Failure criterion for PA 12 multi-jet fusion additive manufactured parts

Abstract

Offering the possibility of producing complex geometries in a compressed product development cycle, it comes as no surprise that additive manufacturing (AM) techniques have become attractive to multiple industries, including the automotive and aerospace segments. Unfortunately, the ubiquitous stratified build approach used by these technologies is responsible for the pain point that hinders their adoption in production of parts that will be subjected to complex loads: the junction of adjacent layers tends to have subpar mechanical properties when compared to those of the bulk material, and thus, assessing the structural integrity of an AM part becomes difficult. In the advent of the industrialization of series production of AM parts for the automotive industry, the necessity to understand and predict how and why AM parts fail under complex stress states becomes of paramount importance. This paper applies a failure criterion for materials with anisotropic properties with stress interactions, to predict failure of multi-jet fusion (MJF) parts manufactured using polyamide 12 powder. The results are compared to the failure surfaces of Selective Laser Sintering (SLS) components. Special test specimens were designed, produced, and tested to measure failure under tensile, compressive, shear, and combined loading scenarios. The results show that much like SLS, MJF parts have a notable difference in tensile and compressive strengths. Unlike SLS however, MJF parts do not exhibit a strong interaction between stresses when under combined loading. The experimental data shows an excellent fit with the failure criterion, precisely capturing the strength behavior of MJF printed parts under complex loading conditions. Of great interest in this study is that the stress interactions with MJF parts were determined to be negligible when compared to SLS specimens, which emphasizes the fact that when performing stress analyses, each one of these powder-based additive manufacturing techniques must be treated differently.

Introduction

Additive manufacturing industry has experienced extensive growth in recent years, which has led to an increased demand for components to be implemented in load bearing structures [1]. This has prompted the need to predict the strength of critical parts, which, for additive manufacturing depends on the orientation of the build due to the anisotropy, as well as the chosen additive manufacturing technology [2]. For example, fused filament fabrication (FFF) is significantly stronger in the printing direction than across the layers [3], [4], [5], [6]. However, a review of the literature related to mechanical properties and anisotropy, and the relation of properties to processing conditions, of FFF parts shows that such work has primarily been in experimental characterization of either strength [7], [8], [9], [10], [11], stiffness [12] or both [13], [14], [15], [16], [17]. Recently, Cuan-Urquizo et al. [6] published a review paper of the experimental characterization as well as modeling work of the mechanical properties of FFF parts.

Uniaxial test results for selective laser sintered parts using polyamide 12 have also been reported by several researchers for neat resins [18], [19], [20], [21] as well as filled composite materials [22], [23]. Less work has been reported on the strength of parts made using the more recently developed multi-jet fusion technique [24], [25], [26]. Powder bed fusion technologies such as selective laser sintering (SLS) or now multi-jet fusion (MJF), while being highly regarded for their strength and mechanical properties in general, also experience varying degrees of anisotropy [27]. SLS parts, for example, exhibit a significant reduction in tensile strength when tested in the Z direction when compared to the X or Y directions. MJF on the other hand, while not achieving the tensile strength of SLS in the X and Y direction, does not lose tensile strength when loaded in the Z direction [28]. Sillani et al. [29] compared SLS and MJF additive manufacturing techniques. From their tensile and notched impact tests, they concluded that SLS parts were stiffer but more brittle than MJF parts.

Although significant research has been done in recent years in the area of mechanical behavior of additive manufactured parts, only recently, have there been attempts to develop a failure criterion [30]. Of the studies that did manage to apply a failure criterion, all but two, did so without differentiating between tension and compression. Of these studies, the majority did not include compressive strength in the analysis [31]. The goal of this research is to build upon the work performed by Obst et al. [32] and Mazzei Capote et al. [33], where the model by Osswald and Osswald [34] was applied to specially designed SLS and Fused Filament Fabrication (FFF) coupons to test tensile, compressive, and shear strengths, as well as strength tensor stress interaction components to allow failure predictions in complex stress fields. Here, the same methodology has been applied to the MJF technology so as to better understand both its potential with respect to mechanical properties, but also to compare this relatively new technology to its more well-known powder bed fusion counterpart, SLS. These test specimens were manufactured out of polyamide 12 (HP PA12 HR) on the Hewlett-Packard (HP) Multi-jet Fusion 4200 and tested under combinations of tension, torsion, and compression, which allow for the determination of the stress interactions.