Effect of Boron on Microstructure and Porosity of Cast Ti-6Al-4V Laser Welded Joint

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Titanium alloys have higher specific strength and fatigue resistance than other metal materials, and are therefore widely used in the aerospace industry where lightweight is required . At present, welding is a key technology for joining parts into large structural parts. Studies have shown that boron can refine the grain of cast titanium alloy, and at the same time refine the coarse columnar β crystal in the additive manufacturing process of titanium alloy. However , the role of boron in laser welding of titanium alloys is still unclear.

[Introduction]

Recently, Sakari Tolvanen (corresponding author) of Chalmers University of Technology published the latest research result "Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron" in me tallurgical and Materials Transactions A. In this paper, the researchers studied the effect of boron on laser welding of Ti-6Al-4V titanium alloy.

[Graphic introduction]

Fig.1 Physical picture of cast Ti-6Al-4V plate surfacing

硼对铸造Ti-6Al-4V激光焊接接头组织和气孔的影响

Figure 2 Cross section of welded joint after corrosion

(a) without B

(b) 0.06% B

(c) 0.11% B

Figure 3 Microstructure of different areas of the welded joint when 0.11% B is added

硼对铸造Ti-6Al-4V激光焊接接头组织和气孔的影响

(a) Fusion zone and heat affected zone microstructure

(b) Fusion zone microstructure

(c) Heat-affected zone microstructure near the fusion line

(d) Heat affected zone microstructure

Figure 4 EBSD orientation map

硼对铸造Ti-6Al-4V激光焊接接头组织和气孔的影响

(a) EBSD pattern of α phase in a welded part with 0.06% boron element added

(b) Reconstructed β phase without adding boron

(c) Reconstructed β phase when 0.06% boron is added

(d) The inverse pole figure color of the α phase and the body centered cubic β phase of the close packed hexagonal structure

Figure 5 SEM image of the fusion zone and heat affected zone

硼对铸造Ti-6Al-4V激光焊接接头组织和气孔的影响

(a) SEM image of the fusion zone without boron

(b) SEM image of the fusion zone when 0.06% boron is added

(c) SEM image of the fusion zone when 0.11% boron is added

(d) SEM image of heat affected zone without boron

(e) SEM image of the heat affected zone when 0.06% boron is added

(f) SEM image of heat affected zone when 0.11% boron is added

Figure 6 Welded joint tomogram and 3D reconstruction

硼对铸造Ti-6Al-4V激光焊接接头组织和气孔的影响

(a) Three-dimensional reconstruction of welded joint at 12 mm without boron element and its enlarged view

(b) Three-dimensional reconstruction of welded joint at 12 mm when adding 0.06% boron element and its enlarged view

(c) Three-dimensional reconstruction of welded joint at 12 mm when adding 0.11% boron element and its enlarged view

【summary】

(1) The grain size of the welded joint to which boron is added is remarkably reduced, and the crystal grains are fine columnar crystals. The TiB particles in the first precipitated β crystal grains in the base material hinder the grain growth in the heat-affected zone.

(2) As the amount of boron element increases, the length of the lath-like α decreases and the thickness does not change.

(3) The TiB particles in the fusion zone and the heat-affected zone near the fusion line are affected by the welding process, and the heat-affected zone away from the fusion line and the TiB particles of the base metal are not affected. Due to the fast cooling rate during the welding process, the size of the affected TiB particles is significantly reduced. The TiB particles are distributed in a band shape between the dendrites in the fusion zone.

(4) Boron has a certain influence on the width of the fusion zone and the distribution of the pores, which indicates that boron affects the flow of the molten pool during the welding process.

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