A study of process-induced grain structures during steady state and non-steady state electron-beam welding of a titanium alloy

doi:10.1016/j.jmst.2021.10.023

Abstract

Abstract:

A detailed microstructural characterisation of the emerging weld-line grain structure, for bead-upon-plate welds in Ti-6Al-4V (Ti64) of differing plate thickness, was performed. The microstructure studied was formed during both steady state and non-steady state sections within the weld path, with the non-steady state portion being taken from the end of the plate as the weld bead and heat source overhang the edge of the plate. This allows for the effects of welding process conditions on the microstructural evolution to be determined. The weld pool geometry and 3D tomography of the weld-induced defects have been investigated. Detailed characterisation of microstructure and texture for different welding parameters and for steady and non-steady states have been used to identify physical parameters for the microstructure predictions that are difficult to obtain otherwise. The different states significantly affect the weld crown shape and formation, weld toe, weld bead depth and width. However, the heat affected zone (HAZ) remains unchanged. Regarding the microstructural evolution, both the steady and non-steady states have similar microstructure and texture. No defects were observed in the steady state section of welds, but sub-surface spherical pores have been observed in the non-steady state section of a weld. Finite element modelling to simulate the thermal-metallurgical-mechanical fields within the steady and non-steady state sections of the welds was considered, and the cooling rates predicted within steady state and non-steady sections were interrogated to improve the theoretical understanding of the microstructure and defect formation differences in these Ti64 EB weld regions.

Key words: Electron-beam fusion welding, Processing, Microstructure, Ti-6Al-4V

Yu Lu, Richard Turner, Jeffery Brooks, Hector Basoalto. A study of process-induced grain structures during steady state and non-steady state electron-beam welding of a titanium alloy[J]. J. Mater. Sci. Technol., 2022, 113: 117-127.

Figures/Tables 13

Table 1. Experimental matrix and analytical penetration depth predictions for the EB welds.

Sample No.	Plate thickness (mm)	EB power (kW)	Travel speed (mm/s)	Zuev penetration H_Z (mm)	Hashimoto penetration H_H (mm)	Lopatko penetration H_L (mm)
1	1.0	0.56	25	1.15	2.16	1.28
2	4.0	3.10	25	3.18	9.17	4.84
3	5.0	3.10	25	2.73	9.17	4.84

Fig. 1. Schematic illustration of specimen cutting from the EBW Ti64 plates.

Fig. 2. Schematic drawings to show the dimensional measurements of weld pool.

Fig. 3. Metallographic cross-sections of Ti64 with different energy per unit area during electron beam welding: steady state and non-steady state.

Table 2. Weld characteristics of EBW Ti64.

	Sample thickness	Crown width	Weld width	Toe width	HAZ	Weld depth	Grain size in FZ
	(mm)	(mm)	(mm)	(mm)	(mm)	(mm)	(µm)
Steady state	1	1.01	0.62	0.67	0.13	0.98	197
	4	4.25	2.64	-	0.37	4.02	170
	5	4.56	2.71	-	0.38	4.47	233
Non-steady state	1	0.89	0.81	0.60	0.16	0.72	70
	4	4.53	2.92	-	0.39	3.09	110
	5	4.87	3.06	-	0.38	3.45	130

Fig. 4. Cooling rates experienced by steady state welding and one instance of transient results from non-steady section.

Table 3. Cooling zone width and peak cooling rates experienced in steady state and non-steady state 1 mm, 4 mm and 5 mm plate welds.

Sample thickness (mm)	Steady state		Non-steady state
Empty Cell	Cooling zone width (mm)	peak cooling rate (K/s)	Cooling zone width (mm)	peak cooling rate (K/s)
1	1.25	28,000	1.65	70,000
4	2.80	20,000	3.25	58,000
5	2.70	12,000	3.65	28,000

Fig. 5. SEM images of EBW Ti64 taken from different locations (FZ, HAZ and BM): (a, d) 1 mm plate, (b, e) 4 mm plate, (c, f) 5 mm plate.

Fig. 6. IPF maps of cross-sections of EBW Ti64 welds in FZ of steady and non-steady states and base material: (a, g, m) 1 mm plate, (b, h, n) 4 mm plate, (c, i, o) 5 mm plate. (d-f), (j-l), (p-r) show the higher magnification of IPF maps in different zones.

Fig. 7. $\left\{ 0001 \right\}$and $\left\{ 10\bar{1}0 \right\}$ pole figures corresponding to the different regions (FZ and BM): (a, d, g) 1 mm palte, (b, e, h) 4 mm plate, (c, f, i) 5 mm plate.

Fig. 8. (a-d) Sample 1 (1 mm); (e-h) Sample 2 (4 mm); (i-l) Sample 3 (5 mm). (a, e, i) Reconstructed 3D X-ray tomography of steady state welds. (b, f, j) Longitudinal sections illustrating the weld shape and absence of pores in the steady state welds. (c, g, k) Reconstructed 3D X-ray tomography of non-steady state welds. (d, h, l) 3D rendering of non-steady state welds showing porosity.

Table 4. Tensile properties of EBW Ti64.

Sample	Yield strength (MPa)	UTS (MPa)	Elongation (%)
1	775±12	1072±7	4.0±0.8
2	885±8	1145±10	7.6±1.1
3	840±9	1090±11	5.4±0.9

Fig. 9. Fracture morphologies of EBW Ti64 welds after tensile testing at room temperature: (a) Sample 1 (1 mm); (b) Sample 2 (4 mm); (c) Sample 3 (5 mm).

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