Laser welding of TiNi shape memory alloy and stainless steel using Ni interlayer

Laser welding of TiNi shape memory alloy wire to stainless steel wire using Ni interlayer was investigated. The results indicated that the Ni interlayer thickness had great effects on the chemical composition, microstructure, gas-pore susceptibility and mechanical properties of laser-welded joints. With an increase of Ni interlayer thickness, the weld Ni content increased and the joint properties increased due to decreasing brittle intermetallic compounds (TiFe₂ and TiCr₂). The joint fracture occurred in the fusion zone with a brittle intermetallic compound layer. The tensile strength and elongation of the joints reached the maximum values (372 MPa and 4.4%) when weld Ni content was 47.25 wt.%. Further increasing weld Ni content resulted in decreasing the joint properties because of forming more TiNi₃ phase, gas-pores and shrinkage cavities in the weld metals. It is necessary to select suitable Ni interlayer thickness (weld composition) for improving the mechanical properties of laser-welded joints.     

Laser-welding behaviour of cast Ni3Al intermetallic alloy

Ductile nickel aluminide, Ni₃Al+B, is an intermetallic alloy with high strength and ductility making it a promising structural material for both elevated temperature and cryogenic temperature applications. In order to be able to use this alloy over a spectrum of temperature-critical applications, it must be capable of being joined or welded. The weldability of a cast nickel aluminide alloy containing boron was studied using laser welding. Welding was carried out at laser beam traverse speeds ranging from 42.33–254 mms^–1 in the bead-on-plate and butt-joint configurations. Two types of surface preparation, namely chemical cleaning and mechanical polishing, were used prior to laser welding. The quality of the laser welds was evaluated through mechanical tests (hardness and tensile), X-ray diffraction and microscopical observations. High-magnification examination of the welds revealed fine columnar structures in the weld zone. The hardness of the weld zone was substantially higher than that of the base metal. Microscopic examination also revealed the welds to contain shrinkage cracks. For a constant set of laser parameters, the chemically etched surfaces provided deeper penetration than the mechanically polished surface. The performance of the laser-welded joint is rationalized.     

Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70 mm × 15 mm × 5 mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.     

A fatigue life prediction method for coke drum base,weld, and HAZ materials from tensile properties

The importance of material fatigue information in design has been well recognized. There are a few existing fatigue life prediction methods based on materials tensile properties. Some of these fatigue life prediction methods can be successfully applied for non-heat affected materials. However, industrial components, such as pressure vessel and pipelines are commonly constructed by welding parts together. The fatigue lives of welded section and its surrounding material could be greatly affected by the welding process. Therefore, it is beneficial to develop a fatigue life prediction model for the weld and surrounding heat affected zone (HAZ) materials based on their tensile testing data. In this paper, fatigue lives of base material and its weld and HAZ materials for constructing coke drums are studied. Mechanical properties are first obtained from the tensile tests. Then, fully-reversed strain-controlled fatigue tests were performed. It is found that the fatigue life of pure base material is roughly twice of the weld and four time of the HAZ at the same strain amplitude. Four-point correlation (FPC) method by Manson can reasonably predict the life of base material. However, it over-predicts the lives of weld and HAZ. By introducing two reduction factors R_plastic and R_elastic for the weld and HAZ material respectively into the FPC method, the over-prediction can be rectified. Therefore, the proposed modified FPC method could be applied in predicting fatigue lives of weld and HAZ materials.     

Microstructure and mechanical properties of laser beam welded Al–Li alloy 2060 with Al–Mg filler wire

Alloy 2060-T8 is a newly developed high-strength Al–Li alloy for applications in aircraft industry. Crack-free welds were obtained in laser beam welding with 5087 filler wire under optimized welding conditions. In this paper, fusion zone microstructure and joint mechanical properties were investigated. Microstructure typical for the weld metal consists of α-Al matrix with a few nanoscale precipitates inside and a coarse icosahedral quasicrystalline T₂ phase at the dendritic and grain boundaries. The quasicrystalline occurred normally in Al–Li–Cu alloys with higher Li contents. Our investigations show that the icosahedral quasicrystalline phase T₂ phase forms in the laser-welded Al–Li alloy 2060 with lower Li content as a result of segregation and replacement of Mg element. The joint tensile strength in as-welded condition is around 317 MPa, about 63% of that of the base metal, and fracture occurs within the fusion zone.     

