Clump-type particle-shape approximation is often employed in discrete-element modelling (DEM). Previous studies have determined the number of spheres in a clump empirically. Herein, a novel approach is proposed to determine the number of spheres in a clump quantitatively on the basis of statistical optimisation using Akaike's information criterion (AIC). AIC quantifies the balance between the approximation accuracy of the clumps and the increase of the number of spheres in a clump. The proposed method is applied to determine a satisfactory number of spheres in a clump value for clumps of Toyoura sand particles. A series of clump-based isotropic-compression DEM simulations is conducted. When the number of spheres in a clump is higher than that determined by the proposed method, the simulated void ratio and the mean contact number converge to their respective values and agree with laboratory test data and X-ray computerised tomography measurements. Therefore, it is concluded that the proposed method is a valid one.

Defects occur in laser powder bed fusion (L-PBF) such as the keyholing, lack of fusion, and the balling depending on the laser power (P) and the scan speed (V). The figure shows that the occupied regions of each defect are the process window and are essentially important to fabricate a high-quality part. This paper is a study of process window generation using single-track experiments and finite-element method simulation of thermal conduction for Inconel738LC alloy. A series of single-track experiments were conducted varying the range of P and V and the results were classified into keyholing, lack of fusion, balling, and good track. A series of simulations were conducted and validated by comparison with the experiments. To quantitively identify the balling, the isolines from the contour map generated by the results of simulations and the balling criteria of the ratio of melt pool length and the depth (L/D) of 7.69 were determined considering the past theoretical studies. The lack of fusion criteria: the ratio of the overlap depth in fabrication using multi-scan (D_ov) and powder layer thickness (t) of 0.1 was obtained. Using the criteria obtained from the experiments and simulation, the process window was generated.