Retreating Side Research Articles

The effect of the welding parameters and tool size on the thermal process and tool torque in reverse dual-rotation friction stir welding

Reverse dual-rotation friction stir welding (RDR-FSW) is a novel variant of conventional friction stir welding (FSW) process. The key feature is that the tool pin and the assisted shoulder are separated and rotate reversely and independently during welding process, thus it has great potential to improve the weld quality and lower the welding loads through adjusting the rotation speeds of the tool pin and the assisted shoulder independently. A 3D model of RDR-FSW process is developed to analyze the effect of welding parameters and tool size on the thermal process and the tool torque quantitatively. The model considers the effect of the welding parameters on the dimensionless slip rate and the friction coefficient between the tool-workpiece contact interfaces. It is found that with an increase of the radial distance, the locations of peak and valley values of heat generation rate at the shoulder-workpiece contact interfaces vary from the retreating side (RS) to the advancing side (AS) and from the AS to the RS, respectively. Although the reverse rotation of the tool pin and the assisted shoulder has little effect on the total heat generation, the corresponding material flow pattern and the distribution of heat generation rate lead to a more homogeneous temperature distribution and a much lower torque exerted on the workpiece in RDR-FSW process. The model is experimentally validated by comparing the measured thermal cycles with the calculated data.

Microstructure feature of friction stir butt-welded ferritic ductile iron

This study conducted friction stir welding (FSW) by using the butt welding process to join ferritic ductile iron plates and investigated the variations of microsturcture in the joined region formed after welding. No defects appeared in the resulting experimental weld, which was formed using a 3-mm thick ductile iron plate and tungsten carbide alloy stir rod to conduct FSW at a rotational speed of 982rpm and traveling speed of 72mm/min. The welding region was composed of deformed graphite, martensite phase, and dynamically recrystallized ferrite structures. In the surface region and on the advancing side (AS), the graphite displayed a striped configuration and the ferritic matrix transformed into martensite. On the retreating side (RS), the graphite surrounded by martensite remained as individual granules and the matrix primarily comprised dynamically recrystallized ferrite. After welding, diffusion increased the carbon content of the austenite around the deformed graphite nodules, which transformed into martensite during the subsequent cooling process. A micro Vickers hardness test showed that the maximum hardness value of the martensite structures in the weld was approximately 800HV. An analysis using an electron probe X-ray microanalyzer (EPMA) indicated that its carbon content was approximately 0.7–1.4%. The peak temperature on the RS, 8mm from the center of the weld, measured 630°C by the thermocouple. Overall, increased severity of plastic deformation and process temperature near the upper stir zone (SZ) resulted in distinct phase transformation. Furthermore, the degree of plastic deformation on the AS was significantly greater than that on the RS, and relatively complete graphite granules and the fine ferrite grains resulting from dynamic recrystallization were observed on the RS.

Effect of weld curvature radius and tool rotation direction on joint microstructure in friction stir welding casting alloys

Curved welds were designed and the effects of the weld curvature radius and tool rotation direction on the microstructure of friction stir welded cast aluminum alloy joints were investigated. Results show that both the weld curvature radius and tool rotation direction have a significant influence on the microstructure of the curved joints during FSW. With decreasing weld curvature radius, the size of the tunnel defect is reduces and the proportion of fine Si particles in the stir zone increases. Si particles are finer and denser in the retreating side (RS) than that in the advancing side (AS) when both the welding direction and tool rotation direction are anticlockwise. However, when the welding direction is anticlockwise while the tool rotates clockwise, the proportions of fine Si particles decrease compared to the former situation. Furthermore, the tunnel defect is more likely to be present in the AS in the former situation.

