R‐curve behaviour of friction stir welds in aluminium‐lithium alloy 2195

ABSTRACT The fracture resistance of friction stir welds in 2195‐T8 is described in this paper. R‐curves were produced for several crack planes, parallel to the welding direction and situated at varying distances from the weld centreline. The friction stir weld was also characterized by hardness and tensile testing. Fracture resistance of the friction stir weld is compared to the base metal and to a variable polarity plasma arc weld. Results show that the material in and around the friction stir weld is tougher than the base metal and the variable polarity plasma arc weld. The friction stir weld fracture performance is discussed in the light of the observed hardness and fractographic data presented.     

Influence of tool geometries and process variables on friction stir butt welding of Al–4.5%Cu/TiC in situ metal matrix composites

Present work describes friction stir welding of in-house produced and hot rolled Al–4.5%Cu/TiC in situ metal matrix composites by using hardened bimetallic tool with varying shoulder surface geometries and other process variables. Joining of the said composite using friction stir welding process has been seen to provide beneficial effects such as grain refinement of the matrix and subsequent redistribution and refinement of reinforcements. A predictive model has also been developed to estimate the weld properties such as tensile strength and ductility with respect to the tool geometry used and input process variables. The X-ray diffraction analysis results of Al–4.5%Cu/TiC butt welds indicated formation of CuAl₂O₄ and CuAl₂ to some extent in the stir zone. Fractography of the weld samples revealed dimpled ductile nature of fracture. Through multi response optimization of the welding parameters and tool geometry, weld strength of 89% that of the base material was achieved.     

Effect of welding speed on microstructural and mechanical properties of friction stir welded Inconel 600

In order to evaluate the properties of a friction stir welded Ni base alloy, Inconel 600 (single phase type) was selected. Sound friction stir welds without weld defect were obtained at 150 and 200 mm/min in welding speed, however, a groove like defect occurred at 250 mm/min. The electron back scattered diffraction (EBSD) method was used to analyze the grain boundary character distribution. As a result, dynamic recrystallization was observed at all conditions, and the grain refinement was achieved in the stir zone, and it was gradually accelerated from 19 μm in average grain size of the base material to 3.4 μm in the stir zone with increasing the welding speed. It also has an effect on the mechanical properties so that friction stir welded zone showed 20% higher microhardness and 10% higher tensile strength than those of base material.     

Shape optimization for tool wear in the friction-stir welding of cast AI359-20% SiC MMC

Tool wear in the friction-stir welding of AI359 + 20% SiC MMC produced a self-optimized shape which when achieved resulted in excellent welds and no additional tool wear. This optimized tool shape was slightly different at weld speeds of 6 and 9 mm/s. Tool wear rate was observed to decrease linearly and to effectively cease above about 11 mm/s weld speed. There was some attrition or comminution of larger SiC particles during welding but the weld zones were very homogeneous in terms of SiC distribution between the base metal and the friction-stir weld zone. There was no weld-related degradation and the weld zone hardness was 30% higher than the base.     

Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

An assessment of microstructure,hardness, tensile and impact strength of friction stir welded ferritic stainless steel joints

Microstructure and mechanical characterization of friction stir welded 409M ferritic stainless steel joint were carried out. Single pass welds free of volumetric defects were produced at a welding speed of 50 mm/min and rotation speed of 1000 rpm. Optical microscopy, microhardness testing, transverse tensile, impact and bend tests were performed. The coarse ferrite grains in the base material are changed to very fine grains consisting duplex structure of ferrite and martensite due to the rapid cooling rate and high strain induced by severe plastic deformation caused by frictional stirring. Tensile testing indicates overmatching of the weld metal relative to the base metal. The joints are also exhibited acceptable ductility and impact toughness.     

Role of nonlinear fracture mechanics in assessing fracture and crack growth in welds

