Effect of Annealing Temperature and Strain Rate on Mechanical Property of a Selective Laser Melted 316L Stainless Steel

doi:10.1007/s40195-021-01342-x

Abstract

Abstract:

In the present work, 316L stainless steel specimens are fabricated by selective laser melting (SLM) via optimized laser process parameters. The effects of two extrinsic factors, i.e., strain rate and annealing temperature, on the mechanical performance of SLM-processed parts are studied. The two intrinsic factors, namely strain rate sensitivity m and work hardening exponent n, which control the tensile properties of the as-built samples, are quantified. Microstructure characterizations show that cellular structure and crystalline grain exhibit apparently different thermal stability at 873 K. Tensile testing reveals that the yield strength decreases from 584 ± 16 MPa to 323 ± 2 MPa, while the elongation to failure increases from (46 ± 1)% to (65 ± 2)% when annealing temperature varies from 298 K to 1328 K. The n value increases from 0.13 to 0.33 with the increase in annealing temperature. Due to the presence of fine cellular structures and high relative density achieved in as-printed 316L samples, a strong dependence between tensile yield strength and strain rate is observed. In addition, the strain rate sensitivity of the SLM-produced 316L part (m = 0.017) is much larger than that of conventional coarse-grained part (m = 0.006), whereas the n value increases slightly from 0.097 to 0.14 with increasing strain rate.

Key words: Selective laser melting, 316L stainless steel, Heat treatment, Strain hardening behaviors, Mechanical property

Hua-Zhen Jiang, Zheng-Yang Li, Tao Feng, Peng-Yue Wu, Qi-Sheng Chen, Shao-Ke Yao, Jing-Yu Hou. Effect of Annealing Temperature and Strain Rate on Mechanical Property of a Selective Laser Melted 316L Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(5): 773-789.

Figures/Tables 16

Fig. 1 a Morphology of the gas-atomized powders, b image of the SLM-produced 316L samples under argon protection, c a sketch of the heat treatment process, d laser scanning strategy, and e sketch of the as-fabricated specimens which the tensile specimen extraction scheme and the position selected for microstructure observation are clearly shown

Table 1 Chemical composition of as-used 316L SS powder (wt.%)

Fe	Cr	Ni	Mo	Mn	Si	C	P	S
Bal	18.84	10.68	2.26	1.05	0.91	≤ 0.03	≤ 0.04	≤ 0.01

Table 2 Heat treatments applied to the 316L SS samples

Samples	Heat treatment conditions
As-built	-
(Heat treatment 1) HT1	873 K + 2 h, furnace cooling
(Heat treatment 2) HT2	1123 K + 2 h, furnace cooling
(Heat treatment 3) HT3	1328 K + 2 h, furnace cooling

Fig. 2 Optical microstructures of heat-treated samples at temperatures of 1328 K, 1123 K, and 873 K, respectively. a, c, e Top view; b, d, f side view

Fig. 3 SEM images of SLM-processed samples heat-treated at various temperatures

Fig. 4 EDS analysis result of the white particles

Fig. 5 Grains maps of as-built and annealed 316L SS samples. The grain maps on the side surfaces are similar to each other; hence only one map paralleling to build direction is presented in these figures. The grain size distribution is obtained from the top surface in the image. a As-built, b 873 K, c 1123 K, d 1328 K. e, f, g, h Corresponding grain size distributions for as-built, 873 K, 1123 K, and 1328 K sample, respectively

Fig. 6 Summary of average grain size and fraction of LAGBs as a function of annealing temperature. a Average grain size, b fraction of LAGBs

Table 3 Summary of grain size changes as a function of the annealing temperature

References	Conditions	Grain size
^aMontero-Sistiaga et al. [36]	The as-built samples are heat-treated under argon atmosphere at 600 °C and 950 °C for 2 h, followed by air cooling, whereas the 1095 °C annealed samples are water cooled	The grain length is ~ 100 μm. There is no difference in grain size in comparison with the as-built samples
^aDing et al. [37]	The as-produced samples are annealed for 3 h at 400 °C and 900 °C, followed by furnace cooling	The difference in grain size between 400 °C annealed sample and as-built sample is little, whereas the grain size is refined after annealing at 900 °C
^bO.O. Salman et al. [27]	The as-built samples are heat-treated under argon atmosphere at 573 K, 873 K, 1273 K, 1373 K, and 1673 K for 6 h	The average grain size increases from 45 μm, 50 μm, 55 μm, 65 μm, 88 μm to 102 μm as annealing temperature increases from 573 K to 1673 K
^cChen et al. [25]	The as-produced samples are annealed for 1 h at 400 °C and 800 °C, followed by water cooling	The average grain size slightly decreases from 5.9 μm to 5.4 μm as annealing temperature increases
^bVoisin et al. [28]	The as-built samples are annealed every 200 °C from 400 °C to 1200 °C for 1 h	The average grain size remains nearly the same up to 800 °C. Overall, the grain size shows an increasing trend (from ~ 9 μm to ~ 18 μm) when annealing temperature is raised from room temperature to 1200 °C
^aSun et al. [15]	The as-built samples are annealed at 650 °C in vacuum condition for 2 h, followed by furnace cooling	There is little sign of grain growth for SLM-produced samples

Fig. 7 Band contrast maps of as-built and annealed 316L SS samples. The misorientation angle distribution is obtained from the top surface in the image. A misorientation angle between 2° and 5° is colored in red, 5°-15° is colored in green, whereas 15°-180° is colored in blue. a As-built, b 873 K, c 1123 K, d 1328 K. e, f, g h Corresponding misorientation angle distributions for as-built, 873 K, 1123 K, and 1328 K sample, respectively

Fig. 8 Effect of annealing temperature on the mechanical property of SLM-processed parts. a and b Representative engineering tensile stress-strain curves and the corresponding true stress-true strain curves at various annealing temperatures. c and d Summary of tensile properties and microhardness of SLM-produced parts as a function of annealing temperature, respectively. e Variation of yield strength with annealing temperature including our work and data reported in the literature. The blue dashed lines indicate the general variation trend. f Ultimate tensile strength as a function of annealing temperature based on reported values in the literature. g Hardness as a function of annealing temperature for our work and researches reported in the literature. h Comparison between the obtained mechanical properties in this study and wrought 316L (including cold finished and solution treated) reported in the literature. The red dashed lines indicate the general trend of mechanical response for samples annealed at different temperatures. The above data are collected from references [2,7,19,22,25,26,28,35,36,37,39,40,41,42,43,44,45,46,47]

Fig. 9 Effect of annealing temperatures on strain hardening behavior of SLM-produced parts. a Normalized work hardening rate curves as a function of annealing temperature, b logarithmic plots of true stress versus true strain for the SLMed parts at various annealing temperatures

Fig. 10 Fracture surfaces of SLMed 316L SS at different annealing temperatures

Fig. 11 a Representative engineering tensile stress-strain curves at different strain rates. b Summary of tensile properties of SLM-produced part at different strain rates. c Logarithmic plots of true stress versus true strain for the SLMed material at different strain rates. d Logarithmic plots of tensile strength versus strain rate for the SLMed material

Fig. 12 Optical micrographs of polished sections showing porosity in as-built sample

Fig. 13 Fracture surfaces of SLM-produced parts at different strain rates

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