PSI - Issue 8

Gianni Nicoletto / Procedia Structural Integrity 8 (2018) 184–191 Author name / Structural Integrity Procedia 00 (2017) 000–000

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control mode at R ≈ 0, the variability is due to: i) the SLM system process parameters; ii) specific surface conditions (i.e. as-built surface, machined, sand blasting etc.) and iii) post-fabrication heat treatments (i.e. no treatment, stress relief, high temperature HT, Hot Isostatic Pressing, etc.). Fig. 1 shows that the worst condition is associated to the as-fabricated SLM material and the best is achieved after optimized heat treatment and surface machining. It is noteworthy that the best performance of SLM Ti6Al4V is comparable to conventional wrought Ti6Al4V.

Fig. 1. Variability of fatigue data for SLM Ti6Al4V tested at R=0, (from Li et al. (2016)).

2.2. About the costs of metal additive manufacturing Cost allocation in the SLM process is difficult. In a recent exercise, Ray (2017) examined the issues of a fictional satellite bracket manufactured using the DMLS process applied to Ti6Al4V alloy: assumptions and issues are used here to discuss the costs of specimen fabrication for the fatigue characterization of SLM metals. In general, the costs associated with additively manufactured components can be broken into four distinct sections—design, production, post-processing and qualification. There are also overhead costs (utilities, rent, shipping, etc.) associated with each step of this process, as well as consumable items (inert gas, filters, protective equipment, etc.). In the case of the production of a batch of fatigue specimens, design and qualification costs can be neglected as the geometry is fixed and process qualification is the objective of the experiments. The main costs are therefore those of specimen production including consumables. Production costs can be broken down into the following areas: materials, setup (part design and machine), machine run time, and final build plate removal. Here additive build failure is not considered assuming a simple-to-fabricate geometry. As far as materials, the cost of powdered metal is significantly higher, in some cases an order of magnitude higher than bar stock, for several reasons including: energy intensive production process (gas atomization); purity; and required powder sphericity and particle size uniformity. The cost for powdered TI6Al4V can range from €200 to €400 per kilogram according to Ray (2017). Material costs are dependent on the component's weight and the scrap rate of powder that cannot be recycled. Part of the setup cost is associated with build area/platform management by an experienced operator. As far as machine run time, the hourly rate on an AM system is determined by the upfront and maintenance costs associated with it, the expected capacity utilization rate, desired profit and payback period or useful life. The cost breakdown in Fig. 2 as proposed by Ray (2017) shows that, by far, the most significant