A Methodology and Analysis of Inoculation in Additive Aluminum Alloys

Metal additive manufacturing, or 3D printing, has the potential to be an incredibly disruptive technology for the fabrication and integration of complex components in a variety of industries, ranging from automotive and aerospace to medical devices and sporting goods. Until recently, additive manufacturing of metals was mostly used for non-critical or prototyping applications, however a global push for improved energy efficiency via light weighting and topological optimization, as well as reduced cost via lower material waste and near net-shape fabrication, is pushing metal additive manufacturing out of the prototype stage and into full scale production. This has led to a large investment in development of additive metals technologies, however this has mostly been focused on the improvement of additive equipment with a focus on better repeatability, quality, and throughput. There has been relatively little focus on improvements of the materials which are used in additive manufacturing, which have been limited to a few “weldable” alloy systems such as Al10SiMg, Ti6Al4V, and Inconel 718. While these alloy systems have provided a good first step in development of the additive industry, they are extremely limited when compared to the >5000 different alloy compositions available in either cast or wrought forms. This limitation is driven by the unique processing conditions of additive manufacturing which differ significantly from conventional bulk material production developed over centuries, if not millennia. This dissertation investigates the unique solidification conditions present during additive manufacturing of aluminum alloys and attempts to understand how novel inoculant methodologies may be used to not only control microstructure evolution of model unalloyed aluminum systems but extend the available alloy systems beyond what was previously considered amenable to the additive process.\tThe potential scope of additive manufacturing both from a technological and applications space is vast. Therefore, this dissertation is focused on a single additive processing route (laser powder bed fusion) and alloy system (aluminum). Aluminum was chosen after an investigation of available additive alloys indicated that the aluminum alloys, in particular, provided the lowest additive material strength (~200MPa, AlSi10Mg) vs their wrought counterparts (>400 MPa, 7000 Series Al). This is driven by the high crack susceptibility of many high strength aluminum alloys during solidification. It was hypothesized, and has been indicated in the literature, that formation of fine equiaxed microstructures can decrease the susceptibility of these systems to solidification cracking. Until now, microstructure control in additive was limited to parametric manipulation of print parameters, however this has been difficult to broadly implement across all alloy systems. This research leveraged the concept of inoculation to aid in the control of microstructure and improve the processing of additive aluminum alloys. This dissertation has been organized to provide the necessary background information to understand the solidification conditions present in laser powder bed fusion and a methodology for inoculation of additive alloys and mechanistic discussion utilizing a model unalloyed aluminum system. Finally, this dissertation will demonstrate that utilization of this inoculation approach can in fact eliminate the crack susceptibility of high strength aluminum alloys (Al7075 and Al6061) and produce crack free additive aluminum with strengths 2X that of the most common commercial Al10SiMg alloy system.