An experimental study was conducted to assess the structural performance of repair welds in an ex-service 1Cr-1Mo-0.25V steam turbine casing material. Material from two weld techniques, one involving a post-weld heat-treatment that produced undermatched welds and the other involving a temper bead welding technique that produced overmatched welds were tested. Both welding techniques were implemented in two base metal conditions giving rise to four different welds and two different base metal conditions. The tests conducted included tensile tests, creep tests, fracture toughness tests, fatigue crack growth tests, creep crack growth tests, and creep-fatigue crack growth tests on the base metal, weld metal and the weldment region.The yield strength of the weld metal in the undermatched condition was approximately 10% lower than the base metal, while the weld metal in the overmatched condition had a yield strength that was 30% higher than the base metal at 565 °C. The creep deformation rates in the undermatched welds were 60 times faster than the base metal at a stress of 207 MPa. In the overmatched welds, the creep rates at 207 MPa were about 2.8 times faster in one case and 2.8 times slower in the other.The crack path in fracture toughness specimens followed the interface between the transition layer and the weaker of the weld metal and the base metal. The J-resistance curves for the weldments at 565 °C showed significant variability among duplicate samples from the same welds. This scatter was caused by the variability in the location of the precrack with respect to the fusion line and the location of the low fracture toughness region in the weldment. This behavior was explained using a novel approach for characterizing the fracture of welds. The creep-fatigue crack growth rates at equivalent (C_t)_avg values in undermatched welds was higher than the crack growth rates in the overmatched weld samples. In all cases under creep-fatigue, the crack appeared to grow in the weaker of the base metal and the weld metal. Recommendations for future work are provided to enhance the theoretical underpinnings of the nonlinear fracture mechanics frame-work to rigorously address fracture and crack growth in welds.     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

Welding of Al-359/20%SiC<Subscript>p</Subscript> metal matrix composites by the novel MIG process with indirect electric arc (IEA)

An Al-based composite reinforced with 20%SiC_p was welded using the MIG welding process with direct and indirect application of the electric arc (DEA and IEA respectively). The welds were made on 12.5 mm thick plates in three welding passes for the DEA joint whereas only one pass was required for the IEA joint. Microstructural examination of the joints revealed DEA welds with light signs of matrix/reinforcement reaction whilst in the IEA welds the SiC particles remained with their initial angular morphology. Mechanical failure occurred consistently and independently of the type of joint in the weld zone and the measured strengths were 209 and 234 MPa for DEA and IEA welds respectively. The greater strength measured for the IEA weld was due to reduced porosity and good incorporation and dispersion of the SiC particles into the weld pool.     

Impact performance of friction welded butt joints between 6061-T6 aluminium alloy and type 304 stainless steel

Abstract

The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al alloy and type 304 stainless steel at high rates of loading were determined using the split Hopkinson bar. Cylindrical tensile specimens machined from as welded butt joints of 13 mm in diameter were used in both static and impact tests. Friction welding was conducted using a brake type friction welding machine under two different welding conditions. The effects of welding conditions and loading rate on the joint tensile properties were examined. Results show that the joint tensile properties were greatly affected by the welding parameters, and were slightly enhanced with increased loading rate. Scanning electron microscope observations revealed that the tensile fracture modes in the butt joint specimens varied with loading rate and depend on welding conditions. Microhardness profiles across the weld interface were measured to investigate the extent of the heat affected zone. The slight enhancement of the joint tensile properties with increasing loading rate is primarily attributed to the strain rate dependence of the thermally softened 6061-T6 Al alloy base material.     

Thermal cycles and their effects during friction stir welding of AA7075 thicker plates with and without in-process cooling

Friction stir welding of AA 7075 plates in three different thicknesses such as 10, 16 and 25 mm at natural convection condition was carried out successfully without defects. Water cooled friction stir welds were also produced on 16 mm thick plates. The thermal cycles at different locations of the plate, during the friction stir welding process, were predicted using a three-dimensional thermal model. Mechanical properties of the welds were evaluated using tensile and hardness tests. Weld microstructures were also examined with optical and transmission electron microscopes. The weld hardness values and tensile properties were found to decrease with increase in plate thickness. The use of water cooling was found to improve the weld properties to some extent, although not to the level of base metal. The reasons for this behavior are discussed, correlating thermal cycles, mechanical properties, fracture locations and precipitate morphology.     

Study of friction welding characteristics of Al/SiC composite and application of grey-based TOPSIS

The present work investigates the possibility of producing friction welded joints with an advanced material like Al/SiC (aluminum–silicon carbide) composite. The study also discloses the multi response optimization in the process of continuous drive friction welding using a hybrid algorithm of grey-based TOPSIS (technique for order of performance by similarity to ideal solution). The friction welding parameters (frictional pressure, upset pressure, burn off length and rotational speed) were optimized considering the multiple performance characteristics such as proof stress, tensile strength, and microhardness. Taguchi’s L₂₇ orthogonal array was used for conducting the welding trials. The confirmation test was conducted at the optimal setting, to sort out the effectiveness of the proposed hybrid algorithm. The macro photographs of the joints and optical micrographs of the weld zone were studied. The scanning electron microscope images of the fractured surface were also examined to identify the failure mode of joints. The significant improvements in the performance characteristics prove the effectiveness of the grey-based TOPSIS method in experimental welding optimization.     

