Towards understanding grain nucleation under Additive Manufacturing solidification conditions

Abstract

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Non- and Quasi-Equilibrium Multi-Phase Field Methods Coupled with CALPHAD Database for Rapid-Solidification Microstructural Evolution in Laser Powder Bed Additive Manufacturing Condition

“…It is well known that the fraction of thermal undercooling increases with the increased cooling rate. It is reported that the fraction of thermal undercooling becomes approximately 80% at a cooling rate of 100 K/s and a temperature gradient of 1000 K/m for the Al-Cu alloy [36]. Figure 12 qualitatively agrees with this relation.…”

“…The results at 10 4 K/s were close to those obtained by the Scheil model, except for the initial undercooling period. Thermal undercooling corresponded to the majority of the solidification interfaces at an increasing cooling rate [36]. Thus, the growth rate under a higher cooling rate accelerated in the early stage and strongly decelerated as it neared the local equilibrium at the interface in the final stage.…”

Grain Refinement of Alloys in Fusion-Based Additive Manufacturing Processes

“…[27,28] It is important to note that V is sometimes denoted as R, as the growth rate of the S/L interface, which is the microstructure response to the isotherm velocity or pull rate. While there is an important difference between the two, [29] in this paper, we are not differentiating the two as they are closely linked. Recent results show that CET can be achieved in some alloys by decreasing G in the liquid region and increasing V without adding alloying elements.…”

“…This suggests a much greater contribution of thermal undercooling to facilitate nucleation on less favorable grains. [29] This means that while at lower V, the solute is a dominant player in providing the required undercooling [5] ; it is the thermal undercooling that acts as a nucleation multiplier at high V and high G. With shorter NFZ at high V and steep G, the role of solute is restricted. This implies that alloys of two different compositions with the same inoculant chemistry and of similar size distribution would be expected to trigger an equally large number of equiaxed nucleation events.…”

Effect of Surface Oxides on the Melting and Solidification of 316L Stainless Steel Powder for Additive Manufacturing

“…In the L-PBF process, the heating and cooling rates range from 10 3 to 10 6 K/s. [26,48] Due to the limited melting time caused by the high scanning speed of the heating source, a much higher heating temperature is required to eliminate the balling behavior induced by the cage effect. However, severe overheating of the metallic powder can inevitably raise the evaporation of low melting point solutes and affect the mechanical performance of the as-built component.…”