Stir zone material flow patterns during friction stir welding of heavy gauge AA5056 workpieces and stability of its mechanical properties

Introduction. Friction stir welding and processing are almost identical processes of severe plastic deformation at elevated temperatures. These technologies differ mainly in the purpose of its use: the formation of a hardened surface layer or producing a welded joint. However, it is known that both during welding and during processing of heavy gauge workpieces temperature gradients occur. As a result, the conditions of adhesive interaction, material plastic flow, and the formation of the stir zone change as compared to thin-sheet workpieces with fundamentally different heat dissipation rates. In this connection, the purpose of the work is to determine the regularities of the structure formation and stability of the mechanical properties in different directions in the material of 35-mm-thick aluminum-magnesium alloy samples produced by friction stir welding/processing. Research Methodology. The technique and modes of friction stir welding and processing of AA5056 alloy workpieces with a thickness of 35 mm are described. Data on the equipment used for mechanical tests and structural research are given. Results and discussion. The data obtained show the excess mechanical properties of the processing zone material over the base metal ones in all studied directions. Material structure heterogeneities after friction stir welding/processing of heavy gauge workpieces have no determining effect on the stir zone properties. At the same time, there is no clear correlation between the tensile strength values and the load application direction, nor is there any significant difference in mechanical properties depending on the location of the samples inside the stir zone. The average ultimate tensile strength values in the vertical, transverse, and longitudinal directions are 302, 295 and 303 MPa, respectively, with the yield strength values of 155, 153 and 152 MPa, and the relative elongation of 27.2, 27.5, 28.7 %.

Role of enhanced surface grain refinement and hardness improvement induced by the combined effect of friction stir processing and ultrasonic impact treatment on slurry abrasive wear performance of silicon carbide particle reinforced A356 composites

In this study, the slurry abrasive wear behavior of silicon carbide particle reinforced A356 composite alloy was investigated after the different surface mechanical attrition treatments. It is known that the aluminum matrix composites produced by the stir casting method have some deficiencies (e.g unfavorable microstructure formation, particle clustering, porosity formation, etc.). These kinds of drawbacks of the composites adversely affect the surface mechanical properties of materials such as wear resistance. For this purpose, the surface properties of the silicon carbide reinforced A356 matrix composites fabricated through the stir casting method were improved by using friction stir processing (FSP) and ultrasonic impact treatment (UIT) in the study. The results indicated that a remarkable increase was observed in the hardness and wear resistance of the cast composite via FSP and ultrasonic impact treatment following friction stir processing (FSP+UIT). The hardness of the stir zone after FSP and FSP+UIT was determined as 82.7+-2 HV and 101.9 +-3 HV0.2, respectively. The stir zone showed a similar tendency also in slurry abrasive wear resistance. FSP increased the wear resistance in the stir zone at the rate of 33.9% while it was determined as 35.5% for FSP+UIT. The microstructural modification of the cast composite that occurred after FSP was clearly demonstrated via optical microscope and scanning electron microscopy (SEM) examinations. Enhanced grain refinement after FSP+UIT was indicated especially by X-ray diffraction analysis (XRD). According to the findings, it was observed that the application of ultrasonic impact treatment following the friction stir processing can be used to obtain an enhanced microstructure and extra hardness increment in the surface of the SiC reinforced A356 alloy, thus resulting in slurry abrasive wear resistance increment.

Effect of friction stir processing on microstructures and mechanical properties of TIG cladding layer on AA7075

AA7075 is a precipitation strengthened Al-Zn-Mg-Cu alloy which has been widely used. As a common way to repair AA7075 components, tungsten inert gas (TIG) cladding generates coarse grains and defects. In addition, the use of other types of filler wires could lead to insufficient rigidity and strength of the cladding layer. In the present work, friction stir processing (FSP) has been applied to the TIG cladding layer on AA7075 to study the effect of process parameters on microstructures and mechanical properties. The macro/micro structural characteristics, elemental distribution, microhardness distribution and tensile properties have been investigated. The macroscopic defects in TIG cladding layer are eliminated and the size of grains is decreases to around 6 μm by FSP. FSP reduces the compositional difference between the stir zone and the base material. Higher rotational speed promotes the grain refinement while the lower traverse speed benefits the microstructural uniformity. FSP on the TIG weld bead brings improvement in tensile properties and hardness. All the fractures for TIG+FSP samples occur at thermo-mechanically affected zone of the advancing side. The tensile strength of the stir zone increases from 424.2 to 442.8 MPa with the increase in rotational speed and traverse speed.

Influence of Tool Geometry and Process Parameters on Torque, Temperature, and Quality of Friction Stir Welds in Dissimilar Al Alloys

In the current investigation, the influence of the tool geometry, the position of the materials in the joint, the welding speed on the temperature and torque developed, and on the quality of the welds in dissimilar and tri-dissimilar T joints were analysed. The aluminium alloys used were AA2017-T4, AA6082-T6, and AA5083-H111 and the friction stir welds were performed with identical shoulder tools, but with either a pin with simple geometry or a pin with progressive geometry. Progressive pin tools proved to be a viable alternative in the production of dissimilar and tri-dissimilar welds, as they provide a larger tool/material friction area and a larger volume of dragged material, which promotes an increase in the heat generated and a good mixing of the materials in the stir zone, although they require a higher torque. Placing a stronger material on the advancing side also results in a higher temperature in the stir zone but requires higher torque too. The combination of these factors showed that tools with a progressive pin provide sound dissimilar and tri-dissimilar welds, unlike single-pin tools. The increase in the welding speed causes the formation of defects in the stir zone, even in tri-dissimilar welds carried out with a tool with a progressive pin, which impairs the fatigue strength of the welds.

