African Fusion June 2018

Welding of thick titanium alloys

Laser-MIG hybrid welding of thick-section titanium TC4 alloy: The relationship between microstructure and tensile performance Xuan Su, Wang Tao, Yanbin Chen This paper, presented last year at the IIW2017 International Conference in Shanghai, China, presents research from the Harbin Institute of Technology (HIT), the principal institution of China’s State Key Laboratory of Advanced Welding and Joining on the welding of thick section TC4 titanium alloy materials using Laser-MIG hybrid welding.

T itanium alloys have become important structural materi- als in the aerospace industry due to their excellent comprehensive properties such as low density, excel- lent thermal stability and high cor- rosion resistance [1,2]. Among these titanium alloys, thick-section TC4 alloy is extensively used in industrial fields. To achieve practical solutions, thick- section TC4 alloy components need to be welded together during fabrication of complex structure. Research on joining TC4 has been well reported using processes such as TIGwelding [3], MIGwelding [4], electron beam welding [5], laser beam welding [6] and laser-MIGhybrid (LAMIG) welding [7, 8]. Among thesewelding techniques, LAMIGwelding is well suited for the join- ing of TC4. LAMIG welding – combining laser beam welding and base metal arc

which absolutely affects themechanical properties of the joints. However, the ef- fect of heat input and layer temperature on the microstructure and mechanical properties are rarely reported. In this study, thick-section TC4 alloy was successfullywelded using a double- sided LAMIGwelding procedure. Themi- crostructure andmechanical properties of the joints under different heat inputs were investigated comprehensively. Materials and methods The basematerial used in this studywas thick-sectionTC4alloyand itsdimension was 15×200×80mm. TC4 standardweld- ing wire ERTi-6Al-4V with a diameter of 1.2mmwas chosen as the fillermaterial. The microstructure of thick-section TC4 alloy is displayed in Figure 1. It was clear that the microstructure of TC4 alloy primarily consisted of equiaxed primary α phase and plate-shaped α + β phases. The chemical compositions andphysical properties at room temperature of TC4 alloy are listed in Table 1 and Table 2, respectively. An IPG YLR-10000 CW fibre laser system with a maximum power of 10 kW and a FRONIUS TPS4000 MIG welding system were used for welding experiments. Figure 2 illustrates the schematic diagram of laser-MIG hybrid welding experimental equipment. The laser focus position was in front of the trailing MIG arc. The angle between the axis of the laser beam and the welding wire was approximately 50°. The laser- MIG arc distance (DLA) was in the range of 2.0 to 4.0 mm. Duringwelding, 99.999%pure argon gas was adopted to protect the molten pool from oxidation. In addition, back shielding gas behind the molten pool was used to protect the solidifying weld seam. Double-V shape grooves were machined from the base materials and

welding – has been proven to be able to effectively resolve the disadvantages of laser beam welding: insufficient gap bridging ability and the required preci- sion on positioning. The interaction of laser and MIG arcs results in good weld- ing stability, high energy density and large heat input area [9]. Recently, some research on the LAMIG welding of thin-plate titanium alloys have been reported. Brandizzi et al . [10] joined 3.0 mm thickness TC4 alloy material using LAMIG welding and the results indicated that the joints produced by LAMIG exhibited higher mechanical properties and lower defor- mation comparedwith the jointswelded by conventional laser beam welding. Li et al [11] stated the LAMIGwelding of Ti-Al-Zr-Fe alloy with 4.0 mm thick- ness and the relationship between the tensile properties and the microstruc- ture was analysed. Satisfactory joints could be achieved due to the improve- ment of themicrostructure of the joints. Unfortunately, the above investi- gations were mainly focused on the laser-MIG hybrid welding of thin-plate titaniumalloys. Therewas little research done regarding the LAMIG welding of thick-section TC4 alloy. For LAMIGweld- ing of thick-section TC4 alloy, multilayer welding is usually preferred. During the multilayer welding process, the temper- ature of the layers is high due to the rela- tively high heat input. As is well known, the microstructure of the welded joints is sensitive to temperature variation,

Figure 1: The microstructure of TC4 alloy material.

Composition (wt %)

Fe

C

N Al

V

H Ti

TC4

≤0.30 ≤0.10

≤0.05 5.5-6.8 3.5-4.5 ≤0.015 Bal.

Table 1: The chemical compositions of the TC4 titanium alloy.

Tensile strength (MPa) Yield strength (MPa) Ductility (%) Reduction of area (%) ≥895 ≥825 ≥10 ≥25

Table 2: Physical properties of TC4 alloy at room temperature.

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June 2018

AFRICAN FUSION