Mechanical characterization of friction stir spot microwelds

Microstructures and failure mechanisms of friction stir spot microwelds in 300-μm thin sheet of aluminum 1050 alloy were investigated. As an alternative to conventional soldering and welding in joining thin metals for electronic, medical and microdevices, friction stir welding may be utilized in order to limit the excessive heat damage. Transmission electron microscopy micrographs of the cross sections of friction stir spot microwelds in lap-shear specimens were examined. These microwelds showed the failure mode of nugget pullout under lap-shear loading conditions. The experimental observations suggested that under lap-shear loading conditions, the failure was initiated near the possible original notch tip in the stir zone and the failure propagated along the circumference of the nugget to final fracture. Microindentation hardness data of base metal, heat affected zone, thermal-mechanical affected zone and stir zone were obtained. The interface between the heat affected zone and the thermal-mechanical affected zone was the softest region, where the cracks of friction stir spot microwelds in the lap-shear specimens under the loadings initiated and leaded to fracture of the specimens.     

Effect of corrosive environment on fatigue property and crack propagation behavior of Al 2024 friction stir weld

In this investigation, 2024 aluminium alloy plates were friction stir welded, a sequence of experiments was performed including fatigue and crack propagation tests in air, under pre-corrosion and in a 3.5% NaCl solution, in combination with fractography analyses of near-threshold region, Paris region and finial fracture region with the aid of scanning electron microscopy (SEM). Results showed that the corrosive environment caused a dramatical decrease in fatigue lives of FS welds, the corrosion fatigue lives of FS welds were almost a half of those of the as-welded specimens. The crack growth rates in FS welds were higher than their counterparts in base materials, under the corrosive environment, the crack growth rate differences between base materials and FS welds become increasingly apparent with the increase of stress intensity factor range ΔK, but the pre-corrosion process had little effect on the FS welds' crack propagation behavior except for shortening the crack initiation lives greatly.     

Influence of microarc oxidation and hard anodizing on plain fatigue and fretting fatigue behaviour of Al-Mg-Si alloy

The present study compares the performance of microarc oxidation (MAO) and hard anodizing (HA) treated Al-Mg-Si alloy (AA6063) test samples under cyclic loading in uniaxial tension with a stress ratio of 0.1 (plain fatigue) and fretting fatigue loading. Fatigue test specimens were treated using MAO and HA techniques. MAO coated specimens were ground to reduce the surface roughness comparable with that in HA coated specimens. In that process the porous outer layer was removed. Characterization of coated and uncoated specimens was done with reference to the coating morphology, microhardness, surface roughness and residual stress. The specimens were tested under plain fatigue and fretting fatigue loading at ambient temperature. While the ground MAO coating exhibited relatively less amount of porosity, HA coating had through thickness cracks. MAO coating had compressive residual stress and it was very hard compared with HA coating. Both types of coated samples exhibited slightly higher friction force than that experienced by the uncoated specimens. Fretted region of the HA coated samples was rougher than that of the MAO coated specimens. Plain fatigue lives of both coated samples were inferior to those of the uncoated specimens. The inferior plain fatigue lives of MAO coated specimens compared with those of the substrate may be attributed to the tensile residual stresses supposedly present in the substrate leading to an early crack initiation in the substrate adjacent to the coating. As friction force of MAO coated samples was higher than that experienced by uncoated specimens, the fretting fatigue lives of MAO coated samples were slightly inferior to those of uncoated samples. As the anodized layer had preexisting through thickness cracks and strong adhesion with the substrate, cracks propagated from HA coating through the interface into the substrate easily. This may be the reason for the HA coated samples exhibiting inferior plain fatigue and fretting fatigue lives compared with MAO coated and uncoated samples.     

