Defect Formation Mechanisms in Selective Laser Melting: A Review

Abstract

Defect formation is a common problem in selective laser melting (SLM). This paper provides a review of defect formation mechanisms in SLM. It summarizes the recent research outcomes on defect findings and classification, analyzes formation mechanisms of the common defects, such as porosities, incomplete fusion holes, and cracks. The paper discusses the effect of the process parameters on defect formation and the impact of defect formation on the mechanical properties of a fabricated part. Based on the discussion, the paper proposes strategies for defect suppression and control in SLM.

Review of defects in lattice structures manufactured by powder bed fusion

“…Pores can also form through the use of hollow powder particles and through ridges formed in previous layers which impede the flow of the melt pool. Incomplete fusion defects are explained by Zhang et al [27] to occur mainly due to insufficient input energy and thus often form as irregular voids containing un-melted powders ( Fig. 4b).…”

“…There are existing reviews in the field of cellular solids which focus on reporting their design, analysis and manufacturing methods [2,4,23]. This paper considers defect formation from the perspective of lattice design, as opposed to process control, which is the focus of other AM reviews, such as [18,27,28]. To refine the discussion on lattice structure defects, several questions were posed, as shown below.…”

“…The heating, melting and cooling mechanisms within the powder bed are at the core of the final quality of the parts produced by PBF and are discussed within several review papers on defects in PBF processes. From [18,27,49], three main categories of defects were identified: porosity/incomplete fusion, residual stresses and surface defects. Porosity is defined by Zhang et al [27] as approximately spherical voids with diameter typically less than 100 μm ( Fig.…”

“…Residual stresses are formed through PBF processes due to the high thermal gradients and cooling rates within the powder bed subjecting the part to rapid expansion and contraction. Residual stresses can cause cracks to form [27], delamination between layers [18] (Fig. 4c) and dimensional inaccuracies through shrinkage or warping [18].…”

Effect of heat treatment on fatigue crack initiation of laser powder bed fusion stainless steel 316L

“…For the as-built condition (Figure 3a), small isolated pores with equivalent circular diameters on the order of 5 μm are present. Pores of this nature are unavoidable for the L-PBF process due to the entrapment of gas bubbles, which could have originated from the evaporation of low melting point elements or oxygen gas pre-existing in the powder feedstock [31]. Relative to the grain boundary defects in Figure 2b, such process- induced pores are smaller in size and could have incurred weaker stress concentration under the cyclic loading, such that they were not critical in causing crack initiation for samples in the as-built condition.…”