Quantitatively characterizing the evolution of hole defects between overlapped aluminum droplets by a two-dimensional solidification model

Abstract

Quantitatively characterizing the effect of process parameters on the evolution of holes in metal droplet 3D printing enables effective prediction and elimination of hole defects in printed parts. In this work, a two-dimensional solidification model is proposed to quantitatively characterize the influence of substrate temperature on hole morphologies between overlapped aluminum droplets. First, a phenomenon is observed in experiments that the surfaces of overlapped droplets show typical "L"-shaped solidification ripples, which indicates that the solidification of overlapped droplets involves two-dimensional solidification. Hole defects are formed in the transition area of two-dimensional solidification. In addition, the appearance of the first L-shaped ripple signifies that the hole is no longer changing. Based on this discovery, a two-dimensional solidification prediction model for the hole size is constructed by combining a hyperbola and the one-dimensional heat transfer model to trace the ripple, which can accurately demonstrate the relationship between the hole size and the parameters (i.e., the thickness of solidified layer X(t) , the two-dimensional solidification angle β , the solidification angle θ ). The hole formed between the two overlapped droplets can be divided into two parts: the hole Ⅰ caused by the two-dimensional solidification and the hole Ⅱ caused by the morphology of the previously deposited droplet. When X(t) ≤ 0 at the end of the spreading of the overlapping droplets on the substrate, the hole can be completely eliminated. This work provides effective theoretical guidance for the prediction and elimination of hole defects in metal droplets printed parts.