强动载荷下焊接钢板力学性能及本构模型研究

为研究强动载荷下船用焊接钢板的力学性能。开展了典型船用焊接钢板母材、焊缝和热影响区的准静态拉伸试验、高温拉伸试验及SHPB动态压缩试验,分析了焊接钢板材料在不同应力状态下的力学行为,基于力学性能试验结果拟合了焊接钢板母材、焊缝和热影响区材料的本构模型。结果表明:准静态条件下,与母材相比,焊缝和热影响区材料的屈服强度与抗拉强度偏大,延伸率偏小;高应变率下,热影响区材料抵抗塑性变形的能力明显强于其他两种材料,且随着应变率的增加抵抗塑性变形的能力呈增强趋势;焊接板母材、焊缝与热影响区材料均表现出应变率效应和温度效应;热影响区是焊接板抗冲击性能相对薄弱的区域。建立的Johnson-Cook模型可以描述强动载荷下焊接钢板的力学性能。     

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.     

Microstructural and mechanical characterisation of laser-welded lap joints with linear and circular beads in thin low carbon steel sheets

This paper presents a microstructural and mechanical characterisation of laser-welded lap joints in low carbon steel thin sheets. Different combinations of steel types (DC05, S355MC) and thickness values are used to assemble welded specimens with linear and circular weld bead. Metallurgical observations and micro-hardness tests are used to characterise the weld microstructure. Mechanical response in tensile test is then used to evaluate the static strength, rotation angle of weld bead and failure mode of welded specimens. Lap-joints with circular weld showed a lower rotation angle compared to linear welds. The fracture in all tested specimens occurred at the base metal, far away from the weld. A simplified mechanical model is finally proposed to derive theoretical formulae for estimating the tensile strength of welded joints as a function of material properties and weld geometry. The analytical results are in good agreement with experimental findings and they estimate an increased strength for circular welds, compared to linear weld with same lateral width. A design chart is also derived to allow a design of laser-welded joints with virtually equal strength of base metal and weld zone.     

Corrosion behavior of a laser-welded NiTi shape memory alloy

Corrosion behaviors of the laser-welded Ni–49.4 at.% Ti shape memory alloy and base metal in 0.9% NaCl solution were investigated by means of electrochemical techniques (the open circuit potential measurement, linear and potentiodynamic polarizations). The results showed that corrosion resistance of the laser-welded NiTi alloy is better than that of the base metal. Compared to the base metal, the laser-welded NiTi alloy exhibits higher open circuit potential, higher polarization resistance, a wider passive region and higher breakdown potential. The improvement of corrosion resistance of the laser-welded NiTi alloy is ascribed to a smoother, defect free surface and an absence of carbides.     

Inertia-friction welding of SiC-reinforced 8009 aluminium

Inertia-drive friction welding (IFRW) of an 8009 Al alloy (Al-8.5 Fe-1.7 Si-1.3 V, wt%) reinforced with 11 volume per cent SiC particles (8009/SiC/11_p) has been investigated. Inertia-drive friction welds were made with constant energy at two levels of axial force. The microstructures of the base material and the welds were characterized using optical and scanning electron microscopy, while the mechanical properties were evaluated using microhardness and tensile testing. Examination of weld sections revealed that the hot deformation experienced during welding produced a homogenized microstructure with a uniform distribution of SiC particles along the bond line. No evidence of a chemical reaction between the SiC and the matrix was found in any of the welds, but cracking of some of the larger SiC particles was observed in the base material as well as in the IFR welds. The average microhardness of the various heat-and-deformation affected zones (HDZs) of the welds did not vary greatly from that of the base material, and no weld induced weak regions were discerned. The room-temperature (RT) tensile strength of the IFR welds exceeded 90 per cent of the base material. The weld tensile specimens failed at the outer edge of the HDZ for all of the welds tested. The fracture surface of the 8009 matrix of tensile samples for both the base material and the welds exhibited a dimpled appearance indicating a ductile failure, while fracture through the SiC appeared to occur in a brittle fashion. IFRW has proven effective in joining 8009/SiC/11_p with little loss in RT hardness and tensile properties.     

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.     

Overload failure behaviour of dissimilar thickness resistance spot welds during tensile shear test

Abstract

Resistance spot welding is the dominant process for joining sheet metals in automotive industry. Even thickness combinations are rarely used in practice; therefore, there is clearly a practical need for failure behaviour investigation of uneven thickness resistance spot welds. The aim of the present paper is to investigate the failure mode and failure mechanism of dissimilar thickness low carbon steel resistance spot welds during tensile shear overload test. Microstructural investigations, microhardness tests and tensile shear tests were conducted. Mechanical properties of the joints were described in terms of peak load, energy absorption and failure mode. In order to understand the failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile shear are examined by optical microscopy. It was found that for well established weld nuggets, the final solidification line is located in the geometrical centre of the joint. In pull-out failure mode, failure is initiated by necking of the base metal at the thinner thickness sheet. Finally, it was concluded that weld nugget size, weld penetration and the strength of the thinner sheet are the main controlling factors of the peak load and energy absorption of dissimilar thickness spot welds.     