Effects of processing time on strengths and failure modes of dissimilar spot friction welds between aluminum 5754-O and 7075-T6 sheets

The effects of the processing time on the strengths and failure modes of two types of dissimilar spot friction welds between aluminum 5754-O and 7075-T6 sheets are investigated by experiments. Dissimilar 5754/7075 and 7075/5754 spot friction welds made at different processing conditions were tested under lap-shear loading conditions. The experimental results indicate that the failure loads of both types of welds in lap-shear specimens increase when the processing time increases for the given ranges of the processing time. The optimal processing times to maximize the failure loads of the 5754/7075 and 7075/5754 welds under lap-shear loading conditions are identified. The maximum failure load of the 7075/5754 welds is about 40% larger than that of the 5754/7075 welds. Selected optical and scanning electron micrographs of both types of welds made at different processing times before and after failure are examined. The micrographs show different weld geometries and different failure modes of the welds made at different processing times. The failure modes of the dissimilar 5754/7075 and 7075/5754 spot friction welds are quite complex and appear to strongly depend on the geometry and strength of the interfacial surface between the two deformed sheet materials.     

双相钢搭接点焊接头疲劳寿命分析

研究了双相钢焊点特征,对不同匹配双相钢搭接焊点进行了疲劳试验,获得了焊点的载荷寿命曲线.研究了双相钢焊点的疲劳裂纹扩展及失效形式,分析和解释了疲劳过程中的现象,并根据裂纹的实际扩展路径,提出了局部等效张开应力强度因子keq,从断裂力学的角度对双相钢焊点的疲劳失效进行了分析.结果表明,keq能够有效地关联具有不同厚度,不同熔核直径的搭接焊点试样的疲劳寿命,是反映双相钢焊点疲劳强度的有效参量,能够用来预测焊点疲劳寿命.     

十字接头恒幅和变载荷历程疲劳试验结果及预测

对十字接头分别进行恒辐轴向载荷和变载荷历程疲劳试验,接头疲劳裂纹通常开始发生在焊缝根部,但如果接头变形引起的弯曲应力较大,疲劳裂纹也可能发生在焊缝趾部。本研究试图找出影响焊接接头疲劳寿命各参数之间的相互关系,并利用作者引伸的焊接接头疲劳裂纹起始－扩展模型较精确地预测复杂焊件的疲劳寿命。该模型把疲劳裂纹起始寿命看成是疲劳裂纹萌生,早期生成并聚合成主疲劳裂纹的循环次数,利用应变控制疲劳数据和Ｐａｌｍｇ     

Static shear strength and fatigue life of refill friction stir spot welded AA 6061-T6 sheets

This study was aimed at evaluating the static shear strength and fatigue properties of the newly developed refilled friction stir spot welded AA 6061-T6 joints. The keyhole, the process disadvantage of conventional friction stir spot welding, was refilled successfully, using an additional filler plate, with specially designed tools. Two different tool profiles, namely, convex and concave, were used for the refilling process. Sound and defect free joints were obtained by the refilling process. Joints refilled with convex tools showed better static shear strength than those with the concave ones. The variation of microhardness in different regions of the weld was analysed. Fatigue tests were conducted on the lap shear specimens at a stress ratio of R?=?0·1. The optical micrographs of the welds after fatigue failure in both the conventional and refilled processes were examined to study the fatigue crack propagation and failure modes.     

Computer-aided experiment on arc welding

Fatigue properties of cast aluminium welded joints by friction stir welding (FSW) and MIG welding were investigated, comparing with that of the base plate. Fatigue crack propagation tests for the da/dN ? ΔK relation and bending fatigue tests for the S–N relation were carried out. Fatigue cracks in both FSW and MIG specimens were accelerated, when the fatigue crack tip reached the stir zone or the weld metal. This behaviour was discussed based on the crack closure induced by the crack surface roughness and the residual stress. In the S–N properties, the influence of specimen surface finishing on fatigue life was also examined. Fatigue lives of the FSW and MIG specimens in the ‘as weld’ condition were in the range of the largely scattered base plate fatigue lives, in spite of the different fatigue crack initiation sites in each specimen such as the porosity in the base plate, the tool mark bottom in the FSW and the weld toe in the MIG. The FSW specimens with the polished surface showed the particular improvement in fatigue strength for finite fatigue life.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

Comparative study on fatigue properties of friction stir and MIG-pulse welded joints in 5083 Al-Mg alloy

The objective of this investigation was to compare the fatigue properties of friction stir welds with those of MIG-pulse welds. The 5083 Al-Mg alloy was welded by single pass friction stir welding（FSW） and double-sided MIG-pulse welding. The results show that friction stir（FS） welds have a better appearance than MIG-pulse welds for the lack of voids, cracks and distortions. Compared with the parent plate, FSW welds exhibit similar fine grains, while MIG-pulse welds display a different cast microstructure due to the high heat input and the addition of welding wire. The S-N curves of FSW and MIG-pulse joints show that the fatigue life of FS welds is 18 - 26 times longer than that of MIG-pulse welds under the stress ratio of 0.1 and the calculated fatigue characteristic values of each weld increase from 38.67 MPa for MIG-pulse welds to 53.59 MPa for FSW welds.     

