴晶粒,前进侧和后退侧热力影响区的晶粒均发生了明显的变形;前进侧热影响区析出η'相,后退侧热影响区因温度较高析出η'相和尺寸较大的η相.     

Hardness,Microstructure, and Residual Stresses in Low Carbon Steel Welding with Post-weld Heat Treatment and Temper Bead Welding

This paper investigates the effects of post-weld heat treatment (PWHT) and temper bead welding (TBW) on hardness, microstructure and residual stresses in multi-layer welding on low carbon steel specimens made with two different weld geometries, viz. (1) smooth-contoured and (2) U-shaped. It was found that the PWHT technique gave overall lower hardness than the TBW technique, but the hardness values in both techniques were acceptable. Microscopy analysis showed that the TBW technique was more effective in tempering the heat affected zone as the grain size decreased slightly at the fusion line in spite of the higher temperature at the fusion line. Residual stresses measured using the hole-drilling method showed that the residual stress is not reduced below yield stress near the last bead solidified in TBW. Only PWHT gives low residual stress results in this area. High tensile residual stresses may result in sensitivity to fatigue loading.     

Effect of Postweld Heat Treatment on Microstructure, Hardness, and Tensile Properties of Laser-Welded Ti-6Al-4V

The effects of postweld heat treatment (PWHT) on 3.2-mm- and 5.1-mm-thick Ti-6Al-4V butt joints welded using a continuous wave (CW) 4-kW Nd:YAG laser welding machine were investigated in terms of microstructural transformations, welding defects, and hardness, as well as global and local tensile properties. Two postweld heat treatments, i.e., stress-relief annealing (SRA) and solution heat treatment followed by aging (STA), were performed and the weld qualities were compared with the as-welded condition. A digital image correlation technique was used to determine the global tensile behavior for the transverse welding samples. The local tensile properties including yield strength and maximum strain were determined, for the first time, for the laser-welded Ti-6Al-4V. The mechanical properties, including hardness and the global and local tensile properties, were correlated to the microstructure and defects in the as-welded, SRA, and STA conditions.     

Understanding the Effects of CoAl2O4 Inoculant Additions on Microstructure in Additively Manufactured Inconel 718 Processed Via Selective Laser Melting

The effect of varying amounts of CoAl₂O₄ inoculant ranging from 0 to 2 wt pct on the microstructure evolution of Inconel 718(IN718) fabricated by selective laser melting (SLM) was evaluated. Characterization of the as-built microstructure revealed that addition of CoAl₂O₄ resulted in a modest degree of grain refinement with a slight increase in microstructural anisotropy. Increasing the total CoAl₂O₄ content beyond 0.2 wt pct resulted in severe agglomeration of the non-metallic particles and the formation of slag inclusions measuring up to 100 μm in size present in the as-built microstructure. In addition to large agglomerates, the inoculant was chemically reduced to form a fine dispersion of submicron-sized Al₂O₃ particles throughout the IN718 matrix. The fine dispersion of oxides significantly hindered grain recrystallization during the post-fabrication heat treatment due to a Zener pinning effect. The findings from this study indicate in order to effectively utilize CoAl₂O₄ as a grain refining inoculant for additive manufacturing, the process parameters need to be optimized to avoid agglomeration of the non-metallic particles and other process-related defects.

     

Study of Dissimilar Header Welding Between 2.25Cr–1Mo Steel and 9Cr–1Mo Steel with 9018 B9 Electrode Under Various Conditions of Post Weld Heat Treatment

Headers are an integral part of the power plant equipment which serves as junction for receiving and distribution of fluid. Headers are routinely used in high temperature applications in which various combinations of steels are used to achieve weight and cost savings thus optimising the use of steels. This paper intends in studying the evolution of microhardness and microstructure in a dissimilar header fillet welding between the base materials 2.25Cr–1Mo steel and 9Cr–1Mo steel when welded using 9018 B9 electrode and a constant preheat of 220 °C. The post weld heat treatment (PWHT) is varied at temperatures from room temperature to 770 °C and the soaking duration is kept at 1 h. The changes in microstructure and microhardness are examined with the help of micrographs, electron dispersive spectrum and scanning electron microscopy analysis. As the PWHT temperatures changes, the variation in microstructure and microhardness becomes very much evident which is detailed out in this paper. Also, carbon migration phenomena and its relation with the PWHT temperatures has been studied in this paper.     

