Abstract
Recent years witnessed progressive broadening of the practical use of 3D-printed aluminium alloy parts, in particular for specific aerospace applications where weight saving is of great importance. Selective laser melting (SLM) is an intrinsically multi-parametric fabrication technology that offers multiple means of controlling mechanical properties (elastic moduli, yield strength, and ductility) through the control over grains size, shape, and orientation. Targeted control over mechanical properties is achieved through the tuning of 3D-printing parameters and may even obviate the need of heat treatment or mechanical post-processing. Systematic studies of grain structure for different printing orientations with the help of EBSD techniques in combination with mechanical testing at different dimensional levels are the necessary first steps to implement this agenda. Samples of 3D-printable Al-Mg-Si RS-333 alloy were fabricated in three orientations with respect to the principal build direction and the fast laser beam scanning direction. Sample structure and proper-ties were investigated using a number of techniques, including EBSD, in situ SEM tensile testing, roughness measurements, and nanoindentation. The as-printed samples were found to display strong variation in Young’s modulus values from nanoindentation (from 43 to 66 GPa) and tensile tests (from 54 to 75 GPa), yield stress and ultimate tensile strength (100–195 and 130–220 MPa) in different printing orientations, and almost constant hardness of about 0.8 GPa. A further preliminary study was conducted to assess the effect of surface finishing on the mechanical performance. Surface polishing was seen to reduce Young’s modulus and yield strength but improves ductility, whereas the influence of sandblasting was found to be more controversial. The experimental results are discussed in connection with the grain morphology and orientation.
Highlights
It seems that grains have almost random crystallographic orientation against principal axes of a sample, but the sizes and aspects of grains are obviously correlated with printing the orientation—main axes of slim elongated columnar grains that are always aligned along the growth direction, and as a result, these grains appear equiaxial in the plane normal to the growth direction
In the planes parallel to the growth direction, elongated columnar grains are not single bouquet-like patterns in the planes parallel to the growth direction. These and other peculiarities of grain patterns should, be thoroughly quantitatively analysed at bigger subsets to validate the number of conceptual models that can be put forward to describe grain pattern formation, which will be addressed by the authors together with FEM modelling of heat fluxes in a separate paper
One can notice that the XY printing orientation returns somewhat different grain patterns than the ZX and XZ printing orientations: thinner and shorter columnar grains, larger number of spots in the planesregions parallel(spots) to growth direction, coarser equiaxial elements of grain pattern–round-like containing veryand small equiaxial grains grains in the plane perpendicular to growth direction
Summary
SLM 3D-printing of metallic materials is a technological process that initially emerged in the early 2000s as a route for rapid prototyping. It has evolved into a mass-production method for the fabrication of highly demanding parts with complex shapes [1,2,3]. Three-dimensionally printable Ti, Ni, and Al alloys are progressively finding applications in the aerospace domain where weight saving issues are of great importance: gas turbine engine components [4,5], fuselage structural elements [6], etc. SLM 3D-printing is a flexibly controlled fabrication technique that is affected by the material composition and multiple process parameters (laser power density, scan speed, layer thickness, etc.) These disparate factors interact in a complex manner and lead to the emergence of various hierarchical microstructures with specific characteristics, e.g., arrangement and size of grains, grain boundaries, pores, and reinforcing phase particles
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