Crack initiation and kinking behaviours of spot welded coach peel specimens under cyclic loading

Abstract

The present study is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the interface plane between the overlapped sheets. Stage II corresponds to kinked crack initiation and propagation through the sheet thickness observed after applying a certain number of loading cycles. Stage III corresponds to crack propagation through the width of the specimens observed at the final step of the fatigue crack propagation. The FE results of the crack paths and crack locations are in good agreement with those of experimental observations.     

Investigation of Fatigue Crack Propagation in Spot-Welded Joints Based on Fracture Mechanics Approach

In this paper, fatigue crack propagation life of resistance spot welds in tensile-shear specimens is investigated based on the calculation of stress intensity factors and J-integral using three-dimensional finite element method. For comparison, experimental works on 5083-O aluminum alloy spot-welded joints have been carried out to verify the numerical predictions of fatigue crack propagation of welded joints. A lot of analyses have been performed to obtain stress intensity factors and J-integral in tensile-shear specimens of spot-welded joints by using commercial software ANSYS. These gathered data have been formulated by using statistical software SPSS. The results of fatigue propagation life and predicted fatigue crack path revealed very good agreement with the experimental fatigue test data and photograph of cross-section of the fatigued spot-weld specimens.     

Effect of Compressive Mode I on the Mixed Mode I/II Fatigue Crack Growth Rate of 42CrMo4

Subsurface cracks in mechanical contact loading components are subjected to mixed mode I/II, so it is necessary to evaluate the fatigue behavior of materials under mixed mode loading. For this purpose, fatigue crack propagation tests are performed with compact tension shear specimens for several stress intensity factor (SIF) ratios of mode I and mode II. The effect of compressive mode I loading on mixed mode I/II crack growth rate and fracture surface is investigated. Tests are carried out for the pure mode I, pure mode II, and two different mixed mode loading angles. On the basis of the experimental results, mixed mode crack growth rate parameters are proposed according to Tanaka and Richard with Paris’ law. Results show neither Richard’s nor Tanaka’s equivalent SIFs are very useful because these SIFs depend strongly on the loading angle, but Richard’s equivalent SIF formula is more suitable than Tanaka’s formula. The compressive mode I causes the crack closure, and the friction force between the crack surfaces resists against the crack growth. In compressive loading with 45° angle, da/dN increases as K _eq decreases.     

Microstructure and fatigue properties of friction stir welded high-strength steel plates

The weldability of high-strength steels (HSSs) is limited by their loss of strength, toughness and fatigue properties. In demanding applications, the fatigue properties of welds are among the strictest requirements. This paper presents a weldability analysis focusing on the microstructure and fatigue properties of 6?mm thick S690 and S355 HSS plates joined by friction stir welding. Their fatigue properties are compared to design recommendations and to fatigue properties obtained with other welding techniques. Results show that the high-quality friction stir welded steel joints outperform high-quality arc welds and FAT80 design recommendations. The fatigue strength of the friction stir welded joints is increased with material yield strength. The mechanisms governing crack initiation at different maximum stress levels are discussed.     

Effect of post-heat treatment on the fatigue behaviour of a friction stir spot-welded Al–Mg–Si alloy

In this paper, the effect of post heat treatment on fatigue behaviour of friction stir spot welded Al–Mg–Si aluminium alloy was investigated. The microstructure of the weld zone was classified into two regions: stir zone (SZ) and mixed zone (MZ), where fine equiaxed grains due to dynamic recrystallization were observed. Two kinds of post heat treatment, namely aging and T6 treatment, were applied to the as-welded joints. The grains in the SZ and MZ were extremely enlarged only by T6 treatment, but some fine grains still remained near the boundary of MZ. Fatigue tests were conducted using lap-shear specimens at a stress ratio R = 0.1. Post heat treatments exhibited little influence on fatigue strength, but fatigue fracture morphology was dependent on both load level and post heat treatment. At high applied loads, fatigue fracture took place through the MZ in the as-welded and aged joints, while along the boundary of MZ in the T6 treated joint. At low applied loads, the fatigue crack initiated at the edge of the nugget and then propagated through the upper sheet in the as-welded joint, but the lower sheet in the aged and T6 treated joints. The dependence of fracture morphology on post heat treatment was attributed to the change of microstructures and hardness distribution around the nugget by post heat treatment.     