Experimental and Computational Investigation of Structural Integrity of Dissimilar Metal Weld Between Ferritic and Austenitic Steel

The structural integrity of dissimilar metal welded (DMW) joint consisting of low-alloy steel and 304LN austenitic stainless steel was examined by evaluating mechanical properties and metallurgical characteristics. INCONEL 82 and 182 were used as buttering and filler materials, respectively. Experimental findings were substantiated through thermomechanical simulation of the weld. During simulation, the effect of thermal state and stress distribution was pondered based on the real-time nuclear power plant environment. The simulation results were co-related with mechanical and microstructural characteristics. Material properties were varied significantly at different fusion boundaries across the weld line and associated with complex microstructure. During in-situ deformation testing in a scanning electron microscope, failure occurred through the buttering material. This indicated that microstructure and material properties synergistically contributed to altering the strength of DMW joints. Simulation results also depicted that the stress was maximum within the buttering material and made its weakest zone across the welded joint during service exposure. Various factors for the failure of dissimilar metal weld were analyzed. It was found that the use of IN 82 alloy as the buttering material provided a significant improvement in the joint strength and became a promising material for the fabrication of DMW joint.     

不同5xxx系焊丝对5182-H111组织和性能的影响

使用3种焊丝ER 5183、ER 5356、ER 5554对6mm 5182-H111铝合金板材进行半自动MIG焊对接试验,通过对焊接接头进行力学性能试验、显微硬度测试及金相组织的观察,探究3种焊丝对5182-H111铝合金组织和性能的影响。结果表明,使用3种焊丝所焊接头中,ER 5183及ER 5356接头抗拉强度均大于ISO 15614-22005要求的焊缝接头系数(1.0),而ER 5554接头焊缝系数仅为0.949;3种焊丝所焊接头硬度均在焊缝区及热影响区有所降低,其中ER 5356所焊接头焊缝强度较其他两种焊丝焊缝区硬度高,为85.5HV;3种焊丝所焊接头组织形貌相近,均存在β(Mg2Al3)强化相,但由于ER 5183与ER 5356焊丝中含有更多的Mg,因此生成了更多的β(Mg2Al3)相,使得焊缝组织更加致密。     

Improvement in Laser Weldability of INCONEL 738 Superalloy through Microstructural Modification

Microstructural study of laser-beam-welded IN 738 superalloy was carefully performed to better understand the causes of heat-affected zone (HAZ) cracking and to determine an improved approach of alleviating the weldability problem. The HAZ cracks in the alloy were intergranular liquation cracks that resulted from the liquation reaction of both secondary solidification products (MC carbides and γ-γ′ eutectic) and solid-state reaction products (γ′ particles) present in the preweld material. In contrast to the expectation based on Chadwick’s equation, a reduction of grain boundary liquid film thickness did not produce a decrease in HAZ cracking owing to increased base alloy hardness that accompanied a preweld heat treatment designed to reduce the intergranular liquation. Moreover, a major factor limiting the effectiveness of an existing preweld heat treatment with low base alloy hardness in reducing HAZ cracking was found to be the formation of intergranular M₅B₃ boride particles during the heat treatment. These borides can widen the HAZ brittle temperature range (BTR) during weld cooling and increase the propensity for cracking. Based on the results, a new preweld heat treatment that induces a moderate hardness and precludes grain boundary boride formation was found and was shown to produce a significant reduction in HAZ cracking in the welded alloy compared to the most effective pre-existing preweld heat treatment.     

Inertia welding nickel-based superalloy: Part I. Metallurgical characterization

This article describes a quantitative study of the microstructure of nickel-based superalloy RR1000 tube structures joined by inertia welding. One as-welded and three post weld heat-treated (PWHT) conditions have been investigated. The samples were characterized mechanically by measuring the hardness profiles and microstructurally in terms of γ grain size, γ′ precipitate size and volume fraction, stored energy, and microtexture. Electron backscatter diffraction (EBSD) was used to characterize high-angle grain boundaries (HAGB) and the variation of microtexture across the weld line. The coherent γ′ precipitates were investigated over a range of scales on etched samples in a field emission gun scanning electron microscope (FEGSEM), using carbon replicas in a transmission electron microscope (TEM) and from thin slices by means of high-energy synchrotron X-rays. Dramatic changes in the microstructure were observed within 2 mm of the weld line. In this region, the hardness profile is influenced by changes in grain size, γ′ volume fraction, γ′ particle size, and the work stored in the material. Further away, the observed hardness variation is still significant although only minor microstructural changes could be observed. In this region, the correlation of microstructure and hardness is less straightforward. Here, a combination of small microstructural changes appears to give rise to a significant change in strength. No significant texture or grain distortion was observed in the extensively plastically deformed region due to recrystallization.     

