LBW/SPF/DB Combined Processing and Microstructure of TA15 Titanium Alloy Four-Layer Sandwich Structure with Square Grid

Jiang Shaosong,Liu Yang,Li Baoyong,Zhang Kaifeng

doi:10.1155/2014/563047

Abstract

The process combining laser beam welding, superplastic forming, anddiffusion bonding (LBW/SPF/DB) was performed to fabricate the TA15 titanium alloy sandwich structure with square grid. The microstructure of different regions before and after superplastic forming was also analyzed. The results showed that the laser welded joints of core structure own good superplastic formability under the condition that the laser welding parameters were optimized. The microstructure near the fusion line is columnar crystal and inside the columnar crystal the acicular martensite appeared. The heat-affected zone is relatively narrow and with no obvious grain growth. At 925°C, under the condition thatthe loading-time curve was optimized, the external outline of the LBW/SPF/DB formed hollow four-layer sandwich structure is clear and the strengthened ribs areupright. It shows good diffusion bonding quality between the ribs and panels. And the bonded ratio between the upright ribs which rely on each other is above 90%. After superplastic forming, the grain size of the TA15 workpieceincreases but remains equiaxed grain. The grain size of laser welded joints also significantly grew up, and the heat-affected zone almost disappeared in this zone. The typical cast microstructure of rapid cooling turned to be the deformed microstructure, creating a better overall performance of the two-state organizations.

Highlights

The multilayer sandwich structure was widely used in the aerospace in order to meet the requirements of both lightweight and high strength [1, 2]
Conventional SPF/DB process has some disadvantages: (1) long DB time at high temperature makes the connection cycle time longer; (2) loading at high ambient temperature makes the material performance lower and the forming die service life shorter; (3) long forming time including heating and cooling time leads to the high cost and low productivity [4]
When SPF/DB is combined with LBW, diffusion bonding of core sheet is replaced by laser welding

Summary

Introduction

The multilayer sandwich structure was widely used in the aerospace in order to meet the requirements of both lightweight and high strength [1, 2]. The LBW/SPF/DB combined process was developed to fabricate the TA15 titanium alloy sandwich structure with square grid and the microstructure of different regions was studied. LBW/SPF/DB combined process consists of four main steps: (1) laser connection of core layers; (2) seal welding of core and face layers; (3) superplastic forming of core and face layers; (4) diffusion bonding between core and face layers. The FEM simulation of superplastic forming process of four-layer sandwich structure was performed using MSC. It can be seen from the color distribution that the sheet region farther away from the joint generates more deformation.

Microstructure of Superplastic Formed Four-Layer Structure

Conclusions