Effect of printing orientation on anisotropic properties in resistance spot welded 316L stainless steels via selective laser melting

Accepted Manuscript Effect of Printing Orientation on Anisotropic Properties in Resistance Spot Welded 316L Stainless Steels via Selective Laser Melting Cheng Luo, Yansong Zhang PII: DOI: Reference:

S0167-577X(19)31073-0 https://doi.org/10.1016/j.matlet.2019.07.087 MLBLUE 26458

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

28 May 2019 17 July 2019 22 July 2019

Please cite this article as: C. Luo, Y. Zhang, Effect of Printing Orientation on Anisotropic Properties in Resistance Spot Welded 316L Stainless Steels via Selective Laser Melting, Materials Letters (2019), doi: https://doi.org/ 10.1016/j.matlet.2019.07.087

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Effect of Printing Orientation on Anisotropic Properties in Resistance Spot Welded 316L Stainless Steels via Selective Laser Melting Cheng Luo, Yansong Zhang Shanghai Key Laboratory of Digital Manufacture for Thin-Walled Structure, Shanghai Jiao Tong University, Shanghai 200240, P.R. China *Corresponding author: Yansong Zhang; E-mail address: [email protected]; Tel. 86-021-34206288

Abstract: The introduction of additive manufactured steels on automotive industry unavoidably involves the resistance spot welding between materials. Due to rapid heating and cooling rates of welding process, the fusion zone shows anisotropic properties in scanning and building directions. This paper aims to investigate the microstructure and mechanical properties of different orientations in the fusion zone of resistance spot welded 316L stainless steels using SEM, EBSD and hardness tests. Microstructure in the building direction exhibited more austenite-to-ferrite transformation, leading to the change of grain orientation of austenite structure from <101> to <111>. All of these result in inferior hardness in comparison with scanning direction. The scanning direction show better mechanical properties than building direction in the spot welds. Keyword: Selective laser melting; Microstructure; Mechanical properties; 316L stainless steel; Welding;

1. Introduction Selective laser melting (SLM) process has attracted industries/researches where it offers fully dense components to meet the standard requirement of bulk material [1-3]. Due to non-equilibrium solidification during printing process caused by superfast heating and melting of powders, metal components fabricated by SLM method have inherent anisotropic characteristics [4-7]. Researches concerning anisotropic properties of SLM-fabricated metal components have been reported [8-10]. One of the important features that is intrinsic to final products is the microstructural difference. Different laser energies and temperature gradient lead to the variation of microstructures in building and scanning directions [11]. 1

Another importance feature is the considerable large and anisotropic distribution of residual stress. Mercelis [12] disclosed that two zones of tensile stresses were formed at top and button of final products, and a large zone of compressive stress was solidified in between along the building direction. Microstructure differences and variation of residual stress drive the anisotropic mechanical properties in SLM-produced steels, both in hardness and tensile properties [13, 14]. AlMangour [15] found that rotating scanning directions by 90° could weaken the anisotropic properties of 316L stainless steels via SLM. Meanwhile, AlMangour [16] improved the mechanical properties of SLM-printed 316L stainless steels by introducing TiB2 nanoparticles. Kurzynowski [17] found that laser energy density and scanning strategy have strong effect on the microstructure and yield strength of SLM-produced 316L stainless steels. Alsalla [18] found that highest fracture toughness was occurred in the building direction of selective laser melted 316L stainless steel. Resistance spot welding (RSW) is one of the most predominant technique across automotive industry. Similar to conventional fabricated steels, the new materials should be weldable. For SLM-printed steels, cellular structure is essential components for efficient interconnected electron transportation [19]. The applied stresses before heating process was recognized to improve constraints for phase transformation [20]. It should be anticipated that the properties in the spot welds will be significantly different from conventional fabricated steels. Nevertheless, no detailed work has focused on the spot welds of SLM-produced steels. This paper investigates the microstructure evolution of resistance spot welded stainless steel via SLM process and its relation to the mechanical properties, aiming to develop a deep characterization of microstructure evolution of SLM-produced steels during welding process.

2. Experimental procedures The materials were 316L stainless steels via SLM process with a thickness of 1.5 mm. Steels were fabricated by EOS280 with 200 W continuous wave Nd:YAG fiber laser, parameters of laser power and scanning speed were 195 W and 1083 mm/s with a layer thickness of 50 μm. The chemical composition and mechanical properties of steels with horizontal printing orientation in manner of ISO 6892 are depicted in Table 1. 2

Table 1. Chemical composition and mechanical properties of stainless steel. Chemical composition

Mechanical properties

Element (wt%)

Fe

Cu

Cr

Ni

Mo

Mn

Yield strength

Tensile strength

Elongation

Stainless steel

Bal.

