Overhanging Features and the SLM/DMLS Residual Stresses Problem: Review and Future Research Need

Abstract

A useful and increasingly common additive manufacturing (AM) process is the selective laser melting (SLM) or direct metal laser sintering (DMLS) process. SLM/DMLS can produce fulldensity metal parts from difficult materials, but it tends to suffer from severe residual stresses introduced during processing. This limits the usefulness and applicability of the process, particularly in the fabrication of parts with delicate overhanging and protruding features. The purpose of this study was to examine the current insight and progress made toward understanding and eliminating the problem in overhanging and protruding structures. To accomplish this, a survey of literature was undertaken, focusing on process modeling (general, heat transfer, stress and distortion, and material models), direct process control (input and environmental control, hardware-in-the-loop monitoring, parameter optimization, and post-processing), experiment development (methods for evaluation, optical and mechanical process monitoring, imaging, and design-of-experiments), support structure optimization, and overhang feature design; approximately 140 published works were examined. The major findings of this study were that a small minority of the literature on SLM/DMLS deals explicitly with the overhanging stress problem, but some fundamental work has been done on the problem. Implications, needs, and potential future research directions are discussed in-depth in light of the present review.

High Cycle Fatigue Properties of as-Built Ti6al4v (Eli) Produced by Direct Metal Laser Sintering

“…When each layer is built, the solidified underlying layers are re-melted and subsequently cooled. The contraction of these re-melted layers upon cooling is restricted by the underlying solidified layers, imposing tension within them as they solidify [9]. The re-melting of underlying layers is repeated several times as the layers are built up, and there is a decrease in temperature with increasing depth of the layers from the surface.…”

“…The re-melting of underlying layers is repeated several times as the layers are built up, and there is a decrease in temperature with increasing depth of the layers from the surface. This temperature gradient results in a thermal stress gradient that starts from compression at the lowest layers to tension at the surface [9,10]. The stress gradient follows the temperature gradient, based on the following equation:…”

Influence of Rapid Solidification on the Thermophysical and Fatigue Properties of Laser Additive Manufactured Ti-6Al-4V Alloy

“…The transient temperature is critically important for determining the thermal stress distribution and residual stress state in additively manufactured parts. According to Patterson et al [8], thermal history generated by a laser beam during additive manufacturing governs the microstructure, properties, residual stress and distortion in fabricated components. In an attempt to overcome or reduce the evolution of residual stresses and the resultant of their unwanted artifacts, modeling and simulation of the additive manufacturing processes is a way to go instead of the time intensive and economic demanding trial and error methods.…”

Discrete Optimization of Internal Part Structure via SLM Unit Structure-Performance Database

“…According to the manufacturability requirements of SLM and the layout of bow structure unit, the paper established the structural parameter constraints of the bow structure unit, as shown in Equations (1)- (5). There are several points that are important to note.…”

“…SLM completely melts the powder material, which enables a density of approximately 100% and can maintain mechanical properties that match or even beat those of conventionally manufactured parts (cutting and casting) [4]. However, residual stress is a serious issue in SLM [5]. As a result, researchers have worked to develop solutions to this problem, such as process modeling and simulation [6], process control and post-processing methods [7,8], design and analysis of overhanging structures [9], and so on.…”