Influence of sampling design on tensile properties and fracture behavior of friction stir welded magnesium alloys

Dissimilar joints of ZK60 and AZ31 alloys were obtained by friction stir welding (FSW). Two types of sampling designs were employed for transverse tensile tests to evaluate the tensile properties and fracture behavior of the joints. One kind of samples was machined excluding the upper part of the weld zone (WZ) and named as the stir zone (SZ) sample. Another kind was machined including the entirety of WZ and named as the entirety sample (EN sample). Both types of samples have strength higher than the base alloy AZ31 but lower than the base alloy ZK60. The tensile properties of the EN sample are slightly better than those of the SZ sample. Significant differences in fracture behaviors were observed between the two types of samples. The fracture of the SZ sample initiated at the boundary of transition zone (TZ)/SZ in retreating side (RS) and propagated toward the SZ. However, the EN sample fractured in advancing side (AS), close to the “hard” material of ZK60 alloy, with more complex fracture morphology. A comprehensive characterization of microstructure and texture distributions was carried out to explain the deformation and fracture behaviors. The grains in the SZ-side are favorably oriented for basal slip or extension twinning, being the weak position during the transverse tensile tests. The grains in the side of crown zone (CZ) have the c-axis tilted toward the normal direction (ND), being harder for extension twinning compared with those in the SZ-side. A triple junction region was clearly observed adjacent to the interface of TZ/WZ in AS, but it was not observed in RS. The grains in the triple junction region were significantly refined but inhomogeneous among the various parts. The texture of the triple junction region was close to that of the CZ-side, but with a tendency to tilt toward that of the SZ-side. This observation confirmed that the triple junction region was located between the SZ-side and CZ-side of the WZ. Significant differences in the grain size and Schmid factor (SF) distributions for slip and twinning were observed among the different parts, which could cause deformation incompatibility, generate stress concentrations and accelerate fracture in the triple junction region in AS.

Surface Improvement by Overlapping in Multipass FSP

Friction stir processing (FSP) is an emerging metal-working technique based on the same fundaments as friction stir welding that allows local modification and control of microstructures, for the purpose of improving surface or in-volume mechanical properties. This paper aims to explore several applications of the FSP technology for surface improvement either by multipass FSP or surface reinforcement with silicon carbide particles. An AA5083-H111 alloy 8 mm thick was tested. Structural and mechanical differences were observed when overlapping by the advancing side (AS) direction or by the retreating side (RS) one. Hardness within the processed layer increased by 8.5 % and was seen to be approximately constant between passes. Toughness under bending was improved by 18 %. The production of particle-reinforced metal matrix composite materials (MMC) was investigated, as the severe plastic deformation inherent to the process promoted the dispersion of 12.5 μm median size SiC particles, resulting in a severe superficial hardness increase.

Thermo-mechanical and metallurgical aspects in friction stir processing of AZ31 Mg alloy—A numerical and experimental investigation

Friction stir processing of AZ31 Mg alloy was investigated by numerical modeling and experiments. A CFD based, fully coupled, 3D, thermo-mechanical model was built to better understand the effect of process parameters on temperature, material flow and strain rate. In order to account for material softening phenomena at elevated temperatures and extremely high strain rates that occur during the FSP process, experimentally measured peak temperatures were utilized to introduce a correction function in the flow stress constitutive relation. The numerical results showed that rotational speed as compared to translational speed had a more dominant effect on temperature field and strain rate. In addition, the asymmetric material flow around the tool axis caused higher peak temperature and strain rate on the advancing side (AS), while the material in the path of tool pin was swept around the retreating side (RS). FSP experiments confirmed peak temperatures measured at sheet surface near shoulder perimeter on AS were always higher than corresponding RS peak temperatures, under the selected range of process parameters. In addition to thermo-mechanical aspects, the metallurgical characteristics of FSP i.e. mainly the grain size evolution was studied by optical and electron microscopy. Experiments revealed that the coarse bimodal microstructure of as-received AZ31 Mg was subdivided into a defect-free, fine grain microstructure at the rotational speed of 1000rpm, while a defect-free but a relatively coarse and bimodal microstructure evolved in the material at rotational speeds higher than 1000rpm. Furthermore, in the selected range of process parameters the increases in translational speed resulted in finer grain sizes without the formation of voids or defects.

Effect of welding parameters on microstructure and mechanical properties of friction stir welded 2219Al-T6 joints

The effect of friction stir welding (FSW) parameters on the microstructure and mechanical properties of 5.6 mm thick 2219Al-T6 joints was investigated in detail. While the sound FSW joints could be obtained under lower rotation rates of 400–800 rpm and welding speeds of 100–800 mm/min; higher rotation rates of 1200–1600 rpm easily led to the tunnel and void defects. The FSW thermal cycle resulted in low hardness zones (LHZs) on both retreating side (RS) and advancing side (AS). The LHZs may be located at the interface between the nugget zone (NZ) and the thermo-mechanically affected zone (TMAZ), at the TMAZ, or at the heat affected zone under the varied welding parameters. The tensile strength of FSW 2219Al-T6 joints increased when increasing the welding speed from 100 to 800 mm/min, and was weakly dependent on the rotation rates from 400 to 1200 rpm. The FSW 2219Al-T6 joints fractured along the LHZ on the RS.