MGH956合金TIG焊原位合金化对其组织性能的影响

采用TIG焊对氧化物弥散强化(ODS)高温合金MGH956进行原位合金化焊接.在相同的焊接条件下,填加两种不同的填充材料:与母材化学成分相似的基体填充材料,以及在基体填充材料基础上加入了合金元素Al和Fe2O3的Al-Fe2O3填充材料.通过对比分析两组试样在焊接过程中发生的原位合金化反应机理,及其对焊缝微观组织和力学性能的影响,研究原位合金化反应对ODS合金TIG焊接头组织与性能的影响.结果表明:在填充材料中加入Al和Fe2O3合金元素时,焊缝处的气孔数量明显减少,气孔尺寸也较为减小;焊缝中原位生成了新的增强相颗粒Al2O3、TiC以及YAlO3,同时,基体中的纳米级增强相Al-Y复合氧化物团聚倾向降低.力学性能试验结果表明,填加Al-Fe2O3填充材料时焊缝显微硬度值明显提高,接头抗拉强度达到了578 MPa,为母材强度的80.3%.     

Evaluation of variation of tensile strength across 316LN stainless steel weld joint using automated ball indentation technique

Tensile strength variation across 316LN stainless steel fusion welded joint comprising of base metal, deposited weld metal and heat affected zone (HAZ) has been evaluated by Automated Ball Indentation (ABI) technique. Automated Ball Indentation tests were conducted on the various zones of the steel weld joint at 300, 523 and 923?K. The flow curves obtained from ABI results were consistent with corresponding conventional uniaxial tensile test results. The HAZ exhibited higher tensile strength than the other regions of the steel weld joint at all investigated temperatures. The ratio of ultimate tensile strength to yield stress (YS), which represents the work hardening behaviour, increased with an increase in temperature for the base metal and HAZ; whereas it remained nearly the same for the weld metal.     

不锈钢母材及其焊缝金属材料单拉本构关系研究

为研究国产双相型S22053和奥氏体S30408不锈钢母材及其焊缝金属材料的单拉本构关系和破坏模式,对4组共12个材性试样进行了单向拉伸试验,并对其破坏截面进行了电镜扫描。基于试验曲线,利用修正的Ramberg-Osgood （R-O）模型对材料本构关系参数进行了拟合,进而对不锈钢母材和焊缝金属的单拉本构关系进行了对比分析。结果表明:不锈钢母材和焊缝金属材料均发生韧性破坏,其本构关系都表现出明显的非线性;焊缝金属材料的屈服强度和极限强度均高于不锈钢母材,而延性低于母材;修正的R-O模型与母材和焊缝金属材料试验曲线吻合良好;在焊缝连接承载力精确分析中,应考虑焊缝金属材料与母材本构关系的差异,分别使用相应的本构关系模型。     

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.     

Microstructural and mechanical characterization of electron beam welded Al-alloy 7020

The electron beam (EB) welding process is used to weld any metal that can be arc welded with equal or superior weld quality. EB welding is carried out in a high-purity vacuum environment, which results in freedom from impurities such as oxides and nitrides. Thus, pore-free joints can readily be achieved in metallic materials, such as Al-alloys and Ti-alloys. However, autogenous EB welding of some aluminium alloys leads to a significant strength reduction (undermatching) in the fusion zone due to the loss of strengthening phases. For such Al-alloys, the local microstructure-property relationships should be established to satisfy the service requirement of a welded component with strength undermatching. Autogenous EB welding was performed on 5 mm thick aluminium alloy 7020 plate. Microstructural characterization of the weld metals was made by optical and scanning electron microscopy. Extensive microhardness measurements were conducted in the weld regions of the joints which exhibited a hardness loss in the fusion zone due to the loss of strengthening phases. Tensile properties of the joints were determined by testing flat transverse tensile specimens at room temperature without machining the weld profiles. Furthermore, elastic-plastic fracture toughness tests (CTOD) were carried out on the base material and welded joints at room temperature.     