Influence of Rotational Speed on Microstructure and Mechanical Properties of Dissimilar Metal AISI 304-AISI 4140 Continuous Drive Friction Welds

 Fundamental investigation of continuous drive friction welding of austenitic stainless steel (AISI 304) and low alloy steel (AISI 4140) is described. The emphasis is made on the influence of rotational speed on the microstructure and mechanical properties such as hardness, tensile strength, notch tensile strength and impact toughness of the dissimilar joints. Hardness profiles across the weld show the interface is harder than the respective parent metals. In general, maximum peak hardness is observed on the stainless steel side, while other peak hardness is on the low alloy steel side. A trough in hardness distribution in between the peaks is located on the low alloy steel side. Peak hardness on the stainless steel and low alloy steel side close to the interface increases with a decrease in rotational speed. All transverse tensile joints fractured on stainless steel side near the interface. Notch tensile strength and impact toughness increase with increase in rotational speed up to 1500 r/min and decrease thereafter. The mechanism of influence of rotational speed for the observed trends is discussed in the torque, displacement characteristics, heat generation, microstructure, fractography and mechanical properties.     

Characterization of Structure–Property Relationship of Incoloy 825 and SAF 2507 Dissimilar Welds

This study was carried out to investigate the evaluation of dissimilar welding between Incoloy 825 Ni-based alloy and SAF 2507 super duplex stainless steel. Welding was conducted by pulsed current (PC) and continuous current (CC) gas tungsten arc welding (GTAW) methods using ERNiCrMo-3 filler wire. The microstructure of weld zones and base metal/weld interfaces as well as mechanical properties of weldments were characterized. The results detailed the formation of Nb, and Mo-rich phases in the inter-dendritic regions of weld metals leading to a decrease in impact resistance of weld zones in comparison to parent metals. Presence of more secondary phases at the CCGTA weld metal resulted in higher hardness and lower toughness than that of the PCGTAW sample. During tensile tests, fracture occurred at the Incoloy 825 base metal, and both weldments also underwent ductile mode of fracture. The research addressed the microstructure–property relationship for dissimilar weld joints.     

Investigation into the Influence of Post-Weld Heat Treatment on the Friction Stir Welded AA6061 Al-Alloy Plates with Different Temper Conditions

In this study, the effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of friction stir butt-joined AA6061 Al-alloy plates both in O and T6-temper conditions was investigated by detailed microstructural investigations and microhardness measurements, in combination with transverse tensile testing. It was determined that the PWHT might result in abnormal grain growth (AGG) in the weld zone particularly in the joints produced in O-temper condition depending on the weld parameters used during friction stir welding. The PWHT generally led to an improvement in the mechanical properties even if AGG took place. Thus, the post-weld heat-treated joints exhibited mechanical properties much higher than those of respective as-welded plates and comparable to those of the respective base plates.     

6061-T6铝合金薄板的搅拌摩擦焊接

采用搅拌摩擦焊(FSW)技术对1 mm厚6061-T6铝合金薄板进行了对接.研究了焊接工艺参数的范围,实验测试了焊接接头的强度、硬度和延伸率,利用金相显微镜、扫描电镜和透射电镜分析了接头的微观组织.结果表明:对于1 mm厚度6061-T6铝合金,FSW的最优工艺参数为旋转速度1 800 r·min^-1,焊接速度1000 mm·min^-1;在此参数下,接头的硬度值达到母材的80%左右,抗拉强度达到母材的103%,延伸率达到母材的54%;接头的力学性能与微观结构相符.     

含钪Al-Zn-Mg-Zr合金薄板材MIG焊接接头的组织与性能

采用新型Al-Mg-Sc-Zr焊丝对含钪Al-Zn-Mg-Zr合金薄板材进行MIG焊,借助显微硬度及拉伸性能测试、OM,SEM,TEM等检测手段对焊接接头的微观组织和性能进行研究。结果表明:焊缝区为典型的铸态组织;热影响区靠近焊缝一侧呈现大量细小等轴晶组织,靠近基材区为纤维状组织和少量再结晶组织;基材区为纤维状组织。焊接接头的显微硬度以焊缝为中心呈近似对称,且中心处硬度值最低;抗拉强度为481 MPa,屈服强度为320MPa,伸长率为10.1%,焊接系数约0.83。同时,焊丝和基材中微量的Sc和Zr元素在合金中形成大量细小且与基体共格的Al3(Sc,Zr)粒子,能显著细化晶粒组织,有效抑制再结晶发生,大大改善焊缝区的力学性能。