Achieving low-porosity and high-strength 2219 aluminum alloy joints through coupling of laser beam oscillation and post-weld heat treatment

Abstract

Laser welding is a highly promising joining method for Al alloys. However, certain limitations such as elevated thermal input and keyhole instability are associated with its application in medium-thickness aluminium alloy plates. To address these issues, circular oscillating laser welding combined with post-weld heat treatment was employed to improve the formation quality and mechanical properties of the welds. The effects of the frequency of circular oscillating laser on the forming quality, microstructure, and properties of the welds were analyzed. At an oscillation frequency of 200 Hz, the grain size in the weld zone was reduced compared to single laser welding, and the maximum tensile strength of the weld was observed to reach (264.96±1.33) MPa, representing approximately 61.19% of the base metal. Following the post-weld heat treatment of "solid solution and artificial aging", the grain boundary segregation was diminished. Nanoscale precipitated phases are present in the weld zone. Furthermore, the tensile strength was augmented to (386.35±5.65) MPa, representing approximately 89.23% of the strength of the base metal. The results of this study can provide a theoretical basis and technological reference for the circular oscillating laser welding of medium-thickness 2219-T6 aluminium alloy plates.

1. Introduction

As a typical 2xxx series (Al-Cu) aluminium alloy, the 2219-T6 aluminium alloy has many advantages, such as thermal stability, high specific strength, superior corrosion resistance, and excellent weldability. They are extensively used in the aerospace field, particularly in the manufacture of fuel tanks for launch vehicles [1, 2]. In contrast to solid-state welding techniques, laser welding is a non-contact melting welding that does not rely on mechanical frictional heat [3−6]. It has unique advantages including concentrated energy density, high welding efficiency, process stability, and dissimilar metal welding capability. It is a major promising welding method for Al alloys and has significant applications in lightweight manufacturing in the automotive and aerospace industries [7, 8].

The laser welding of 2××× series aluminium alloy sheets, particularly in medium-thickness plates, presents technical challenges due to the material's inherent characteristics and process-specific interactions [9]. Firstly, 2××× series aluminium alloys have poor fluidity, which are prone to forming welding defects such as porosity, slumping and spatter [10−12]. Secondly, the main strengthening mechanism of the 2219 aluminium alloy is precipitation strengthening, and the precipitation phase distribution and size are mainly influenced by the temperature and external stress [13]. It is widely accepted that a heat source of the laser with high-energy density can rapidly melt the materials and solidify when the laser beam exits. Regardless of whether laser oscillations occurred, the strengthening phase dissolved and segregated to form a brittle eutectic phase during solidification. This reduced the precipitation strengthening effect. The principal strengthening mechanism of welded joints is transformed into fine-grained strengthening and solution strengthening, which degrades the tensile mechanical properties of the welds [14].