Abstract
High cooling rates within the selective laser melting (SLM) process can generate large residual stresses within fabricated components. Understanding residual stress development in the process and devising methods for in-situ reduction continues to be a challenge for industrial users of this technology. Computationally efficient FEA models representative of the process dynamics (temperature evolution and associated solidification behaviour) are necessary for understanding the effect of SLM process parameters on the underlying phenomenon of residual stress build-up. The objective of this work is to present a new modelling approach to simulate the temperature distribution during SLM of Ti6Al4V, as well as the resulting melt-pool size, solidification process, associated cooling rates and temperature gradients leading to the residual stress build-up. This work details an isotropic enhanced thermal conductivity model with the SLM laser modelled as a penetrating volumetric heat source. An enhanced laser penetration approach is used to account for heat transfer in the melt-pool due to Marangoni convection. Results show that the developed model was capable of predicting the temperature distribution in the laser/powder interaction zone, solidification behaviour, the associated cooling rates, melt-pool width (with 14.5% error) and melt-pool depth (with 3% error) for SLM Ti6Al4V. The model was capable of predicting the differential solidification behaviour responsible for residual stress build-up in SLM components. The model-predicted trends in cooling rates and temperature gradients for varying SLM parameters correlated with experimentally measured residual stress trends. Thus, the model was capable of accurately predicting the trends in residual stress for varying SLM parameters. This is the first work based on the enhanced penetrating volumetric heat source, combined with an isotropic enhanced thermal conductivity approach. The developed model was validated by comparing FEA melt-pool dimensions with experimental melt-pool dimensions. Secondly, the model was validated by comparing the temperature evolution along the laser scan path with experimentally measured temperatures from published literature.
Highlights
Additive manufacturing (AM) techniques form threedimensional components directly from a digital model by joining materials layer by layer [1, 2]
The developed isotropic enhanced thermal conductivity model for selective laser melting (SLM) Ti6Al4V treated the laser as a penetrating volumetric heat source and was capable of predicting the melt-pool width and melt-pool depth
Considering enhanced laser penetration to account for heat flow in the melt-pool due to Marangoni convection is a valid approach for modelling the SLM Ti6Al4V melting behaviour
Summary
Additive manufacturing (AM) techniques form threedimensional components directly from a digital model by joining materials layer by layer [1, 2]. The expanded geometric freedom of the process, low material wastage and rapid product development cycles make these technologies attractive to a variety of industries [2]. Due to the rapid heating and cooling cycles of successive layers, large thermal gradients are generated which in turn can create high residual stresses within fabricated components [3]. The process-induced residual stresses may lead to in process part failure due to geometric distortion, built-in cracking or premature failure of parts subjected to alternating loading or corrosive environments [3,4,5,6,7,8,9]. The complex nature of the layer-bylayer building process and thermal cycling requires a robust understanding of the numerous physical phenomena associated with the selective laser melting (SLM) process in order to be able to control residual stress and improve the quality of parts [10].
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