No abstract

With the advent of disruptive additive manufacturing (AM), there is an increasing interest and demand of high mechanical property aluminium parts built directly by these technologies. This has led to the need for continuous improvement of AM technologies and processes to obtain the best properties in aluminium samples and develop new alloys. This study has demonstrated that porosity can be reduced below 0.035% in area in Al-Mg samples manufactured by CMT-based WAAM with commercial filler metal wires by selecting the correct shielding gas, gas flow rate, and deposition strategy (hatching or circling). Three phase Ar+O2+N2O mixtures (Stargold®) are favourable when the hatching deposition strategy is applied leading to wall thickness around 6 mm. The application of circling strategy (torch movement with overlapped circles along the welding direction) enables the even build-up of layers with slightly thicker thickness (8 mm). In this case, Ar shielding gas can effectively reduce porosity if proper flow is provided through the torch. Reduced gas flows (lower than 30 Lmin) enhance porosity, especially in long tracks (longer than 90 mm) due to local heat accumulation. Surprisingly, rather high porosity levels (up to 2.86 area %) obtained in the worst conditions, had a reduced impact on the static tensile test mechanical properties, and yield stress over 110 MPa, tensile strength over 270 MPa, and elongation larger than 27% were achieved either for Ar circling, Ar hatching, or Stargold® hatching building conditions. In all cases anisotropy was lower than 11%, and this was reduced to 9% for the most appropriate shielding conditions. Current results show that due to the selected layer height and deposition parameters there was a complete re-melting of the previous layer and a thermal treatment on the prior bottom layer that refined the grain size removing the original dendritic and elongated structure. Under these conditions, the minimum reported anisotropy levels can be achieved.

No abstract

Maraging steel 300 processed by laser powder bed fusion (PBF-LB/M) is widely used for manufacturing inserts and moulds with conformal cooling channels. For such applications, high fracture toughness and strength are required to withstand thermal fatigue and any defect like pores, lack of fusion and cracks should be avoided so that the durability of the mould is not reduced. Maraging steel is prone to be cracked due to thermal gradients which arise during PBF-LB/M. Moreover, it was recently observed that slight variations in composition led to the crack's presence, which implied lower mechanical properties. In this study, two powder batches, from the same powder producer, were employed with the same processing parameters. The effect of small changes in minor elements composition of the powder batch on cracking was studied and the preheating temperature was varied to significantly reduce the crack density. The presence of cracks significantly reduced the ductility of the parts.