Finite Element Analysis of Materials Flow Behavior in Friction Stir Welding of 7075 Aluminum Alloy Plate

A 3D elastic-plastic and coupled thermo-mechanical FE model for friction stir welding(FSW) of 7075 aluminum alloy plate was developed based on the dynamic explicit code ABAQUS/explicit, then the FSW process of 7075 aluminum alloy plate was simulated and the materials flow behavior was analyzed. The results show that in the horizontal direction of the plate, two patterns of materials migration are produced: (1) the material rotates with the tool and finally deposits the tentative cavity behind the pin; (2) the material transfers in the mode of laminar flow. Furthermore, the streamlines of material in advanced side (AS) are bent along the reverse welding direction. After bypassing the pin, the material of retreating side (RS) moves backward it, and extends to AS.

Fatigue Crack Growth Rates in Friction Stir Welding Joints of 7075-T6 Al Alloy and Fatigue Life Prediction Based on AFGROW

In this work, a study of fatigue crack propagation (FCP) behavior of friction stir welding (FSW) joints of 7075-T6 aluminum alloy was carried out. Fatigue crack growth rate curves were determined for cracks growing in different locations of the welding lines, including prefabricated crack through welding seam center, advancing side (AS), retreating side (RS), and vertical to welding seam. A computational simulation of fatigue crack propagation was conducted by AFGROW with different stress ratios R, and the effects of R on FCP rate were analyzed. Results showed that the FCP rate in RS of the Heat Affected Zone (HAZ) was the lowest and the highest was in the region vertical to the welding seam. In the low stress intensity factor range (△K) region, the FCP rate in Nugget Zone (NZ) was lower than that in AS of the HAZ, but in the high △K region, the situation was contrary. Reasons of the results were analyzed. Compared with the standard of International Institute of Welding (IIW), FCP rates in all regions were lower and it concluded nice fatigue properties of FSW 7075-T6. The simulation made in terms of crack propagation rate (da/dN) versus △K generally showed a good agreement with the measured values. The study of effects of different R on FCP rates based on AFGROW indicated that FCP rates increased with increasing R.

Effect of overlapping direction in multipass friction stir processing

This paper aims to study the effect of overlapping direction in multipass friction stir processing (FSP). An AA 5083-H111 alloy 8 mm thick was tested. Structural and mechanical differences were observed when overlapping by the advancing side (AS) direction or by the retreating side (RS) one. The surface morphology was analysed and the processed zone was observed by optical microscopy, hardness measurements and bending tests were performed to evaluate surface ductility. Overlapping by the retreating side was found to generate smoother surfaces, while overlapping by the advancing side led to more uniform thickness layers. Hardness within the processed layer increased by 8.5% and was seen to be approximately constant between passes. The mechanical resistance and toughness under bending were improved by 18% and 19%, respectively.

Microstructure and microtextural studies of friction stir welded aluminium alloy 5052

The present investigation presents a composite picture of the microstructural developments in a friction stir welded (FSW) AA5052. Optimized, defect free and chemically homogeneous, FS weld was generalized in four regions – base material (BM), nugget, advancing side (AS) and retreating side (RS), using standard nomenclatures. Each region had its signature of microstructural features. AS had clear indications of shear and of grain fragmentation. The nugget region, on the other hand, had nearly equiaxed grains, with strong in-grain misorientation and presence of grain-interior dislocation structure ruling out contributions from static recrystallization. Equiaxed grains of the nugget region had typical onion ring structure – each ring did approximately correspond to one dominant family of orientation. Microstructural developments, as obtained from relative grain refinement, in-grain misorientation development, relative banding, etc., were most significant in nugget followed by AS and then by RS. Heterogeneous plastic deformation and thermal activation through localized heating/friction were the apparent causes. Most of the friction stir welded specimen fractured away from the nugget and showed ductile mode of failure.

