Down-facing surfaces in laser powder bed fusion of Ti6Al4V: Effect of dross formation on dimensional accuracy and surface texture

Amal Charles,Veit Hagenmeyer,Steffen Scholz,Umberto Paggi,Ahmed Elkaseer,Lore Thijs

doi:10.1016/j.addma.2021.102148

Abstract

Down-facing surfaces are one of the most challenging features in metal parts produced by laser powder bed fusion (LPBF). A combination of reasons, primary of which are residual stresses and overheating cause these features to have the worst surface finish and dimensional accuracy of all LPBF surfaces. In order to examine this phenomenon, a Design of Experiments (DoE) study is conducted for three different inclination angles, namely 45°, 35° and 25° and for two different layer thicknesses of 60 µm and 90 µm. The results from the DoE are used to establish quadratic regression equations that can be used to predict the quality marks of surface roughness and the relative dimensional error.This fundamental investigation helps to explain the reasons for the major defects in down-facing surfaces of parts produced with Ti-6AL-4 V material, namely the dross formation and attempts to improve the predictability of quality within the region. Further to the establishment of the quadratic equations, a discussion is conducted on the thermomechanical processes involved in the mechanism of dross formation and explanations are given on the reasons behind the observed physical phenomena. The trend of the propagation of (Root Mean Square) RMS Surface roughness (Sq) and the relative dimensional error with respect to the Volumetric Energy Density (VED) is discussed in detail. The respective quadratic equations are then tested by a second round of validation prints, and the results confirm the feasibility of the developed quadratic models to accurately predict process outcomes especially when operating near the suggested optimal printing zones. The high roughness of low VED printing is attributed to the formation of ‘inverse mushroom’ structures, and the low roughness of high VED surface is attributed to the formation of large flat regions formed as adjacent meltpools that can fuse together at various locations.

Highlights

Additive Manufacturing (AM) technologies refer to the processes of layer-by-layer manufacturing that are creating a new paradigm in the way parts are produced and value is added to new/modified products [1]
The results from the Design of Experiments (DoE) are used to establish quadratic regression equations that can be used to predict the quality marks of surface roughness and the relative dimensional error.This fundamental investigation helps to explain the reasons for the major defects in down-facing surfaces of parts produced with Ti-6AL-4 V material, namely the dross formation and attempts to improve the predictability of quality within the region
Volu metric Energy Density (VED) can be used as it gives an indication on the degree of over heating within the down-facing area which directly relates to dross formation

Summary

Introduction

Additive Manufacturing (AM) technologies refer to the processes of layer-by-layer manufacturing that are creating a new paradigm in the way parts are produced and value is added to new/modified products [1]. This is largely due to the benefits offered by AM technology in terms of both scale and scope [2,3]. Down-facing surfaces (as seen in Fig. 1) of the L-PBF process are especially critical. Patterson et al [6] studied 143 different publications related to the L-PBF process and concluded that only a small minority deals with the over hanging problem, where only limited fundamental work has been done

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