PSI - Issue 17

Thomas Simson et al. / Procedia Structural Integrity 17 (2019) 843–849

844

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

2

process, these properties change and affect i.a. the flowability and achievable packing density of the powder. This factors has the most significant effect on the final density of the manufactured parts German, R.M. (1989); Olakanmi, E. (2013). The mechanical properties achieved with SLM may differ from those of the wrought material for a variety of reasons. Due to the small melting pools during laser processing and the high scanning speeds, short interaction times result and lead to high cooling rates and high thermal gradients. The high cooling rates may result in the formation of non-equilibrium phases, quasi-crystalline phases and new crystal phases with extended composition ranges Rombouts, M. (2006). At sufficiently high cooling rates, finer microstructures may be observed in comparison to conventional manufacturing methods. Many material properties, such as yield strength, thermal conductivity or ductility strongly depend on the microstructural properties Yasa, E. et al. (2009). Fracture toughness is one of the properties that reflects the ability of a structural material to inhibit crack propagation. Changes in chemical composition and highly localized grain boundary separation may result in loss of ductility Brandon, D., and Kaplan, W. D. (2013).

Nomenclature AM

Additive manufacturing

d

Layer thickness

DownSkin

Areas without further areas below

EDX

Energy dispersive X-ray Elongation at failure

ε tot

G

Gage length Hatch spacing Heat-treated

h

HT

In Fill LPBF MAR

Inner area

Laser powder bed fusion

Maraging steel Laser power

P

PSD RMS

Particle size distribution

Root mean square

Rm

Ultimate tensile strength

Rp 0.2

Yield strength

Rz

Average surface roughness Scanning electron microscope

SEM

t

Thickness

UpDown

Areas without further areas above

v

Scan speed

W

Width

2. Experimental procedure

2.1. Materials and LPBF process

The material used for this investigation is martensitic steel MAR 300 (EN 1.2709) the chemical composition was provided by the producer (Tab. 1). The powder was bought by Electro Optical Systems (EOS) Finland Oy and manufactured by gas atomization resulting in spherical particles. According to manufacturer the average particle size was 30 µm. The current study compares the properties of three different powder states, i.e. “ new ” powder, repeatedly used one and “ waste ” powder. The particle size distributions of the powders have been measured with a laser granulometer (Sympatec HELOS (H2852) with dry dispersion unit RODOS). The morphology of the powders and chemical compositions of individual particles have been examined via a LEO 1450 VP SEM with an Oxford Instruments X-Max EDX. For the determination of mechanical properties, test samples were produced under nitrogen atmosphere in two different building directions (horizontal and vertical) by using an EOS M290 metal laser melting system. The laser scanning strategy was in zig-zag pattern with 67° rotation between the superimposed layers. The applied process parameters laser power ( P ), scan speed ( v ), hatch spacing ( h ) and layer thickness ( t ) are listed in Tab. 2. Before starting the LPBF process, the base plate was preheated to 40 °C by a heater placed inside the building platform, and the oxygen content in the process chamber was maintained below 0.04% by continuous flow of nitrogen. After the manufacturing process, the samples were first solution annealed at 820 °C for 1 h in argon atmosphere and air-cooled, followed by aging at 490 °C for 6 h in ambience atmosphere. For the solution annealing a gas-tight chamber furnace of the PKR 35/11 type from LAC is used. The aging is carried out using a 18 kW/70 liters chamber furnace from Linn High Therm GmbH with the type designation HK-70.