Ductile fracture mechanisms in shielded metal-arc and gas tungsten-arc welds of Type 347 stainless steels

The ductile fracture behavior of two different welds of Type 347 stainless steel, which are made by SMAW (shielded metal arc welding) and GTAW (gas tungsten arc welding) processes was characterized by J-integral testing and microstructural evaluation techniques. Both welds by SMAW and GTAW processes showed significantly low fracture toughness compared with that of the base metal. Metallographic and fractographic examinations revealed that different micromechanisms are operative in the fracture process of the two welds. In the SMAW weld, the fracture was dominated by void initiation and growth at the inclusions that are homogeneously distributed in the matrix. On the other hand, in the GTAW weld, a large number of Nb(CN) particles precipitated on the austenite/ferrite interface as long rod shapes and the fracture proceeded by void initiation at these particles and accompanying decohesion of the interface. It is recommended that the C and Nb contents be reduced in weld metal itself as well and that the welding atmosphere be controlled.     

The investigation of abnormal particle-coarsening phenomena in friction stir repair weld of 2219-T6 aluminum alloy

The single-pass friction stir weld of aluminum 2219-T6 with weld-defects was repaired by overlapping friction stir welding technique. However, without any post weld heat treatment process, it was found that the phenomena of abnormal particle-coarsening of Al₂Cu had occurred in the overlapping friction stir repair welds. The detecting results of non-destructive X-ray inspection proved that not only one group of repair FSW process parameters could lead to occurrence of the abnormal phenomena. And the abnormally coarsened particles always appeared on the advancing side of repair welds rather than the retreating side where the fracture behaviors occurred after mechanical tensile testing. The size of the biggest particle lying in the dark bands of ‘Onion-rings’ was more than 150 μm. After the related investigation by scanning electron microscope and X-ray energy spectrometer, three types of formation mechanisms were proposed for reasonably explaining the abnormal phenomenon: Aggregation Mechanism, Diffusion Mechanisms I and II. Aggregation Mechanism was according to the motion-laws of stir-pin. Diffusion Mechanisms were based on the classical theories of precipitate growth in metallic systems. The combined action of the three detailed mechanisms contributed to the abnormal coarsening behavior of Al₂Cu particles in the friction stir repair weld.     

6009铝合金光纤激光-MIG电弧复合焊和光纤激光焊工艺对比研究

采用光纤激光-MIG 复合焊和光纤激光焊分别对6009铝合金进行焊接,研究两种焊接方式下焊接接头的成型性、显微组织、拉伸性能、显微硬度、断口形貌的不同。研究表明：激光电弧复合焊的焊接速度是激光焊接的3倍;相比于激光焊,激光电弧复合焊焊缝中心显微组织更加细小;接头的抗拉强度达到母材的63%以上,而激光焊接的抗拉强度仅仅只有母材的38%;显微硬度试验表明：复合焊存在软化区,而激光焊接几乎没有软化区;断口分析表明：复合焊和激光焊的拉伸断口都是典型的韧窝状态,但是复合焊接的韧窝更加均匀。     

304不锈钢局部干法水下激光填丝焊接工艺及焊缝性能研究

目的 采用自主研制的水下激光填丝焊接装备,在304奥氏体不锈钢板材表面进行U形坡口激光填丝焊接试验,为304不锈钢水下修复工作提供技术参考。方法 在功率为5 600 W、焊接速度为6 mm/s、送丝速度为205 cm/min、保护气体流量为15 L/min、排水气体流量为30 L/min的条件下进行焊接试验,并对空气和水下环境下的焊缝进行对比检测分析。通过光学显微镜分析2种环境下焊缝的显微组织;对2种焊缝进行拉伸、弯曲等力学性能测试;采用显微硬度计测试1 kg载荷下不同区域的显微硬度;使用VersaSTAT3F电化学工作站测定在3.5%（质量分数）的NaCl溶液中2种焊缝的开路电位和极化曲线。结果 2种环境下的焊缝均无明显裂纹、气孔等缺陷;显微组织主要由奥氏体和铁素体组成,但2种环境下焊缝的奥氏体晶粒大小和铁素体形状均略有差别,焊缝拉伸断口均为典型的韧性断裂形貌且抗拉强度符合304不锈钢标准。2种环境下焊缝的微观组织和晶粒大小不同,水下焊缝硬度高于空气的。通过分析2种环境下焊缝的开路电位和极化曲线,可知水下焊缝的耐腐蚀性略高。结论 所开发的局部干法水下激光填丝焊接工艺可以满足实际工程中...     