Evolution of Microstructure in Friction Stir Processed Dissimilar CuZn37/AA5056 Stir Zone

Dissimilar friction stir processing on CuZn37/AA5056 was performed to study structural and phase evolution of a friction stir zone. Formation of 5–10 μm intermetallic compounds (IMCs) such as Al2Cu was the main type of diffusion reaction between copper and aluminum. Other alloying elements such as Mg and Zn were forced out of the forming Al2Cu grains and dissolved in the melt formed due to exothermic effect of the Al2Cu formation. When solidified, these Zn-enriched zones were represented by α-Al+Al2Cu+Zn phases or α-Al+Al2Cu+Zn+MgZn regions. Eutectic Zn+MgZn was undoubtedly formed the melt after stirring had stopped. These zones were proven to be weak ones with respect to pull-off test since MgZn was detected on the fracture surface. Tensile strength of the stirred zone metal was achieved at the level of that of AA5056.

Effects of FSW Tool Plunge Depth on Properties of an Al-Mg-Si Alloy T-Joint: Thermomechanical Modeling and Experimental Evaluation

One of the main challenging issues in friction stir welding (FSW) of stiffened structures is maximizing skin and flange mixing. Among the various parameters in FSW that can affect the quality of mixing between skin and flange is tool plunge depth (TPD). In this research, the effects of TPD during FSW of an Al-Mg-Si alloy T-joint are investigated. The computational fluid dynamics (CFD) method can help understand TPD effects on FSW of the T-joint structure. For this reason, the CFD method is employed in the simulation of heat generation, heat distribution, material flow, and defect formation during welding processes at various TPD. CFD is a powerful method that can simulate phenomena during the mixing of flange and skin that are hard to assess experimentally. For the evaluation of FSW joints, macrostructure visualization is carried out. Simulation results showed that at higher TPD, more frictional heat is generated and causes the formation of a bigger stir zone. The temperature distribution is antisymmetric to the welding line, and the concentration of heat on the advancing side (AS) is more than the retreating side (RS). Simulation results from viscosity changes and material velocity study on the stir zone indicated that the possibility of the formation of a tunnel defect on the skin–flange interface at the RS is very high. Material flow and defect formation are very sensitive to TPD. Low TPD creates internal defects with incomplete mixing of skin and flange, and high TPD forms surface flash. Higher TPD increases frictional heat and axial force that diminish the mixing of skin and flange in this joint. The optimum TPD was selected due to the best materials flow and final mechanical properties of joints.

Bobbin Tool Friction Stir Welding of Aluminum Thick Lap Joints: Effect of Process Parameters on Temperature Distribution and Joints’ Properties

Bobbin tool friction stir welding (BT-FSW) is characterized by a fully penetrated pin and double-sided shoulder that promote symmetrical solid-state joints. However, control of the processing parameters to obtain defect-free thick lap joints is still difficult and needs more effort. In this study, the BT-FSW process was used to produce 10 mm AA1050-H14 similar lap joints. A newly designed bobbin tool (BT) with three different pin geometries (cylindrical, square, and triangular) and concave shoulders profile was designed, manufactured, and applied to produce the Al alloy lap joints. The experiments were carried out at a constant tool rotation speed of 600 rpm and a wide range of various welding travel speeds of 200, 400, 600, 800, and 1000 mm/min. The generated temperature during the BT-FSW process was recorded and analyzed at the joints’ center line, and at both advancing and retreating sides. Visual inspection, macrostructures, hardness, and tensile properties were investigated. The fracture surfaces after tensile testing were also examined. The results showed that the pin geometry and travel speed are considered the most important controlling parameters in BT-FSW thick lap joints. The square (Sq) pin geometry gives the highest BT-FSW stir zone temperature compared to the other two pins, cylindrical (Cy) and triangular (Tr), whereas the Tr pin gives the lowest stir zone temperature at all applied travel speeds from 200 to 1000 mm/min. Furthermore, the temperature along the lap joints decreased with increasing the welding speed, and the maximum temperature of 380 °C was obtained at the lowest travel speed of 200 mm/min with applying Sq pin geometry. The temperature at the advancing side (AS) was higher than that at the retreating side (RS) by around 20 °C. Defect-free welds were produced using a bobbin tool with Cy and Sq pin geometries at all the travel welding speeds investigated. BT-FSW at a travel speed of 200 mm/min leads to the highest tensile shear properties, in the case of using the Sq pin. The hardness profiles showed a significant effect for both the tool pin geometry and the welding speed, whereas the width of the softened region is reduced dramatically with increasing the welding speed and using the triangular pin.