Improvement of impact toughness of AWS E6010 weld metal by adding TiO2 nanoparticles to the electrode coating

The effect of TiO₂ nanoparticles in the electrode coating on the impact toughness of three weld metals prepared by the shielded metal arc welding process was investigated and the main factors affecting the impact toughness were discussed. The microstructure, mechanical properties and fracture surface morphology of the weld metals have been evaluated and the results are compared. When the content of TiO₂ nanoparticles in the composition of electrode coating is increased, the morphology of ferrite in the microstructure of columnar zone will change from Widmanstätten ferrite to acicular ferrite. This finally changes to allotriomorphic ferrite when the amount of TiO₂ nanoparticles in the electrode coating goes relatively high. Furthermore, the addition of TiO₂ nanoparticles is effective in refining the ferrite grain size of the reheated microstructures of weld metals. This effect is attributed to the increased number of nucleation sites on the oxide nanoparticles. The impact toughness of the weld metal was improved by adding TiO₂ nanoparticles, especially when a medium TiO₂ nanoparticle content was used in the electrode coating. A significant increase in the impact toughness of weld metal was shown to be due to the increased percentage of acicular ferrite and refinement of microstructure.     

Comparative study on local and global mechanical properties of 2024 T351, 2024 T6 and 5251 O friction stir welds

The effect of the microstructure heterogeneity on the global and local tensile properties of friction stir welded joints in 5251 (O temper) and AA2024 (T351 and T6 tempers) aluminium alloys has been investigated. Micro-tensile tests parallel to the welding direction have been carried out in the regions representative of the main microstructural zones. The digital image correlation technique (DIC) has been used during transverse tensile tests for mapping the strain distribution and to determine the local stress–strain curves. A 3-D finite elements model has been developed to predict the weld behaviour from the tensile curves of the individual regions of the weld.The tensile properties of the 5251 O weld are relatively homogeneous leading to high ductility and fracture in the base material. In contrast, the tensile properties of the various regions of the 2024 T351 and 2024 T6 welds are very heterogeneous and essentially controlled by the state of precipitation. The thermo-mechanically affected zone is the weakest region where the strain localises during a transverse tensile test. The 2024 T6 base material is stronger than the 2024 T351 alloy, leading to a more pronounced strain localisation during transverse tensile tests and a lower overall ductility. Local tensile data obtained by strain mapping are in good agreement with the curves obtained by micro-tensile tests, and these results can be safely used in a finite elements model to predict the behaviour of the overall weld assembly.     

Characterization of inertia-friction welds between a rapidly-solidified Al-Fe-Mo-V alloy and IM 2024-T351

The structures, mechanical properties and fracture behaviour of inertia-friction welds produced between rapidly-solidified/powder metallurgy (RS/PM) Al-9Fe-3Mo-1V (wt %) and ingot metallurgy (IM) 2024-T351 aluminium were investigated. Visual examination showed the axial displacement experienced by the specimens during welding and the degree of metal expulsion from the weld interface (i.e. flash) to increase with an increase in axial force. The weld flash was observed to originate principally from the IM 2024-T351, which was consistent with the lower elevated-temperature strength of this precipitation-hardened alloy. Although the weld interface region remained nearly flat in welds produced using low axial force, this surface became increasingly curved (concave into the Al-9Fe-3Mo-1V alloy) with an increase in axial force. Microstructure analysis using both light and analytical electron microscopy characterized the heat- and deformation-affected zones (HDZs) in each of the base metals and the weld interface regions. The HDZ directly adjacent to the weld interface in the IM 2024-T351 exhibited fine, recrystallized alpha aluminium grains and an absence of S precipitates present in the base metal microstructure. The HDZ directly adjacent to the weld interface in the Al-9Fe-3Mo-1V exhibited fine alpha grains and fine, spherical and acicular dispersoids, which in part originated from the plastic deformation and fracture of coarse base metal dispersoid particles. The extent of this dispersoid-refined region was greatest at the centre of the weld as opposed to the outer periphery, and in the low rather than the high axial force weld. At the weld interface in the vicinity of the axial centre line, the occurrence of highly localized mechanical mixing between the two alloys was determined using both light and electron microscopy and electron-microprobe analysis techniques.Microhardness traverses showed relatively little variation in hardness across the weld interface and an absence of hardness degradation at any location relative to the unaffected base metals. Room-temperature transverse-weld tensile testing showed tensile strengths to range between 85 and 90% of the RS/PM base metal, with fracture occurring in the Al-9Fe-3Mo-1V HDZ remote from the weld interface. Three-point guided bend testing also revealed fracture to occur in the Al-9Fe-3Mo-1V HDZ. SEM fractographic analysis of the fracture surfaces found fracture in the Al-9Fe-3Mo-1V to involve microvoid formation at dispersoid/alpha aluminium interfaces and subsequent ductile rupture in the alpha aluminium matrix.