Empirical methodology to determine inherent strains in additive manufacturing

Abstract

Part distortion is a critical issue during Additive Manufacturing (AM) of metallic parts since it prevents this technology from being implemented at industrial level. To this regard, distortion prediction even from design stage has become crucial. Actually, numerical modelling methodologies play an important role here. Different modelling approaches have been developed but one of the most computationally efficient methodology to predict distortion is the so called inherent strain method. In this work an empirical methodology to determine inherent strains is presented. This is the input data in simplified Finite Element (FE) models in order to predict distortion and residual stress fields. These inherent strains are calculated considering layer lumping strategies that might be adopted in the numerical model as well. The procedure has been developed and validated using the wellknown twin-cantilever beam structure. Ti-6Al-4V beams have been manufactured by LPBF technology following different scanning strategies. Distortion after support removal has been measured in order to be compared against numerical results. The methodology has been applied at coupon level giving accurate results and providing a preliminary validation.

CMT-Based Wire Arc Additive Manufacturing Using 316L Stainless Steel: Effect of Heat Accumulation on the Multi-Layer Deposits

“…where T is the unknown temperature, C p is the heat capacity, ρ is the mass density, k is the conductivity, t is the time, and q is the heat input. C p , ρ, and k are temperature-dependent variables; the values for STS316L were applied with reference to existing literature [13,14]. Furthermore, the latent heat required for phase change was considered by modifying C p according to the temperature.…”

Design against distortion for additive manufacturing

“… A thin layer of elements (100µm) was modelled in the FE mesh of the impeller, in ABAQUS, to account for surface residual stresses induced by the machining tool.  Referring to estimated induced stresses due to machining in literature [12], stated to be in the region of 500MPa at the surface for stainless steel 316L, a uniform in-plane tensile stress profile of 500MPa (in both transverse directions to the surface) was mapped onto the modelled 100µm thickness of material in ABAQUS using published mapping techniques [13]; this mapping assumption was validated against OMI distortion, measured at three points, as stated in section 2.3.  The residual stresses due to AM and the estimated induced stresses due to machining in the FE model were then allowed to relax in ABAQUS and achieve a new state of distortion and residual stresses that took both AM and machining into account.…”

“…Li et al [11] describe the modelling of SLM by applying the inherent strain method to four different scanning strategies, validating their numerical work against experimental results. Setien et al [12] also applied the inherent strain method to simulate distortion in an AM component, highlighting the high efficiency that can be achieved by such an approach. Liang et al [13] numerically simulated distortion in depositions of a single wall and a rectangular contour wall models with different number of layers deposited by a representative directed energy deposition (DED) process.…”

Understanding powder bed fusion additive manufacturing phenomena via numerical simulation

“…In other words, by imposing the inherent strain as plastic strain tensor on the part and by imposing the equilibrium conditions it is possible to obtain the residual stress field with a linear elastic analysis. With application to AM process simulation, this approach involves calculating the inherent strain using experiments [154] or a meso-scale model with a sufficient number of layers in order to reach the steady-state conditions. The inherent strain is then applied as an initial condition over a full layer on a macro-scale simulation of the full part, layer-by-layer according to the building strategy, with the advantage to carry out at each layer activation a simple liner elastic analysis [155], thus bypassing the thermal analysis.…”