PSI - Issue 19

Wolfgang Schneller et al. / Procedia Structural Integrity 19 (2019) 556–565 Author name / Structural Integrity Procedia 00 (2019) 000–000

557

2

are identified, such as the unprocessed surface (as-built) condition as well as post treatments and residual stresses. With a rough surface topography, the fatigue strength of engineering components decreases significantly as roughness valleys act like micro-notches [7–9]. Changes to the microstructure due to post treatments like solution annealing and hot isostatic pressing (HIP) affect the fatigue crack propagation and consequently the fatigue resistance in a great measure. An applied HIP treatment not only reduces porosity, which enhances the fatigue strength, but also Fe-rich precipitates and Si-agglomerations are formed within the microstructure due to the heat input during the process [10– 12]. This microstructural evolution, which is also observed for a solution annealing treatment, positively affects the fatigue strength [12]. Furthermore, one can not neglect residual stresses within the material since such stresses can significantly alter the present stress state at the failure initiating feature [13,14]. Accounting for residual stresses is necessary in order to ensure a conservative design of safety-sensitive components. This study provides an overview of influencing factors on the fatigue strength, which should be considered within the design process of AM components. 2. Materials and Methods The used AlSi10Mg powder for specimen manufacturing shows the following chemical composition, which corresponds to the standard DIN EN 1706:2010, given in Table 1 [15].

Table 1. Chemical composition of the AM powder in weight %

Material AlSi10Mg

Si

Fe

Cu

Mn 0.45

Mg

9.0-11.0

0.55

0.05

0.20-0.45

The specimens are manufactured using an EOS M290 system with a Yb fiber laser , a power of 400W, and a beam diameter of 100 µm. The structures are fabricated with the standard parameter set given by the system and powder manufacturer EOS. All specimens are built in vertical direction. In order to characterize the influence of the surface roughness on the fatigue strength, each investigated test series consists of nine machined and five not-machined (denoted as unprocessed surface condition) specimens. The following test series are investigated:

 As-built test series with no additional post treatment

o Machined surface condition – abbreviated as M-AB o Unprocessed surface condition – abbreviated as UP-AB  HIP test series with HIP post treatment and subsequent age hardening applied o Machined surface condition – abbreviated as M-HIP o Unprocessed surface condition – abbreviated as UP-HIP  Solution annealed test series with additional age hardening process

o Machined surface condition – abbreviated as M-SA o Unprocessed surface condition – abbreviated as UP-SA Table 2 provides a detailed overview of the received treatments for each test series. The post treatment is applied directly after the building process and before further machining of selected specimens.

Table 2. Parameter of subsequent post treatments

Treatment

T [°C]

P [MPa]

Time [h]

HIP

above 500 above 500 below 200

above 100

2 6 7

Solution annealing

- -

Age hardening