Impact of contour scanning and helium-rich process gas on performances of Alloy 718 lattices produced by laser powder bed fusion

Contour scanning and process gas type are process parameters typically considered achieving second order effects compared to first order factors such as laser power and scanning speed. The present work highlights that contour scanning is crucial to ensure geometrical accuracy and thereby the high performance under uniaxial compression of complex Alloy 718 lattice structures. Studies of X-ray computed tomography visualizations of as-built and compression-strained structures reveal the continuous and smooth bending and compression of the walls, and the earlier onset of internal contact appearance in the denser lattices printed with contour. In contrast, the effect of addition of He to the Ar process gas appears to have limited influence on the mechanical response of the lattices and their microstructure as characterized by electron backscattered diffraction. However, the addition of He proved to significantly enhance the cooling rate and to reduce the amount of the generated spatters as evidenced by in situ monitoring of the process emissions, which is very promising for the process stability and powder reusability during laser powder bed fusion

Microstructure, mechanical properties and fracture mechanisms in a 7017 aluminium alloy tailored for powder bed fusion – laser beam

This study addressed a 7017 Al-alloy tailored for powder bed fusion – laser beam (PBF-LB) process. The alloy was prepared by mixing 3 wt% Zr and 0.5 wt% TiC powder to standard pre-alloyed 7017 grade aluminium powder. This made printing of the alloys possible avoiding solidification cracking in the bulk and achieving high relative density (99.8 %). Such advanced alloys have significantly higher Young\u27s modulus (>80 GPa) than conventional Al-alloys (70–75 GPa), thus making them attractive for applications requiring high stiffness. The resulting microstructure in as-printed condition was rich in particles originating from admixed powders and primary precipitates/inclusions originating from the PBF-LB process. After performing a T6-like heat treatment designed for the PBF-LB process, the microstructure changed: Zr-nanoparticles and Fe- or Mg/Zn- containing precipitates formed thus providing 75 % increase in yield strength (from 254 MPa to 444 MPa) at the cost of decreasing ductility (∼20 % to ∼9 %). In-situ tensile testing combined with SXCT, and ex-situ tensile testing combined with fracture analysis confirmed that the fracture initiation in both conditions is highly dependent on defects originated during printing. However, cracks are deflected from decohesion around Zr-containing inclusions/precipitates embedded in the Al-matrix. This deflection is seen to improve the ductility of the material

An assessment of subsurface residual stresses analysis in SLM Ti 6Al 4V

Ti-6Al-4V bridges were additively fabricated by selective laser melting (SLM) under different scanning speed conditions, to compare the effect of process energy density on the residual stress state. Subsurface lattice strain characterization was conducted by means of synchrotron diffraction in energy dispersive mode. High tensile strain gradients were found at the frontal surface for samples in an as-built condition. The geometry of the samples promotes increasing strains towards the pillar of the bridges. We observed that the higher the laser energy density during fabrication, the lower the lattice strains. A relief of lattice strains takes place after heat treatment

Influence of a 265 C heat treatment on the residual stress state of a PBF LB M AlSi10Mg alloy

Laser Powder Bed Fusion PBF LB M additive manufacturing AM induces high magnitude residual stress RS in structures due to the extremely heterogeneous cooling and heating rates. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their generation and evolution after post process heat treatments. In this study, one of the few of its kind, the RS relaxation induced in an as built PBF LB M AlSi10Mg material by a low temperature heat treatment 265 C for 1 h is studied by means of X ray and neutron diffraction. Since the specimens are manufactured using a baseplate heated up to 200 C, low RS are found in the as built condition. After heat treatment a redistribution of the RS is observed, while their magnitude remains constant. It is proposed that the redistribution is induced by a repartition of stresses between the amp; 945; aluminium matrix and the silicon phase, as the morphology of the silicon phase is affected by the heat treatment. A considerable scatter is observed in the neutron diffraction RS profiles, which is principally correlated to the presence or absence of pockets of porosity developed at the borders of the chessboard patter

Connecting Diffraction Based Strain with Macroscopic Stresses in Laser Powder Bed Fused Ti 6Al 4V

The laser powder bed fusion LPBF production process often results in large residual stress RS in the parts. Nondestructive techniques to determine RS are badly needed. However, a reliable quantification of macro RS i.e., stress at the component level by means of diffraction based techniques is still a great challenge, because the link between diffraction based strain and macro RS is not trivial. In this study, we experimentally determine by means of in situ synchrotron radiation diffraction this link for LPBF Ti 6Al 4V. We compare our results with commonly used models to determine the so called diffraction elastic constants DECs . We show that LPBF materials possess different DECs than wrought alloys, simply because their microstructural and mechanical properties are different. We also show that the existing models can be used to calculate DECs only if high accuracy of the RS values is not required. If the peculiarities of the microstructure have to be taken into account as is the case of additively manufactured materials , a radically new approach is desirabl

Explaining Deviatoric Residual Stresses in Aluminum Matrix Composites with Complex Microstructure

The residual stresses in multiphase metal matrix composites with both random planar-oriented short fibers and particles were studied by neutron diffraction and by a model based on the reformulation of classic Maxwell’s homogenization method. Contrary to common understanding and state-of-the-art models, we experimentally observed that randomly oriented phases possess non-hydrostatic residual stress. The recently developed modeling approach allows calculating the residual stress in all phases of the composites. It rationalizes the presence of deviatoric stresses accounting for the interaction of random oriented phases with fibers having preferential orientation

The role of reinforcement orientation on the damage evolution of AlSi12CuMgNi plus 15 Al2O3 under compression

Internal damage of an AlSi12CuMgNi alloy reinforced with planar randomshort fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load

The role of reinforcement orientation on the damage evolution of AlSi12CuMgNi + 15% Al2O3 under compression

Internal damage of an AlSi12CuMgNi alloy reinforced with planar random short fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load.Peer Reviewe