2A12-T4铝合金回填式搅拌摩擦点焊工艺及机理研究

Abstract

Refill Friction stir spot welding (RFSSW) as a new type of solid phase connection technology can not only avoid the defects caused by conventional welding, but also eliminate the keyholes brought by conventional friction stir spot welding. Therefore, it has great advantages for connecting lightweight materials such as aluminum alloys. In this paper, the joint formation, microstructure, mechanical properties and the material flow behavior of RFSSW for the 2 mm thick Alclad 2A12-T4 aluminum alloy are studied. Connection mechanism of RFSSW was revealed, which provides a theoretical basis for the application of RFSSW.

In the range of 900-1700 rpm welding speed, 2.0-3.5 mm penetration depth and 0.6-1.8 mm/s insertion speed, the RFSSW joint has good refilling effect. The welding spot has no keyhole and the surface is flat. Joints can be divided into three zones: Stir Zone (SZ), Thermal-mechanically Affected Zone (TMAZ) and Heat Affected Zone (HAZ). The SZ outer interface is curved upwards to form a Hook profile, and the overlapping surface cladding aluminum layer is redistributed within the SZ to form an arc-shaped cladding aluminum layer. As the heat input to the weld increases, the Hook height increases first and then decreases, and the length of the clad aluminum layer decreases. Dynamic recrystallization occurs in SZ. The grain boundary angle is dominated by high-angle grain boundaries, and the grains are fine equiaxed grains. The GPB zone is completely converted to the S phase and dissolution and melting occur in the subsequent process. The molten S phase is distributed in the grain boundary and shrinks during cooling, forming liquefied cracks. The larger the welding heat input, the more dissolved and melted the S phase, the lower the precipitation phase density and the increase of liquefied cracks. Some dynamic recrystallization of TMAZ occurred in the grain, and a large number of low-angle grain boundaries were distributed in the grain. The GPB zone was completely transformed into S'' and S' phases, and the precipitated phase and dislocation density were large. As the heat input to the weld increases, the S'' phase decreases, the S' phase increases, and both the precipitated phase and the dislocation density decrease. Under the effect of welding thermal cycling, HAZ transforms the precipitated phase from GPB zone to S'', and with the increase of welding heat input, the precipitation phase coarsens.

The joint hardness is affected by the grain size, dislocation density and precipitation phase density. The hardness of joint distribute as W-shaped. The SZ grain size is small, so the hardness of it is higher. The GPB region in TMAZ and HAZ transforms and grows, and thus softens in the welding process. As the heat input to the

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weld increases, the hardness of each zone decreases. The tensile and shear properties of the joint are also affected by the Hook height, the length of the clad aluminum layer, and the strength of the SZ material. When the Hook height is low, the length of Alclad layer is large , the joint failure is shear fracture and the strength is relatively low. When the Hook height is large, the length of the Alclad layer is short, the joint fuilure is tensile shear fracture, and the strength was higher. As the Hook height increased, the strength decreased. The parameters were optimized using the surface response method. The best process parameters were: 1385 rpm-2.66 mm-0.97 mm/s, and the corresponding joint tensile strength was 10.096 kN.

As the interaction between the welding tool and the material increases, the temperature rise rate and peak temperature of the joint increase, and the welding process torque and pressure decrease. Into the stage, the material at the bottom of sleeve plasticizes, and flows along the inner wall of the sleeve to pin affect zone. During the refill stage, the plastic material at the bottom of the pin affected zone is flowing to the surrounding sleeve affected zone and the material at the top entirely migrates downward. Throughout the process, the plastic material moves in the longitudinal, transverse and circular directions at the same time, so that the actual flow path of the material is a conical spiral curve. Finally, under the influence of thermal cycling and plastic deformation, some of the original interfaces broke down to form a connection, and a new connection interface is formed at SZ/TMAZ.

Keyword: Refill Friction Stir Spot Welding, Aluminum alloys, Microstructure, Mechanical property, Material Flow

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