Discrete dislocation plasticity modeling of short cracks in single crystals

The mode-I crack growth behavior of geometrically similar edge-cracked single crystal specimens of varying size subject to both monotonic and cyclic axial loading is analyzed using discrete dislocation dynamics. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The fracture properties are specified through an irreversible cohesive relation. Under monotonic loading conditions, with the applied stress below the yield strength of the uncracked specimen, the initiation of crack growth is found to be governed by the mode-I stress intensity factor, calculated from the applied stress, with the value of K_init decreasing slightly with crack size due to the reduction in shielding associated with dislocations near a free surface. Under cyclic loading, the fatigue threshold is ΔK-governed for sufficiently long cracks. Below a critical crack size the value of ΔK_I at the fatigue threshold is found to decrease substantially with crack size and progressive cyclic crack growth occurs even when K_max is less than that required for the initiation of crack crack growth in an elastic solid. The reduction in the fatigue threshold with crack size is associated with a progressive increase in internal stress under cyclic loading. However, for sufficiently small cracks, the dislocation structure generated is sparse and the internal stresses and plastic dissipation associated with this structure alone are not sufficient to drive fatigue crack growth.     

7050铝合金搅拌摩擦焊接头超高周疲劳性能

采用超高周疲劳试验系统研究7050-T7451铝合金搅拌摩擦焊接头的超高周疲劳性能. 试验结果表明,焊接接头在107周次以上仍然会发生疲劳失效,S-N曲线在108周次左右出现转折点,呈折线型下降;通过SEM对超高周疲劳断口形貌进行观察发现,当应力范围较高时,试件的疲劳裂纹往往在表面萌生,随着应力范围的降低,裂纹有亚表面和内部萌生的倾向;裂纹萌生位置取决于表面起裂和内部起裂相互竞争的结果;试件的断裂位置多为焊接接头的热力影响区和热影响区,EBSD和接头硬度的分析结果表明断裂位置与接头组织不均匀密切相关.     

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.     

Discrete dislocation modeling of fatigue crack propagation

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small-scale yielding are carried out using discrete dislocation dynamics. Cracks along a metal–rigid substrate interface and in a single crystal are studied. The formulation is the same as that used to analyze crack growth under monotonic loading conditions, differing only in the remote stress intensity factor being a cyclic function of time. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. The cyclic crack growth rate log(da/dN) versus applied stress intensity factor range log(ΔK_I) curve that emerges naturally from the solution of the boundary value problem shows distinct threshold and Paris law regimes. Paris law exponents in the range 4 to 8 are obtained for the parameters employed here. Furthermore, rather uniformly spaced slip bands corresponding to surface striations develop in the wakes of the propagating cracks.     

8090Al—Li合金的疲劳裂纹扩展

研究了8090型Al—Li合?疲劳裂纹扩展行为以及组织结构和环境的影响,结果表明,在空气中,自然时效和欠时效?现出最好的疲劳裂纹扩展阻力,其次是峰时效状态,而过时效状态的疲劳裂纹扩展阻力最低,在3.5%NaCl水溶液中,时效状态对疲劳裂纹扩展行为有着同空气中相同的影响规律,但在同样的时效条件下,在3.5%NaCl水溶液中的疲劳裂纹扩展阻力要比空气中的低,在相同的应力强度因子范围△K作用下,缺口短裂纹和物理短裂纹均表现出比长裂纹高的扩展速率,用位错的平面滑移性和循环滑移可逆性解释了时效的影响,用裂纹的闭合效应和裂纹尖端塑性区尺寸说明了长短裂纹扩展行为的差别。