Weldability, microstructure and mechanical properties of laser-welded selective laser melted 304 stainless steel joints

doi:10.1016/j.jmst.2019.04.017

Abstract

Abstract:

Laser welding is a promising process for joining small components produced by selective laser melting (SLM) to fabricate the large-scale and complex-shaped parts. In the work, the morphology, microstructure, microhardness, tensile properties and corrosion resistance of the laser welded stress-relieved SLMed 304 stainless steel joints are investigated, as the different sections of stress-relieved SLMed 304 stainless steel are joined. Results show that the SLMed 304 stainless steel plates have a good laser weldability. The microstructure of laser-welded joints consists of the cellular dendrites in austenite matrix within columnar grains, exhibiting a coarser dendrite structure, lower microhardness ($\widetilde{2}$20 HV) and tensile properties (tensile strength of $\widetilde{7}$50 MPa, and area reduction of $\widetilde{2}$7.6%), but superior corrosion resistance to those of SLMed plates. The dendrite arm spacing of the joints varies from $\widetilde{3}$.7 μm in center zone, to $\widetilde{5}$.0 μm in fusion zone, to $\widetilde{2}$.5 μm in epitaxial zone. The SLMed anisotropy shows a negligible effect on the microstructure and performance of the laser-welded joints. The laser welding along the building directions of the SLMed base plates can induce a slightly finer dendritic structure and higher tensile properties.

Key words: Selective laser melting, Laser welding, 304 stainless steel, Anisotropy

Jingjing Yang, Yun Wang, Fangzhi Li, Wenpu Huang, Guanyi Jing, Zemin Wang, Xiaoyan Zeng. Weldability, microstructure and mechanical properties of laser-welded selective laser melted 304 stainless steel joints[J]. J. Mater. Sci. Technol., 2019, 35(9): 1817-1824.

Figures/Tables 16

Table 1 Chemical compositions of 304 stainless steel powders (wt%).

C	Cr	Ni	Mo	Mn	Si	P	S	Fe
≤0.07	17.0-19.0	8.0-11.0	≤0.03	≤2.0	≤1.0	≤0.035	≤0.03	Bal.

Fig. 1. Morphology of metal powders (a) and macro-morphology of SLMed plates (b) of 304 stainless steel.

Table 2 Parameters of laser welding.

Laser power	Welding speed	Laser defocused distance	Gas flow rate
3000 W	33.3 mm/s	+2 mm	15 L/min

Fig. 2. Schematic drawings of SLMed plates (a) and different laser welding types (b).

Fig. 3. Surface morphologies of laser-welded joints under types 1 (a, b), 2 (c, d) and 3 (e, f) in front (a, c, e) and back (b, d, f) views.

Fig. 4. Cross-section morphologies of joints under type 1 (a), 2 (b) and 3 (c).

Fig. 5. XRD patterns of SLMed sample and three laser-welded joints.

Fig. 6. Three-dimensional optical microscopy composite view (a) and microstructures of stress-relieved SLMed 304 stainless steel on XOY (b, c) and XOZ (d, e) sections.

Fig. 7. Microstructures of laser-welded joints under type 1 (a, b), 2 (c, d) and 3 (e, f).

Table 3 Dendrite arm spacing and calculated cooling rate of SLMed sample and three laser-welded joints at various locations.

Zone	Weld type	Dendrite arm spacing, DAS (μm)	Cooling rate, ? (K/s)
Center zone (CZ)	Type 1	3.6	1.2 × 10⁴
	Type 2	3.7	1.1 × 10⁴
	Type 3	4.0	0.8 × 10⁴
Fusion zone (FZ)	Type 1	4.6	5.7 × 10³
	Type 2	4.9	4.7 × 10³
	Type 3	5.5	3.7 × 10³
Epitaxial zone (EZ)	Type 1	2.3	4.7 × 10⁴
	Type 2	2.7	2.9 × 10⁴
	Type 3	2.7	2.9 × 10⁴
Base metal	-	0.3	$\widetilde{2}$.6 × 10⁷

Fig. 8. EBSD IPF figure (a) and grain size distribution (b) of laser-welded joint.

Fig. 9. Microhardness of laser-welded joints under three types.

Fig. 10. Tensile properties of 304 stainless steel samples under various processes.

Fig. 11. Photos of tensile test samples after tensile tests (a) and OM image showing fracture path of laser-welded joint under type 2 (b).

Fig. 12. Tafel plots (a) and potentiodynamic curves (b) of 304 stainless steel samples under various processes in 3.5 wt% NaCl solution.

Fig. 13. Corrosion potential and corrosion current density (a), corrosion rate (b), and pitting potential (c) of 304 stainless steel samples in 3.5 wt% NaCl solution.

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