Fatigue behavior of friction stir welded Al–Mg–Sc alloy

The effect of Friction Stir Welding on the fatigue behavior of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.     

The microstructure and mechanical properties of friction stir welded Al–Zn–Mg alloy in as welded and heat treated conditions

High strength aluminium alloys generally present low weldability because of the poor solidification microstructure, porosity in the fusion zone and loss in mechanical properties when welded by fusion welding processes which otherwise can be welded successfully by comparatively newly developed process called friction stir welding (FSW). This paper presents the effect of post weld heat treatment (T₆) on the microstructure and mechanical properties of friction stir welded 7039 aluminium alloy. It was observed that the thermo-mechanically affected zone (TMAZ) showed coarser grains than that of nugget zone but lower than that of heat affected zone (HAZ). The decrease in yield strength of welds is more serious than decrease in ultimate tensile strength. As welded joint has highest joint efficiency (92.1%). Post weld heat treatment lowers yield strength, ultimate tensile strength but improves percentage elongation.     

Tensile properties of friction stir welded and friction stir welded-superplastically formed Ti–6Al–4V butt joints

A University and Industry collaborative research project was undertaken to evaluate the performance of as friction stir welded (FSW) and friction stir welded-superplastically formed Ti–6Al–4V alloy sheets. The purpose of this particular effort was to evaluate the tensile properties of friction stir welded and superplastically formed friction stir welded Ti–6Al–4V. Welds were produced out of both standard grain and fine grained titanium and tested in the as welded, stress relieved (SR) and superplastically formed (SPF) conditions. The preliminary results of the FSW and post FSW–SPF joint were found to be close to that of as received titanium with respect to strength, but elongations were decreased.     

Effect of abnormal grain growth on tensile strength of Al–Cu–Mg alloy friction stir welded joints

An Al-4.5%Cu-1.5%Mg aluminum alloy with a T4 temper was friction stir welded, and the effect of the abnormal grain growth on the tensile strength of joints was investigated. Abnormal grain growth usually happens during post weld heat treatment. It is found that the tensile strength and elongation of the heat-treated joint will increase significantly if this phenomenon completely happens in stir zone. On the other hand stable grains in the stir zone have no effect on the mechanical properties of heat-treated joint.     

Challenges in dissimilar friction stir welding of aluminum 5052 and 304 stainless steel alloys

In this study, dissimilar friction stir welding of aluminum 5052 and stainless steel 304 has been carried out with different process parameters. This investigation provides a better insight regarding the defect formation of the weld joints with tilt angles ranging from 0 ° to 2.5 °. The experiments were conducted according to Taguchi L9 orthogonal array by changing the tool rotational speed, and welding speed. The tool pin was kept 70 % towards the aluminum with the tool rotational speed ranging from 800 min⁻¹ to 1200 min⁻¹ with a varying traverse speed of 5 mm/min to 15 mm/min. The bottom part of the stir zone was perfectly welded without any defects. Tunnel defect was detected just above the bottom welded surface. Microstructural analysis reveals that the weld between both materials is formed on the retreating side, whereas on the advancing side, the weld was formed with void defects. Mostly, the stir zone is filled with irregular shaped aluminum and steel parts which were detached from the base material. Several other defects such as voids, cracks, and fragmental defects were observed in the stir zone irrespective of the process parameters. It was observed from the experimental investigations that the tunnel defect can be reduced by increasing the tilt angle.     

Thermal characteristics and corrosion behaviour of Mg–xZn alloys for biomedical applications

The thermal parameters of Mg–xZn cast alloys with 0·5–9 wt% Zn were evaluated by using computer aided cooling curve thermal analysis (CA–CCTA), whereas the corrosion behaviour was investigated by potentiodynamic polarization and immersion tests. Thermal analysis results revealed that the dendrite coherency temperature (T _DCP ) decreased from 642·2 to 600 °C with the addition of Zn from 0·5 to 9 wt%. The liquid fraction at coherency point ( ${f}_{ L}^{ DCP}$ ) increased by 72% when Zn was increased up to 9 wt%. MgZn intermetallic phase was observed in samples with <3 wt% Zn. At higher percentages of Zn, the Mg ₅₁Zn ₂₀ intermetallic phase was also detected in addition to α-Mg and MgZn by first derivative cooling curves under non-equilibrium solidification. All these phases were observed along the grain boundary when Zn was rejected from the solid/liquid interface and enriched in the triple conjunction of grain boundary. The grain size decreased from 185·2 to 71·5 μm when Zn content was increased. The addition of Zn content had a significant effect on the corrosion rate and the corresponding mechanisms. The corrosion rate decreased from 2·1 to 1·81 mmpy as Zn content increased from 0·5 to 3 wt%; afterwards, however, this value increased with further increase of Zn. Mg–3Zn also had the lowest degradation rate and highest corrosion resistance which can be fully utilized for biodegradable orthopedic applications.     