0.5

17-19

13-15

2.3-3.0

2.00

497 MPa

626 MPa

28 %

Spot welds were produced using a MFDC welding machine. The electrodes were Cu-Cr-Zr alloy with dimensions specified in ISO 5821. The welding parameters were referred to AWS D8.9M [21]. 100 ms of squeezing time, followed by 400 ms of welding time and 100 ms of holding time were applied with an electrode force of 3.0 kN and welding current of 10 kA. Cross sections of spot welds were mechanical polished and etched in a solution of 5 g CuCl2+100 ml HCl+200 ml CH3COOH. The microstructure was examined by JEOL JSM-7800F with operating voltage of 5 kV under second electron test conditions. EBSD analysis was conducted with an operating voltage of 20 kV and a step size of 0.1 μm using Tescan Mira 3. The hardness was measured by a Vickers hardness tester at 100 g with a dwell time of 10 s (referring to ISO 1427). The specimens selected for tensile testing were selected according to ASTM E8/E8M-08 [22] and the strain distribution during tensile process was captured by DIC device.

3. Results and Discussion The microstructure of as-printed stainless steels is depicted in Fig. 1(a). Elongated grains are observed in scanning direction and hexagonal grains and is formed in building direction. As shown in Fig. 1(b)-(c), austenite with different grain sizes are observed in scanning direction of fusion zone due to different ratios of temperature gradient (G) and grain growth rate (R). During the rapid cooling process, high temperature gradient at bottom of spot welds attributes to planar extension growth and formation of larger grain size. Ascending from the bottom, the increase of R leads to the value decrease of G/R rate, resulting in the formation of finer structure. In the case of building direction revealed in Fig. 1(d)-(e), the grains change from lath structure to elongated dendritic with the increment of building layers. Because that compressive stresses could affect the phase transformation from austenite to ferrite [20], this phenomenon could be ascribed to the instinct characteristic of unevenly distributed residual stresses in SLM-produced stainless steels. 3

(b)

(a)

(c)

2 μm 5 μm

5 μm

(d)

(e)

5 μm

2 μm

5 μm

Fig. 1. (a) Microstructure in scanning direction and building direction of as-fabricated stainless steel. (b)-(c) Microstructure in scanning direction of fusion zone. (d)-(e) Microstructure in the buidling direction of fusion zone.

Fig. 2(a)-(b) show the grain sizes distribution in fusion zone and base metal. The average grain size of fusion zone is 2 μm and it is smaller than base metal. Large temperature gradient in fusion zone leads to insufficient time for grain growth, resulting in formation of finer structure. Therefore, the variation of grain sizes in scanning direction of fusion zone could be ascribed to different temperature gradients during welding process. Fig. 2(c)-(d) displays grain orientations distribution of austenite structure in the fusion zone and base metal in scanning and building directions. In the building direction, the base metal shows preferred orientation of <101>. During the welding process, large compressive residual stress in the direction attributes to the austenite-to-ferrite transformation. Release of residual stress leads to the change of austenite orientation, resulting in preferred orientation along <111> in the fusion zone, which is the slipping direction of ferrite structure. Due to the lack of compressive stress, the austenite grains share the same same preferential orientation of <001> in the scanning directions of fusion zone and base metal.

4

(a)

(c)

0.6 0.5

Frequency

001

BD

001

SD

Min: 0.2 μm Max: 15 μm Mean: 2 μm

0.4 0.3 0.2 0.1 0.0

111

111 0

1.5

3.0

4.5

6.0

7.5

9.0

101

10.5 12.0 13.5 15.0

101

Grain size (μm)

(b)

Min: 2 μm Max: 71 μm Mean: 15 μm

(d)

001

001 001

BD

111 101

SD

111

101

Fig. 2. (a)-(b) The grain size distribution in fusion zone and base metal. (c)-(d) The grain orientations of austenite structure along scanning direction and building direction of the two zones.

Fig. 3(a)-(b) depict the hardness distribution in fusion zone and base metal of spot welds. Lower hardness is found in fusion zone due to austenite-to-ferrite transformation during welding process. Fig. 3(c) illustrates the hardness curves along scanning direction and building direction in fusion zone. As displayed, the hardness increases from 182 Hv to 217 Hv along scanning direction due to the grain refinement. Hardness decrease is observed in building direction because that more austenite-to-ferrite transformation occurs along the direction. According to the hardness curves of base metal shown in Fig. 3(d), hardness increase is also observed in the scanning direction. The same preferred austenite structure attributes to the similar hardness distribution in the scanning direction of fusion zone and base metal. In the case of building direction, the hardness increases with the distance above 0.4 mm. The phenomenon is different from the hardness decrease in the fusion zone due to the change of preferred orientation.

