PSI - Issue 13

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 311–316 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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powder in contrast to pure EN AW-7075 particle size distributions (PSDs) were determined using M ALVERN M ASTERSIZER 2000. Comparing both PSDs the mixed alloy shows a higher content of fine particles. The EN AW-7075 alloy shows a mean particle size of about 43.5 µm while the mixed alloy has 37.8 µm (see Fig. 2a). To analyze the particle shape scanning electron microscope (SEM) images are taken. Regarding the particle shape, the aluminum alloy EN AW-7075 (see Fig. 2b) exposed several spherical as well as longitudinal particles. There are also adhesions at the main particles, which have an irregular form. In Fig. 2c the particle shape of the manufactured mixed aluminum alloy is shown.

Fig. 2. (a) Particle size distributions (b) particle shape of aluminum alloy EN AW-7075 (c) particle shape of mixed aluminum alloy.

In a first step cubes (10 mm x 10 mm x 10 mm) are manufactured on a SLM 280 HL (SLM Solutions Group AG) system with a build chamber of 280 mm x 280 mm x 350 mm, a 400 W Yttrium laser and in inert gas atmosphere. The platform was pre-heated to 673.15 K, according to Brüggemann et al (2017). Following SLM processing the chamber was flooded with nitrogen and the oxygen content is reduced below 0.2 %. In order to find the best set of parameters for high-densed structures the four process parameters of the volumetric energy density (laser energy P , scan speed v , hatch distance h , layer thickness t ) were varied, see equation 1.

vol E P 

(1)

v h t  

The valuation of the best parameter set is performed by density measurements with optical micrographs. For the optical micrographs, each specimen was mechanically polished and as appropriate etched. With this detected set of parameters tensile specimens based on the norm DIN 50125 (2008) and CT specimens with a thickness of 2.5 mm, a width of 20 mm and a V-shaped notch at a length of 4 mm according to the ASTM 647-08 (2008) standard were manufactured to examine the mechanical and fracture mechanical properties. Two different material conditions were investigated. The condition “as - built” corresponds to the state immediately after the laser melting process without any subsequent heat treatment. In order to achieve a better mechanical performance and a comparability to the conventionally manufactured EN AW-7075 T651 alloy, the other specimens were heat treated, see Holt et al. (2000). These specimens were solution annealed for 1.5 hours at 753.15 K following by rapid quenching in water (293.15 K) and then aged for 6 hours at 443.15 K. A universal testing machine, I NSTRON 5569, was used for the characterization of the quasi-static properties. The tensile tests with a minimum of three specimens for each condition were conducted displacement controlled with a crosshead speed of 5 mm/min according to the DIN EN ISO 6892-1 (2009) standard. The elongation at room temperature (293.15 K) was measured using an optical extensometer. For determination of the fracture mechanical properties by fatigue crack growth experiments an I NSTRON testing machine, Electro Puls E10000, was chosen. The tests were performed at ambient conditions under mode-I loading. A minimum of three specimens was tested in each test series. The sample was set to a sinusoidal loading at a stress ratio of R = 0.1 The direct current potential drop method was utilized with the measurement system M ATELECT