Microstructural characteristics and mechanical properties of friction stir welded joints of Ti–6Al–4V titanium alloy

The α + β titanium alloy, Ti–6Al–4V, was friction stir welded at a constant tool rotation speed of 400 rpm. Defect-free welds were successfully obtained with welding speeds ranging from 25 to 100 mm/min. The base material was mill annealed with an initial microstructure composed of elongated primary α and transformed β. A bimodal microstructure was developed in the stir zone during friction stir welding, while microstructure in the heat affected zone was almost not changed compared with that in the base material. An increase in welding speed increased the size of primary α in the stir zone. The weld exhibited lower hardness than the base material and the lowest hardness was found in the stir zone. Results of transverse tensile test indicated that all the joints had lower strength and elongation than the base material, and all the joints were fractured in the stir zone.     

Microstructures and wear property of friction stir welded AZ91 Mg/SiC particle reinforced composite

The microstructures and wear property of friction stir welded AZ91 Mg alloy/SiC particle reinforced composite (AZ91/SiC/10p) were investigated. The initial microstructures of the AZ91/SiC/10p were composed of irregularly distributed β-phases (Al₁₂Mg₁₇) and agglomerated SiC particles, while the friction stir weld zone was characterized by the homogeneous distribution of SiC particles, the recrystallized grain structure and the dissolution of β-phase. Thank to the microstructural modification, an improvement in the hardness and wear property of the weld zone were observed as compared to those of the base metal. The hardness near the weld zone was a higher and more homogeneously distributed and the wear resistance within the weld zone, as evaluated by the specific wear loss, was superior, as compared with the base metal.     

Mikrostrukturelle und mechanische Eigenschaften von laserstrahlgeschweißtem Magnesiumfeinblech AZ31‐HP mit und ohne Zusatzwerkstoffe

Microstructural and mechanical properties of laser welded sheets of magnesium AZ31‐HP with and without filler wires This paper describes Nd:YAG laser beam welding experiments carried out on rolled 2.5 mm thick magnesium sheet AZ31‐HP. For the butt welds in flat position, filler wires AZ31X and AZ61A‐F were used, diameter 1.2 mm. The microstructure and mechanical properties of the different laser beam welded joints were examined and compared with one another. The obtained results show that the laser beam welding of AZ31‐HP sheet is possible without hot crack formation, both without and with filler wires. The determined tensile strength, ductility, fracture toughness and microhardness of laser beam welded joints without filler wire were not effected by AZ31X nor AZ61A‐F. By use of these filler wires loss of zinc was minimized and the shape of weldments was optimized. The values of fracture strength, yield strength and microhardness of the joints and base material are quite similar. It is found that the ductility of the joints is lower than the base materials due to the heterogeneous microstructure of the fusion zones and geometrical notches of the weld seams. Both, weld and base material of AZ31‐HP, showed stable crack propagation. Furthermore, for base material slightly lower fracture toughness values CTOD than for the joints were determined.     

A comparative study of the MIG welding of AI/TiC composites using direct and indirect electric arc processes

Metal inert gas welding of Al-1010/TiC/50p composites was carried out on 9 mm thick square bars by applying the electric arc directly and indirectly. Three pre-heating temperatures were used, 50, 100 and 150°C but only direct electric arc (DEA) was applied at room temperature. Welds were microstructurally examined and tested under tensile load. Complete penetration was achieved using both DEA and IEA methods. Uniform welds were obtained using indirect electric arc (IEA), meanwhile broadening was observed in the upper part in DEA welds facilitated by mixture of the base material with the filler. Microstructural observations showed good lateral fusion of the parent composites, little or no dissolution of TiC by IEA and only slight dissolution by DEA, which led to TiAl _x formation during solidification. The presence of Al₄C₃ was not detected. Microhardness weld profiles revealed that the use of IEA reduces the heat affected zone (HAZ). Mechanical failure of the samples was consistently in the weld zone. Mechanical strength in IEA welds (182–186 MPa) was consistent irrespective of the pre-heating conditions and dependant only of the consumable (Al-2024). The mechanical strength of DEA welds was affected to some extent by the incorporation of the reinforcing particles into the weld region and wettability aspects inherent to the welding conditions. The use of IEA seems to be a suitable route for joining Al-based composites even when the reinforcement content is high.