Effect of process parameters on the metallurgical characteristics of additive-manufactured alloys

Abstract

The production of metallic parts layer by layer, well-known as Additive Manufacturing (AM), has increased the flexibility and freedom to design metallic products. This area undergone a huge development in the last years thanks to the possibility to produce near-net-shape components, with customized density and complex shapes (sometimes impossible to achieve with traditional manufacturing methods) and to the development of new AM technologies with competitive prices. Products fabricated by AM are already in use in the aeronautic, automotive and biomedical industries. To make use of all the AM potential, it is necessary to develop new alloys specifically for these processes, as was done in the past by the casting and forming industries, to adjust the solidification mode to the thermal characteristics of AM. Several challenges still exist with use of the available metallic alloys as the formation of macro and micro defects during build up, residual stresses generated during process, composition control during melting, formation of texture and hot tears, formation of microstructural heterogeneities, among others. Most of these defects occur because the alloy composition is not adequate for the AM process. The products obtained must satisfy specifications of composition, surface quality, damage tolerance, fatigue, strength and other properties, which are very sensitive to changes in composition and/or microstructure and defects fraction. Nowadays, only a few additive manufacturing alloys as AlSi10Mg, TiAl6V4, CoCr and Inconel 718 can be used in critical applications where high strength and good fatigue and toughness properties are required. Most of the 5500 alloys available today cannot be used because the melting and solidification dynamics promote the formation of long cellular/dendritic grains with cracks periodically distributed in the material. Taking this into consideration, this project aims to investigate the effect of processing parameters in the metallurgical characteristics of parts produced by AM, trying to understand how these parameters affect the solidification behaviour and thermal flow during processing. Two alloys were chosen for this investigation: tool steel AISI A2 and aluminum alloy A2017. Powders will be obtained by gas atomization and it will be used to produce parts by the additive manufacturing processes of Powder Bed Fusion (PBF) e Directed Energy Deposition (DED). It will be investigated the influence of scanning speed, laser power, hatching, scanning strategy and powder characteristics (with and without additions of nucleant particles) in the metallurgical behaviour of the samples obtained. These samples will be characterized by X-ray Diffraction, Differential Scanning Calorimetry, Optical Microscopy, Scanning Electron Microscopy and Transmission Electron Microscopy, Computer X-ray Tomography, chemical analyses, tensile, hardness, fatigue and toughness tests. Simulations of phase formation and solidification by THERMOCALC and numerical modeling of the heat flow will be carried out and compared with the experimental results. The processing parameters used will be correlated with the microstructure, phase formation and properties of the AM samples. (AU)