Non-dimensional process maps for residual stress in laser directed energy deposition

Abstract

The nature of the residual stress and substrate melting produced by laser directed energy deposition (DED) affects the quality and reliability of the additively manufactured part. The residual stresses are induced because of temperature gradients, differences in thermal expansion coefficients and the elastoplastic behavior of the deposit / substrate material. In addition, the high cooling rate in DED can lead to additional strains, such as transformation induced plasticity and volumetric dilatation. If tensile residual stress is induced in the deposited layer, it may compromise the service life due to accelerated fatigue. The residual stress evolution depends on the process parameters and selection of right process window can yield favorable residual stresses. A validated metallo-thermomechanical finite element model for directed energy deposition is used to predict the temperature distribution and residual stress for different process conditions. The model predictions at different process conditions have been used to develop a non-dimensional empirical relationship between the residual stress and the process parameters via a nonlinear regression analysis. Isopleth of residual stress free (~0 stress) state at the deposition-substrate interface is employed to divide the process parameter subspace into two distinct domains: a detrimental tensile zone and a desirable compressive zone. It may also be noted that in order to form a sound metallurgical bond a minimal dilution (Dmin) is required which can ensure that the entire deposition melts with minimal substrate melting. Isopleth of normalized dilution of unity (D/Dmin= 1) can produce sound metallurgical bond between the deposited layer and substrate. These two isopleths in combination are crucial in identifying favorable and unfavorable domains. Such novel process maps serve as a theoretical framework in the choice of suitable process parameters to ensure the quality and integrity of the deposition.