The 7-series aluminum alloy is a heat-treatable aluminum alloy widely used in aerospace, rail transit, and other fields because of its excellent specific strength. Laser beam swing welding is a new method developed from conventional laser-welding technology. It can reduce the temperature gradient, stabilize the welding process, and inhibit the formation of pores and other defects in the weld. The addition of alloying elements is currently the focus of research for improving the weld performance. Many researchers worldwide have shown that adding rare-earth elements or Zr, Ti, and other elements to the weld can improve the mechanical properties. However, research on the effect of Ti addition on the microstructure and properties of 7075 aluminum alloy weld is not yet comprehensive. In this study, a systematic investigation of the effect of the thickness of a Ti metal interlayer on the microstructure and mechanical properties of 7075 aluminum alloy weld joints is reported.
In this study, 2 mm thick 7075 aluminum alloy and Ti foils of different thicknesses were used. First, welds with different thicknesses of Ti metal intermediate layers were prepared using laser beam swing welding. Then, methods, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD), were used to analyze the effects of different Ti interlayer thicknesses on the microstructures and phases of the weld metal. Finally, by analyzing the fracture location and considering the changes in the tensile strength of the joint, the influence of adding different thicknesses of Ti metal intermediate layers on the mechanical properties of the joint was determined.
When the thickness of Ti intermediate layer is 0.02 mm, the Ti content in the liquid metal is relatively low. During the cooling solidification process, Al grains precipitate simultaneously with TiAl3, and the TiAl3 phase forms a short rod-shaped distribution near the interface of the aluminum grains during the solidification process. When the thickness of Ti intermediate layer is 0.03 mm, the liquid metal first precipitates a high-melting-point TiAl3 phase during the solidification process. The precipitated phase is small and cross shaped. As the temperature continues to decrease, the TiAl3 phase dispersed in the molten pool becomes the substrate for heterogeneous nucleation. When the thickness of Ti intermediate layer is 0.04 mm, excess Ti cannot be dissolved in liquid Al, and a large area of unmelted Ti is retained. At the interface between this phase and the Al grains, a short rod-shaped TiAl3 phase forms to envelop the unmelted Ti. When the thickness of the Ti intermediate layer is 0.02 mm, the influence of Ti on the fusion line is relatively small. The fusion line on both sides of the weld becomes the weak position of the welded joint owing to the reduction in alloy element segregation and the strengthening phase. When the thickness of the Ti interlayer increases to 0.03 mm, the interface between a small amount of Ti gathering area and the Al grains in the weld becomes the crack source. When the thickness of the Ti interlayer continues to increase to 0.04 mm, a large number of cracks are generated in a large area of the Ti gathering area, and these cracks continue to extend and connect with each other, resulting in joint failure.
By studying the effects of Ti intermediate layer thickness on the weld formation, microstructure, and mechanical properties of the weld after adding different thicknesses of Ti metal intermediate layers in 7075 aluminum alloy during laser beam swing welding, the following conclusions can be drawn.
(1) With the increase in Ti intermediate layer layer thickness from 0.02 to 0.04 mm, a short rod-like TiAl3 phase, cross-shaped TiAl3 phase, and large area of Ti gathering area appear in the weld. The EBSD results indicate that the microstructure of the weld area with the addition of the Ti intermediate layer consists of fine equiaxed grains. When a 0.03 mm thick Ti intermediate layer is added, the average equivalent circular diameter of the equiaxed grain area in the weld is 3.34 μm. The weighted average value of the grain area is 1.7% of the grain size of the base material.
(2) The fine TiAl3 phase is mainly distributed within the Al grains owing to the formation of the high-melting-point TiAl3 phase as the heterogeneous core of the Al grains during the solidification process and the segregation of Zn, Mg, and Cu at the grain boundaries, resulting in the formation of hard and brittle phases, such as Al2CuMg, which weakens the grain boundaries.
(3) As the thickness of the intermediate Ti metal layer increases from 0.02 to 0.04 mm, the average tensile strength of the joint shows a pattern of first increasing and then decreasing. When the thickness of the Ti intermediate layer is 0.03 mm, the average tensile strength is the highest, reaching about 377.8 MPa, which is 69% of the base material strength.
(4) When the thickness of the Ti intermediate layer is 0.02 mm, the weak position of the joint are located at the grain boundaries of the fusion zone and the heat-affected zone. When the thickness of the Ti intermediate layer is 0.03 mm, the weak position of the joint is in the fine equiaxed crystal zone on both sides of the weld. When the thickness of Ti interlayer is 0.04 mm, the weak position of the joint is in the Ti gathering area in the middle of the weld.
