Study of the effect of residual stresses and surface roughness of additively manufactured components on the coatability and fatigue strength of the composite system

By employing additive manufacturing (AM) in general and selective laser melting (SLM) in particular, it is possible to produce metallic parts and components of high complexity. In order to reach the efficiency of conventionally manufactured parts, additively produced parts must fulfil at least the same requirements. Therefore, basic investigations for coating and composite systems are essential to obtain a comprehensive understanding of the process-microstructure-mechanical properties of IN 718 and 316L alloys processed by SLM.

Surface quality after SLM
Surface quality of SLM process is still a key issue during the fabrication of the metallic parts. The surface roughness depends not on the process parameter itself, but also on the orientation of the parts in the building chamber. The surface roughness of AM parts has been in the focus of several studies. However, only little research has been performed to characterize the impact of the surface roughness on the coatability of AM parts as well as on the fatigue strength of the overall composite system. Therefore, a deep understanding of the surface properties after SLM is required in determining their effects on the mechanical properties and in designing components with improved performance.

Residual stresses
In order to apply AM components under conditions comparable with conventionally produced components in the future, equivalent prerequisites must be created with regard to the post-treatment and the further manufacturing process steps. Most of the metallic components used in high performance conditions, under heat, wear, or corrosive load, are enhanced by adapted heat treatments (e.g. hardening) or (protective) coatings. Through targeted surface functionalization by means of coatings and the adaptation of the layers to the substrate material, the performance characteristics of the entire component are decisively influenced. Most of the components produced by SLM are intended for applications with high mechanical, thermal or corrosive loads. Residual stress can be found in components manufactured by SLM. Therefore, in this project the reason for the evolving of residual stresses and their influence on the coatability of AM parts are explored. Different SLM specimens will be coated by means of High Velocity Oxy-Fuel (HVOF) spraying, Physical Vapour Deposition (PVD) as well as Atmospheric Plasma Spraying (APS). Afterwards, experimental methods are used to measure the residual stress profiles in a set of test specimens with different conditions.

Microstructural characterization
The microstructure evolution in SLM material strongly influences the resulting mechanical properties such as strength, ductility, hardness etc., and consequently has to be thoroughly studied. Especially, the microstructure stability under high temperature quasi-static and cyclic loading for IN 718 is of high interest, as their stability is crucial for the envisaged applications. The stability is significantly influenced by precipitates formed artificially by aging or during thermo-mechanical testing.

Mechanical Testing
Mechanical testing will be performed under different loading conditions. In a first step, the characterization of the residual stresses profiles in various specimen conditions will be defined. Consequently, the mechanical tests will be extended to different coated specimen and loading conditions. The main focus will then be in characterization of the overall composite system under cyclic loading at elevated temperatures. It will be of high of interest to clearly show the influence of residual stresses and surface roughness on the crack initiation.