Fatigue behavior of dissimilar friction stir welds between cast and wrought aluminum alloys

Cast aluminum alloy, AC4CH-T6, and wrought aluminum alloy, A6061-T6, were joined by means of friction stir welding (FSW) technique. The effect of microstructure and post heat treatment on fatigue behavior of the dissimilar joints was investigated. Near the weld centre, Vickers hardness was lower than in the parent metals and the hardness minima were observed along the trace route of FSW tool’s shoulder edge. Tensile fracture took place on A6061 side where the hardness was minimal, resulting in the lower static strength of the dissimilar joints than AC4CH or A6061. Fatigue fracture occurred on AC4CH side due to casting defects and the fatigue strength of the dissimilar joints was similar to that of AC4CH, but lower than that of A6061. Friction stir process (FSP) and post heat treatment successfully improved the fatigue strength of the dissimilar joints up to that of the parent metal, A6061. __________ Translated from Problemy Prochnosti, No. 1, pp. 150–154, January–February, 2008.     

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.     

焊接参数对搅拌摩擦焊接质量的影响

采用基于固体力学的有限元方法建立了搅拌摩擦焊接过程的三维数值模型,研究了在焊接参数不同的情况下搅拌摩擦焊接过程中力学特征的变化．数值模拟结果和试验结果都表明,等效塑性应变能近似地反映焊接构件焊缝区域材料显微结构的演化,较高的搅拌头转速和较低的焊速有利于提高焊缝的质量．焊接构件特定的等效塑性应变等值线可以较好的对应不同焊接区域的边界．随着搅拌头转速的提高,等效塑性应变随之增大,但搅拌探针与焊接构件交界面上的接触压力随之减小．等效塑性应变随着搅拌头平移速度的增大而减小．     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

Fatigue of friction stir welds in aluminium alloys that contain root flaws

Although the vast majority of friction stir welds will be free of flaws, it is not always possible to assume that they are. The properties of welds with flaws are needed to enhance confidence in the design and application of friction stir welded joints. The monotonic strength and fatigue behaviour of single-sided butt welds in 6–7 mm thick AA5083-O, AA5083-H321 and AA6082-T6, both without and with root flaws, was investigated.

Examination of the root flaw faces showed that there was bonding between the flanks of the flaws but the bonding was of poor quality and incomplete. This meant that the strength and ductility of the flaws were lower than the surrounding material. However, the comparison of the mechanical test results suggests that root flaws up to a certain size are tolerable without a significant loss in performance when compared to nominally flaw-free welds. These data also suggest that even friction stir welds with root flaws exceed the design life for equivalent fusion welds set out in the draft Eurocode 9 and that a higher rating may be warranted. Limited test results produced for this work need to be supplemented with a wider range of tests.     

Improvement of refill friction stir spot welds by auxiliary material addition

An innovative technology to improve the bonding of refill friction stir spot welding by auxiliary material addition was proposed. The annular groove was eliminated and the energy absorption was increased. Weld bonding was enhanced because the improved technology affected the hook, retractable line and stir zone (SZ) shape significantly: (1) the effective bearing thickness of the welds increased as the hook penetration through the sheet thickness was reduced significantly, (2) the retractable line was eliminated as the dynamic recrystallisation was enhanced along the path of the sleeve moving and (3) fracture propagation was impeded by the bending interface between the SZ and the base materials.     

搅拌摩擦与氩弧焊铝合金接头疲劳性能的比较

建立了铝合金焊接接头的S-N曲线,对比分析了搅拌摩擦和氩弧焊两种工艺对其焊接接头疲劳性能的影响,结果表明:在载荷相同的条件下,铝合金搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头,搅拌摩擦焊接头的疲劳寿命N=106次的疲劳强度为59-65 MPa,搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能,铝合金焊接接头的缺陷是主要的疲劳裂纹源.     

镁合金搅拌摩擦焊接工艺参数优化

为了优化镁合金搅拌摩擦焊接工艺参数,对5 mm厚镁合金AZ31B板材的搅拌摩擦焊接技术进行了试验研究,利用SN比实验设计,对镁合金AZ31B搅拌摩擦焊接工艺参数进行了方差分析,优化了搅拌头的材料、结构,最终确定搅拌头的材料为W6Mo5Cr4V2,结构为凹面圆台形.轴肩尺寸为12 mm.探针的根部直径为5.5 mm,端部直径为2.5 mm,长度为4.7 mm.获得镁合金AZ31B搅拌摩擦焊的工艺参数显著性顺序为旋转速度、横向速度和压力;确定了镁合金AZ31B搅拌摩擦焊的最优工艺参数为1500 r/min、47.5 mm/min、3kN.     