Effect of fixed location variation in friction stir welding of steels with different carbon contents

Dissimilar friction stir welding between steels with different carbon contents was carried out by placing the parent materials at two different fixed locations: the advancing side (AS) and the retreating side (RS). The phase transformation and the depth and width of the weld were affected by the fixed location. A smaller weld nugget was formed, and more martensite transformation occurred when the stronger SK5 steel was fixed at AS, compared to that at RS. This variation was attributed to the difference of temperature and materials flow between AS and RS.

Friction stir welding using unthreaded tools: analysis of the flow

Friction stir welding (FSW) is a solid-phase welding process. Material flow during FSW is very complex and not fully understood. Most of studies in literature used threaded pins since most industrial applications currently use threaded pins. However initially threaded tools may become unthreaded because of the tool wear when used for high melting point alloys or reinforced aluminium alloys. In this study, FSW experiments were performed using two different pin profiles. Both pins are unthreaded but have or do not have flat faces. The primary goal is to analyse the flow when unthreaded pins are used to weld thin (4 mm) plates. Material flow with unthreaded pin was found to have the same features as material flow using classical threaded pins: material is deposited in the advancing side (AS) in the upper part of the weld and in the retreating side (RS) in the lower part of the weld; a rotating layer appears around the tool. The plunge force and the rotational speed were found to affect the thickness of the shoulder dominated zone. This effect is reduced using the cylindrical tapered pin with flats.

Understanding the material flow path of friction stir welding process using unthreaded tools

Material flow during friction stir welding is very complex and not fully understood. Most of studies in literature used threaded pins since most industrial applications currently use threaded pins. However, initially threaded tools may become unthreaded because of the tool wear when used for high melting point alloys or reinforced aluminium alloys. In this study, FSW experiments were performed using two different pin profiles. Both pins are unthreaded but have or do not have flat faces. The primary goal is to analyse the flow when unthreaded pins are used to weld thin plates. Cross-sections and longitudinal sections of welds were observed with and without the use of material marker (MM) to investigate the material flow. Material flow with unthreaded pin was found to have the same features as material flow using classical threaded pins: material is deposited in the advancing side (AS) in the upper part of the weld and in the retreating side (RS) in the lower part of the weld; a rotating layer appears around the tool. However, the analysis revealed a too low vertical motion towards the bottom of the weld, attributed to the lack of threads. The product of the plunge force and the rotational speed was found to affect the size of the shoulder dominated zone. This effect is reduced using the cylindrical tapered pin with flats.

Microstructural Characterization and Temperature Analysis in Friction Stir Welding of A383/5052 Dissimilar Aluminum Alloys

Microstructural characterization and temperature analysis have been performed in friction stir welding (FSW) of A383 and 5052 dissimilar aluminum alloys. Marked difference in microstructure was observed between joints with different arrangements of materials. The temperature at four points on each side of the joint line was measured during FSW in various conditions. In addition, an analytical model assumed that the work generated by the rotation of the tool led to the work for stirring materials and heat generation of the material and the tool. The temperature of the retreating side (RS) for the joint of the advancing side (AS):A383/RS:5052 was about 50K higher than that of AS, while the temperatures of AS and RS for the joint of AS:5052/RS:A383 were almost the same. The experimental temperature could be calculated reasonably by using the model with assumption of the work for stirring the material.

Temperature analysis in friction stir welding of ADC12/5052 dissimilar aluminum alloys

Temperature analysis has been performed in friction stir welding (FSW) of ADC12 and A5052 dissimilar aluminum alloys. The temperature at the stir zone and four points on each side of the joint line was measured during FSW in various conditions. In addition, an analytical model assumed that the work generated by the rotation of the tool led to the work for stirring materials and heat generation of the material and the tool. The temperature of the stir zone (SZ) where A5052 were FSWed was about 30 K higher than that of ADC12. The temperature of SZ of the advancing side (AS) : A5052/the retreating side (RS) : ADC12 was about 30 K higher than that of AS : ADC12/RS : A5052. The flow stress of the materials in FSW was calculated based on the temperature obtained in the experiment. The analytical model could predict appropriately that the motor power consisted of the heat generated by friction and the work generated by the rotation of the tool.