Microstructure and interface characterization of dissimilar friction stir welded lap joints between Ti–6Al–4V and AISI 304

The feasibility of dissimilar friction stir welding (FSW) in overlap configuration between Ti–6Al–4V alloy (Ti64) and AISI 304 austenitic stainless steels (304SS) was investigated. Sound joints were achieved when placing titanium as the upper workpiece. Joints were successfully produced by employing a welding speed of 1 mm/s and rotational speeds of 300 and 500 rpm. A lamellar microstructure was formed in the stir zone of Ti64, where grain size was found to increase with increasing rotational speed, and austenitic equiaxed grains were obtained near the interface of 304SS coupon. Energy dispersive X-ray spectroscopy (SEM-EDS) of the interface revealed a thin intermixed region and suggested intermetallic compound formation. Microhardness data in the titanium weld zone for both rotational speeds exhibited slightly lower values than the base material, with the lowest values in the heat affected zone, whereas the microhardness values in the stainless steel side around the weld center were found to be higher than those obtained for the base material.     

Microstructural characteristics and mechanical properties of non-combustive Mg–9Al–Zn–Ca magnesium alloy friction stir welded joints

Non-combustive Mg–9Al–Zn–Ca magnesium alloy was friction stir welded with rotation speeds ranging from 500 to 1250 rpm at a constant welding speed of 200 mm/min. Defect-free joints were successfully produced at rotation speeds of 750 and 1000 rpm. The as-received hot extruded material consisted of equiaxed α-Mg grains with β-Mg₁₇Al₁₂ and Al₂Ca compounds distributed along the grain boundaries. Friction stir welding produced much refined α-Mg grains accompanied by the dissolution of the eutectic β-Mg₁₇Al₁₂ phase, while Al₂Ca phase was dispersed homogeneously into the Mg matrix. An increase in rotation speed increased the α-Mg grain size but not significantly, while microstructure in the heat affected zone was almost not changed compared with the base material. The hardness tests showed uniform distributed and slightly increased harness in the stir zone. Results of transverse tensile tests indicated that the defect-free joints fractured at the base material, while longitudinal tensile tests showed that the strength of the defect-free welds was improved due to microstructural refinement and uniform distribution of intermetallic compounds.     

Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding

5A02 aluminum alloy and pure copper were joined by friction stir welding (FSW). A defect-free joint was obtained when one of process parameters, i.e. the traverse speed was lowered from 40 mm/min to 20 mm/min. A good mixing of Al and Cu was observed in the weld nugget zone (WNZ). A large amount of fine Cu particles were dispersed in the upper part of the WNZ producing a composite-like structure. In the lower part, nano-scaled intercalations were observed and identified by transmission electron microscopy (TEM). These layered structures were subsequently confirmed as Al₄Cu₉ (γ), Al₂Cu₃ (ε), Al₂Cu (θ), respectively. Formation of these microstructures caused an inhomogeneous hardness profile. Particularly, a distinct rise in hardness was noticed at the Al/Cu interface. Excellent metallurgical bonding between Al and Cu gave rise to good behaviors in the tensile and bending strength.     

Analysis of crack propagation resistance of Al–Al2O3 particulate-reinforced composite friction stir welded butt joints

This work is devoted to the analysis of fatigue crack propagation resistance of particulate metal-matrix composites butt joints obtained by friction stir welding. Two different aluminum alloy matrices reinforced with alumina particles were examined. Tests were conducted on both parent material and welded joint for comparison. Fatigue crack propagation was carried out both within the weld nugget and in the thermo-mechanically altered zone at the side of the weld. The comparison between parent material and joint showed that the welding process affects fracture toughness and fatigue crack growth rate differently depending on the material. The analysis of crack path roughness helped to understand those differences in the fatigue crack growth rate. Therefore, roughness-induced crack closure arguments have been introduced to discuss data obtained under different testing conditions (parent material/joint, R-ratio, crack location, crack growth regime). Both the classical Elber’s approach and more recent approaches based on partial crack closure concept (adjusted compliance ratio, ACR, and 2/π methods) were considered. The results showed that, using partial crack closure, all of the data collapse within a reasonable scatterband.     

Effects of tool wear on friction stir welded joints of Ti–6Al–4V alloy

Tool wear behaviour on microstructure and mechanical properties of friction stir welded zones of Ti–6Al–4V alloy was evaluated. SEM examination, EDS analysis and X-ray diffraction results indicated that severe wear of the tool is indicated by the presence of WC-Co particles in the stir zone at rotational speed of 630?rev?min^?1 and travel speed of 8?mm?min^?1. Micro-hardness, tensile tests and fractographical examinations also reflected that these particles make the material more brittle and reduce the mechanical strength by 40%. However at travel speed of 22?mm?min^?1, tool wear is less, hardness distribution is more uniform and enhanced ductility and strength is achieved.     

Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets

5 mm-Thick dissimilar AA2024-T3 and AA7075-T6 aluminum alloy sheets were friction stir lap welded in two joint combinations, i.e., (top) 2024/7075 (bottom) and 7075/2024. The influences of process conditions (welding speed and joint combination) on defects (hook and voids) features and mechanical properties of joints were investigated in detail. It was found that the hook deflects largely upwards into the stir zone (SZ) at lower welding speeds (50, 150 mm/min) in both combinations. The process conditions significantly affect the hook geometry which in return affects the lap shear strength. In all 2024/7075 joints, voids appear and the joints fracture from the tip of hook on AS along the SZ/TMAZ (thermomechanically affected zone) interface in lap shear test (tensile fracture mode). In 7075/2024 joints, the hook on RS horizontally extends a large distance into the bottom stir zone at higher welding speeds (225, 300 mm/min). The joints fracture in three modes: shear fracture along the lap interfaces, tensile fracture and the mix fracture of both. In both joint combinations, the lap shear strength generally increases with the increase of welding speed. 7075/2024 Joints show higher failure load than 2024/7075 joints at lower welding speeds while the opposite result appears at higher welding speeds.     

Microstructural evolution and superplastic behavior in friction stir processed Mg–Li–Al–Zn alloy

A Mg–Li–Al–Zn alloy was friction stir processed (FSP) under water, and the microstructures and superplastic behavior in the FSP alloy were investigated. The FSP Mg–Li–Al–Zn alloy consisted of a mixed microstructure with fine, equiaxed, and recrystallized α (hcp) and β (bcc) grains surrounded by high-angle grain boundaries, and the average grain size of the α and β grains was ~1.6 and ~6.8 μm, respectively. The fine α grains played a critical role in providing thermal stability for the β grains. The FSP Mg–Li–Al–Zn alloy exhibited low-temperature superplasticity with a ductility of 330 % at 100 °C and high strain rate superplasticity with ductility of ≥400 % at 225–300 °C. Microstructural examination and superplastic data analysis revealed that the dominant deformation mechanism for the FSPed Mg–Li–Al–Zn alloy is grain boundary sliding, which is controlled by the grain boundary diffusion in the β phase.     

Thermo-mechanical and microstructural issues in dissimilar friction stir welding of AA5086–AA6061

In this research, thermo-mechanical behavior and microstructural events in dissimilar friction stir welding of AA6061-T6 and AA5086-O have been evaluated. The thermo-mechanical responses of materials during the process have been predicted employing a three-dimensional model together with a finite element software, ABAQUS. Then, mechanical properties and microstructures of the weld zone were studied with the aid of experimental observations and model predictions. It is found that the mixing of material in the weld nugget is performed more efficiently when AA5086 is in the advancing side and also the temperature field is distributed asymmetrically resulting in larger thermally affected region in the AA6061 side. Besides, the microstructural studies shows that the microstructures of stirred zone consist of fine equiaxed grains where finer grains are produced in AA6061 side compared to AA5086 side.     

Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

Abstract

PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y₂O₃ particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air).

High-resolution SuperSTEM images indicate that the Y₂O₃ can transform to Y₃Al₅O₁₂ garnet (YAG) and YAlO₃ perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y₂O₃ or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.     

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.     

Large load friction stir welding of Mg–6Al–0.4Mn–2Ca magnesium alloy

Friction stir welded (FSW) magnesium alloys usually exhibit a lower yield strength and elongation compared with base materials. In this study, large load FSW associated with an extremely low welding speed and rotation rate were applied to a non-combustive Mg–6Al–0.4Mn–2Ca magnesium alloy to modify the microstructure and texture in the weld zone and improve the mechanical properties of the joint. The twin structure in the stir zone provided adequate barriers for dislocation motion for strengthening and created more local sites for nucleating and accommodating dislocations, thereby elevating ductility and strain hardening in the transverse tensile test. The results showed that the yield strength and elongation of the joint were enhanced to 98% and 126% of the base material levels, respectively.     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of a super high strength Al–Zn–Mg–Cu aluminum alloy

Samples made of a super high strength aluminum alloy with high Zn content were friction stir welded with rotation rates of 350–950 rpm and welding speeds of 50–150 mm/min. The effect of welding parameters on the microstructure and mechanical properties was investigated. It was observed that the grain size of the nugget zones decreased with the increasing welding speed or the decreasing tool rotation rate. Most of the strengthening precipitates in the nugget zone were dissolved back and the intragranular and grain boundary precipitates in the heat affected zone coarsened significantly. The greatest ultimate tensile strength of 484 MPa and largest elongation of 9.4 were obtained at 350 rpm−100 mm/min and 350 rpm−50 mm/min, respectively. The ultimate tensile strength and elongation deteriorated drastically when rotation rate increased from 350 to 950 rpm at a constant welding speed of 100 mm/min.     

Calculations of electrical resistivity for Al–Cu and Al–Mg–Si alloys

Abstract

A system for thermodynamical calculations (Thermo-Calc) was used to derive the solid solubility of the alloying elements in commercial Al–Cu and Al–Mg–Si alloys. The electrical resistivity was then calculated using a model developed by the authors based on the Matthiessen's rule. The calculated resistivity agreed with the observed resistivity within ±2.5 nΩ m for the Al–Mg–Si alloys and ±2 nΩ m for the Al–Cu alloys, except for Al–Mg–Si alloys containing boron or chromium and Al–Cu alloys with special compositions.