Interface strengthening in dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys by adjustable probes

Dissimilar welding of AZ31/ZK60 magnesium alloys with a thickness of 2 mm was successfully carried out by the double-sided friction stir spot welding with adjustable probes.A dissimilar joint bearing flat surfaces on both sides without a keyhole was obtained and the shear failure load of 8.7±0.5 kN was reached.The role of the adjustable probes has been revealed in detail.In the center of the stir zone,the welding interface structure was heterogeneous around which some distinct oxides still remained,leading to a weak interface strength.On the contrary,the welding interface structure around the shoul-der/probe interface was homogeneous with no oxides giving rise to a strong interface strength,which is attributed to the severe material flow introduced by the adjustable probes.In addition,the vicinity outside the shoulder/probe interface,where the fracture occurred during the shear tensile tests,was also strengthened owing to the shearing and torsion by the adjustable probes.Therefore,a stable plug failure can be obtained and the joint properties can be improved.     

Effect of weld orientation on static strength and failure mode of friction stir stitch welds in lap‐shear specimens of aluminium 6022‐T4 sheets

Stitch friction stir spot welding (FSSW) is performed on 6022‐T4 Al alloy using a concave shoulder tool with cylindrical pin. Stitch FSSW is an extension of the conventional spot FSW process where an elongated (oval) spot is produced instead of a circular spot. The main advantage of this process is that it gives appreciably higher strength than conventional spot FSW due to an increase in the joint area. In this research, an experimental and numerical approach is taken to understand the failure mechanism of stitch welds made in lap‐shear configuration. There are four ways (orientations) in which specimens can be welded to produce a lap‐shear specimen – two in transverse direction and two in longitudinal direction. The static strength of welds made with these orientations was found to be different. For stitch welds made in the longitudinal orientation, the failure always occurred near the keyhole at the tool retract position. For welds made in the transverse orientation, failure always occurred in the region of the highest stress. This difference in the weld strength can be attributed to the hook geometry and interface bond strength. The results are explained using a kinked cracked model approach and calculation of stress intensity factor at the hook geometry.     

Material flow during friction stir spot welding of dissimilar Al2024/Al materials

In the present paper, the material flow and intermixing during friction stir spot welding of dissimilar Al2024/Al materials were investigated. The dissimilar materials had quite different strength. The microstructural evolutions taking place during a series of lap and butt welds were observed. The effect of penetration depths, dwell time, rotational speed and tool geometry were systematically investigated. The material flow and formation of the intermixed region were explained by a modified model.     

Effect of friction stir welding parameters on morphology and strength of acrylonitrile butadiene styrene plate welds

The aim of this study is to examine the effect of main friction stir welding (FSW) parameters on the quality of acrylonitrile butadiene styrene (ABS) plate welds. Welds were carried out in a FSW machine, using a tool with a stationary shoulder and no external heating system. The welding parameters studied were the tool rotational speed which varied between 1000 and 1500 (rpm); the traverse speed which varied between 50 and 200 (mm/min), and the axial force ranging from 0.75 to 4 (kN). The major novelty is to study the influence of the parameter axial force on FSW of polymers. Produced welds have always a tensile strength below the base material, reaching the maximum efficiencies of above 60 (%) for welds made with higher rotational speed and axial force. Good quality welds are achieved without using external heating, when the tool rotational speed and axial force are above a certain threshold. Above that threshold the formation of cavities and porosity in the retreating side of the stir zone is avoided and the weld region is very uniform and smooth. For low rotational speed and axial force welds have poor material mixing at the retreating side and voids at the nugget. For this reason the strain at break of these welded plates is low when compared with that of base material.     

Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel

Thin sheets of aluminum alloy 6061-T6 and one type of Advanced high strength steel, transformation induced plasticity (TRIP) steel have been successfully butt joined using friction stir welding (FSW) technique. The maximum ultimate tensile strength can reach 85% of the base aluminum alloy. Intermetallic compound (IMC) layer of FeAl or Fe₃Al with thickness of less than 1 μm was formed at the Al–Fe interface in the advancing side, which can actually contribute to the joint strength. Tensile tests and scanning electron microscopy (SEM) results indicate that the weld nugget can be considered as aluminum matrix composite, which is enhanced by dispersed sheared-off steel fragments encompassed by a thin intermetallic layer or simply intermetallic particles. Effects of process parameters on the joint microstructure evolution were analyzed based on mechanical welding force and temperature that have been measured during the welding process.     

Refill friction stir spot welding of 5083-O aluminum alloy

In this work,refill friction stir spot welding(RFSSW) was used to weld 2 mm-thick 5083-O alloy.The Box–Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property.Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects.Microhardness of the stir zone(SZ) was higher than that of the base material(BM) and the hardness decreased with increasing the heat input during welding.The optimum failure load of 7.72 k N was obtained when using rotating speed of 2300 rpm,plunge depth of 2.4 mm and refilling time of 3.5 s.Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.     

