Keywords: solid-state, friction stirring, process, in situ monitoring, temperature
Summary:Most metal additive manufacturing today is based on selective melting and rapid solidification of powders or wires. Despite great research progresses, fundamental challenges persist in minimizing the cost, heat input, thermal gradient, residual stresses, and the risk of hot cracking. Recently, a series of solid-state metal additive manufacturing technologies emerge, which exploit deformation bonding to enable material addition and adhesion. Among them, MELD stands out as a purely additive technology that enables site-specific build-up of metals with good quality, fine equiaxed microstructures, and excellent mechanical properties. MELD has also drawn great attention in the fields of multi-material additive manufacturing, selective-area cladding, and additive repair. Remaining at an early stage, a good understanding of the physical processes underlying MELD—most notably temperature evolution and material deformation—has been elusive, critically preventing us from the control of the microstructure and properties of the as-printed material. In this talk, we will discuss the most recent understanding of MELD process fundamentals, in which we explore the physical processes via complimentary in situ monitoring of (i) temperature evolution, (ii) material deformation flow, and (iii) feed force evolution. We focus on two materials with distinct stacking fault energies (SFEs), Cu with medium SFE and Al with high SFE. Generic trends of temperature evolution with respect to the processing condition have been revealed. Quantitatively, different power-law relationships are found between the peak temperature and MELD processing parameters in Cu and Al. This difference suggests a change of the dominant heat generation mechanism in these two materials, which is supported by our observation of their distinct material flow behavior and feed force evolution. These findings unveil the vital roles of SFE and the dynamic recovery rate in governing the heat generation mechanisms in MELD, paving the road toward material-orientated processing control.