5

(a)

(c) FZ

HAZ

210

220 Hv220 215

275

0.8

1

210

Hardness Hardness(Hv) (Hv)

280

0.8

0.7

215

0.5

0.4

265

205

200

260

0.40.6

195 190

0.20.4

185

245

0 0

0.1

0.2

0.3

180

240

0.2

0.4

0.5

0

0.2

0.6

0.4

0.7

0.6

0.8

0.8

0.9

1.0

BD (mm) 0

(b)

200

190

195

250

0.1

0

200

255

0.3

0.2

210

205

0.60.8 SD

SD (mm)

270

0.6

Building direction Scanning direction

BM

1.01.2

0.9

220

0

0.2

0.4

0.6

190

180 185

0.0

0.2

0.8

1

1.2

BD

(d) 280

0.6

0.8

1.0

0.8

1.0

Building direction Scanning direction

FZ BM

HAZ

0.4

Distance (mm) Distance (mm)

180

270

275 Hv

0.9

275 275

1.0

0.8

0.8

270

270 270

0.8

0.7

0.7

265

0.6

SD (mm)

0.6

0.5

0.4

0.3

265 265

0.6

0.5

260

260 260

0.4

0.4

255

0.3

250

250

0.2

250 250

0.1

0.1

0

260

255 255

0.2

0.2

Hardness (Hv) Hardness (Hv)

0.9

0

0.1

0.2

0.3

0.4

0.5

0

0

0

0

0.1 0.6

0.2

0.2

0.30.7 0.4

0.4

0.5

0.6

0.6 0.8

0.7

0.8

0.8 0.9

0.9

240

245

0.0

245 245

1.0

0.2

0.4

0.6

Distance (mm) (mm) Distance

BD (mm)

Fig. 3. (a)-(b) The hardness mapping results in fusion zone and base metal of stainless steels. (c)-(d) The hardness curves along scanning and building directions of fusion zone and base metal.

Fig. 4(a) shows the strain distribution of tensile specimens printed by 0° and 90°, in which the loading direction is scanning direction and building direction, respectively. The strain concentrates between the layers in the specimen printed by 90° and the strain was scatter distributed in the specimen printed by 0°. There was a large zone of intermediate compressive stress between layers in the building direction of SLM-produced components. Compressive stress in the specimen printed by 90° facilitate phase transformation from austenite to ferrite, leading to the strain concentration between layers. Besides, the maximum strain is larger in the specimen with printing orientation of 90° due to the austenite-to-ferrite transformation. As depicted in Fig. 4(b), larger elongation and inferior tensile strength are found in the specimen printed by 90° orientation, indicating that the ductility increases at the cost of strength with printing orientation changing from 0° to 90°. Large compressive stresses in building direction could facilitate austenite to ferrite transformation, leading to the improvement of ductility. Besides, due to the phase transformation, grains changes from cubic structure in scanning direction shown in Fig. 4(d) to balling structure in building direction shown in Fig. 4(c). 6

(a)

Loading direction

0.68

BD

Loading direction

0.57

0.10

0.47

0.08

0.36

0.06

0.25

0.04

0.14

Loading direction

0.11

SD

0.04

(c)

0.02

Loading direction

0

800

(b)

Engineering stress (MPa)

0° (SD) 90° (BD)

1 μm

600

(d) 90°

400

Building direction

200

0° 1 μm

0 0

10

20

30

40

50

Engineering strain (%)

Fig. 4. (a) The strain distribution in tensile specimens with different printing orientations. (b) The engineering stress-strain curves in scanning and building directions. (c)-(d) The fractured interfaces in the two specimens.

4. Conclusions In conclusion, microstructure and mechanical properties of resistance spot welded stainless steel via SLM were characterized. Compared to scanning direction, microstructure of building direction experienced more austenite-toferrite transformation with change of crystallographic orientation due to comparable compressive stresses, leading to the inferior hardness and tensile properties. These results suggest that the SLM-produced steels could be welded along scanning direction to improve mechanical properties. The paper investigated the anisotropic properties of SLMproduced 316L stainless steel with fixed welding parameters. The effect of welding parameters on the microstructure and mechanical properties of SLM-produced steels needs further investigation.

Acknowledgements This research has been supported by National Natural Science Foundation of China (Project 51675338).

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights

1.

Fusion zone microstructure was characterized in scanning and building directions.

2.

Change of grain orientation was observed in the building direction of fusion zone.

3.

Grain refinement was found in the scanning direction of fusion zone.

4.

Spot welds generated by scanning direction show better mechanical properties.

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