6082-T6回填式搅拌摩擦点焊热-流耦合分析

为了解释了回填式搅拌摩擦点焊的连接机理,本文根据6082-T6铝合金回填式搅拌摩擦点焊焊接过程的特点,建立了简化的热源模型,利用有限元分析软件ANSYS模拟出焊接过程中的温度场,进而耦合得到其应力场.结果表明:随着焊接过程的进行,铝合金6082-T6最高温度分布在袖筒1/2处,焊点处粘塑性金属的最大流动速度出现在铝合金上表面袖筒内侧区域;通过分析模拟过程中流体流动的流线与试验测量所得接头形貌照片,得到流场的分布规律.     

Effect of friction stir processing and hybrid reinforcements on copper

In this research, a copper based surface composite was fabricated through dispersing hybrid composite particles onto its surface through friction stir processing (FSP) technique. Optical micrographs and scanning electron microscopy images indicates finer refinement of grains and particles dispersion into matrix along with its bonding and particle separation. As per the outcomes of microhardness analysis, hardness of the developed surface composite shows increment with increase in dispersion of volume fraction of hybrid particles. Strength of the developed copper surface composite exhibited a positive trend with introduction of hybrid reinforcement particle onto the surface of the composite but yet again ductility reduced. Wear resistance of the composite increased with reinforcement addition and the same was supported through worn out surface morphology. Fluctuations in friction coefficient value reduced with increase in particles, as for the presence in BN particles while the average frictional coefficient value was observed increasing. A reduction in corrosion rate was observed with increase in reinforcement particle dispersion onto copper matrix through FSP.     

Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy

In this work, a third generation Al-Li alloy has been successfully spot welded with probeless friction stir spot welding (P-FSSW), which is a variant of conventional friction stir welding. The Box-Behnken experimental design in response surface methodology (RSM) was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83 kN was obtained under a dwell time of 7.2 s, rotation speed of 950 rpm and plunge rate of 30 mm/min. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction (EBSD) and differential scanning calorimetry (DSC) were used for microstructure evolution and property analysis. The precipitation of GP zone and Al₃Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.     

Dissimilar friction stir welding between magnesium and aluminum alloys

Dissimilar friction stir welding between magnesium and aluminum alloy plates with thicknesses of 2 mm was performed. The tool for welding was rotated at speeds ranging from 800 to 1600 rpm under a constant traverse speed of 300 mm/min. For tool rotation speeds of 1000, 1200, and 1400 rpm, defect-free welds were successfully obtained and the surface morphology of the welds became smoother as the tool rotation speed was increased. The relatively simple bonded interface was clearly evident and had a zigzag pattern. A mixed microstructure of magnesium and aluminum alloys was formed near the bonded interface. The maximum tensile strength of about 132 MPa was obtained at the tool rotation speed of 1000 rpm. However, there were not noteworthy changes in the tensile strength as a function of the tool rotation speed. The elongation was 2% or less, regardless of the tool rotation speed.     

Barium titanate reinforced nickel aluminium bronze surface composite by friction stir processing

Friction stir processing (FSP) is a surface modification technique, which can be used for the fabrication of surface composites. In the present work, a surface composite was prepared by introducing a piezoelectric ceramic powder (PZT; BaTiO₃) to a nickel aluminium bronze (NAB) metal matrix using FSP. BaTiO₃ powder was placed in holes drilled at the centre of a NAB plate and FSP was carried out. Microstructural characterisation of the surface composite was carried out using optical microscopy and scanning electron microscopy. The microhardness and tensile behaviour of the surface composite were investigated, together with the cavitation erosion behaviour. The results are discussed in light of the microstructural modification.     

Morphology and strength of acrylonitrile butadiene styrene welds performed by robotic friction stir welding

The aim of this study is to examine the main factors affecting friction stir welding (FSW) of acrylonitrile butadiene styrene (ABS) plates, performed by a robotic system developed to this purpose. Welds were carried out using a tool with stationary shoulder and an external heating system. The welding parameters studied were the axial force, rotational and traverse speeds and temperature of the tool. The major novelty is to perform FSW of a polymer in a robotic system and to study the influence of the axial force on weld quality. In a robotic solution the control of axial force allows to eliminate robot positional errors and guarantee the contact between the FSW tool and the work pieces. Strength and strain properties of the welds are evaluated and correlated with the morphology of the welded zone. A comparison between welds produced in the robotic FSW system and in a dedicated FSW machine is presented. It is shown the feasibility of robotic FSW of ABS without deteriorating the mechanical properties of the welds in relation to those produced in the dedicated FSW machine.