Understanding the effect of laser scan strategy on residual stress in selective laser melting through thermo-mechanical simulation

TLDR: In this article, the authors used a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in selective laser melting (SLM).

Abstract: Selective laser melting (SLM) is an attractive technology, enabling the manufacture of customised, complex metallic designs, with minimal wastage. However, uptake by industry is currently impeded by several technical barriers, such as the control of residual stress, which have a detrimental effect on the manufacturability and integrity of a component. Indirectly, these impose severe design restrictions and reduce the reliability of components, driving up costs. This paper uses a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in SLM. A complex interaction between transient thermal history and the build-up of residual stress has been observed in the two laser scan strategies investigated. The temperature gradient mechanism was discovered for the creation of residual stress. The greatest stress component was found to develop parallel to the scan vectors, creating an anisotropic stress distribution in the part. The stress distribution varied between laser scan strategies and the cause has been determined by observing the thermal history during scanning. Using this, proposals are suggested for designing laser scan strategies used in SLM.

Figures

Figure 11 - Transient probability density distribution P(T,t) for consolidated elements normalised to the scanning time of the hatched region. Distributions are shown for the alternating strategy with sizes a) 1 mm x 1 mm, b) 2 mm x 2 mm and c) 3 mm x 3 mm, and unidirectional strategy with sizes d) 1 mm x 1 mm, e) 2 mm x 2 mm and f) 3 mm x 3 mm.

Figure 16 - Profile view of surface normal stresses a) σyy and b) σxx when using unidirectional (left) and alternating (right) scan strategies

Figure 17 – Mirrored profile views of a) the temperature, b) the temperature gradient in the Y direction, and c) σyy taken at t = 0.0320s (left half) and at t = 0.156s (right half).

Frequently asked questions

Q1. What are the contributions in "Understanding the effect of laser scan strategy on residual stress in selective laser melting through thermo-mechanical simulation" ?

This paper uses a thermo-mechanical model to better understand the effect of laser scan strategy on the generation of residual stress in SLM. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Understanding the Effect of Laser Scan Strategy on Residual Stress in Selective Laser Melting through Thermo-Mechanical Simulation L. Parry, I. A. Ashcroft *, R. D. Wildman Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, UK * Corresponding author email: ian. ashcroft @ nottingham. Using this, proposals are suggested for designing laser scan strategies used in SLM. 

Q2. What is the effect of nonuniform thermal expansions and contractions in the heat affected?

The effect of nonuniform thermal expansions and contractions in the Heat Affected Zone (HAZ) result in the formation of residual stresses in the finished part. 

Q3. Why was the latent heat excluded in this work?

This was excluded in this work because accounting for latent heat requires a small enough time-step to ensure the temperature change does not overshoot this interval and is relatively insignificant compared with loss of heat through radiation. 

Q4. What is the common reason for failure of a part produced by SLM?

Parts produced by SLM generally require additional support structures to constrain the part to restrict ‘curling’ or distortion during manufacture. 

Q5. What is the potential for a performance improvement for simulating SLM?

a performance improvement for simulating SLM could be gained by modelling regions far from the melt-pool using a steady-state analysis, but retaining the transient behaviour in region near the melt-pool. 

Q6. What is the role of the model in determining the effects of the laser scan strategy?

The model is then used to determine the implications of the temperature history, created by the choice of laser scan strategy and scan area size, on the development of residual stress during selective laser melting. 

Q7. What is the potential for reducing the rate of cooling?

this will enable control over the build-up residual stress and generation of the microstructure by elevating the temperature and reducing the rate of cooling [42]. 

Q8. What is the common method of reducing residual stress?

After manufacture, the relief of residual stress requires further post processing either by heat treatment or hot isostatic pressing (HIP) [8]. 

Q9. What is the sizing effect of the scan?

Depending on the substrate (powder or solid), varying the scan area size will enable control over the sustained temperature achieved in a scanned region. 

Q10. How does the overall modelling strategy replicate the SLM process?

The overall modelling strategy attempts to replicate the SLM process by directly simulating the machine build files to enable a direct comparison with experiments. 

Q11. How did Hodge and his colleagues develop a multi-phase stress term?

Hodge et al. [18] advanced this area by incorporating a multi-phase stress term using volumetric fractions, and a phase expansion term to account for volumetric shrinkage during phase change between powder and consolidated form. 

Q12. What is the effect of the laser on the thermal mass of the scan track?

it is evident in Figure 9 that the heating effect from the laser in previously consolidated areas diminishes between three to four adjacent scan tracks, and this is also visible in Figure 12. 

Q13. What is the stress distribution in the XY plane?

The stress distributions in the XY plane are mostly dominated by the σyy component which decreased in magnitude from the start to the end of the hatched region, as shown in Figure 16(a).