Development of an empirical process model for adjusted porosity in laser-based powder bed fusion of Ti-6Al-4V

Abstract

A promising approach to address the mismatch of bone and implant stiffness, leading to the stress-shielding phenomenon, is the application of functionally graded materials with adjusted porosity. Although defect formation and porosity in laser-based powder bed fusion of metals (PBF-LB/M) are already widely investigated, so far there is little research on the influences and parameter interactions regarding the pore characteristics. This work therefore aims to provide an empirical process model for the generation of gas porosity in the PBF-LB process of Ti-6Al-4V. Parts with closed locally adjusted porosity of $\sim $ ∼ 6 % achieved through gaseous pores instead of lack of fusion defects or lattice structures were built by PBF-LB. Parameter variation and evaluation of relative density, pore size and sphericity was done in accordance with the design of experiments approach. A parameter set for maximum gas porosity (laser power of 189 W, scanning speed of 375 mm/s, hatch spacing of 150 μm) was determined for a constant layer thickness of 30 μm and a spot diameter of 35 μm. Tensile tests were conducted with specimens consisting of a core with maximum gas porosity or lack of fusion porosity, respectively, and a dense skin as well as fully dense specimens. Whereas lack of fusion defects can lead to significant reduction of stiffness of 32.2 %, the elastic modulus remained unchanged at 110.0 GPa when implementing spherical pores. Nevertheless, the found superior strength and ductility of specimens with gas porous core (> 1100 MPa and > 0.05 mm/mm, respectively) underline the advantages of adjusted porosity for the application in functionally graded materials and lightweight applications.

Additive Manufacturing of AlSi10Mg and Ti6Al4V Lightweight Alloys via Laser Powder Bed Fusion: A Review of Heat Treatments Effects

“…Finally, the LOF and keyhole are grouped under the name melting-related defects due to their irregular shape ( Figure 5 a,b), while gas pores are considered separately due to their spherical morphology (<100 μm) [ 51 , 94 ]. The main causes of gas pore formation ( Table 2 ) are the presence of gas wrapped into the gas atomized powder, or the dissolution/entrapment of gas present within the build chamber [ 51 , 74 , 75 , 76 , 77 , 78 , 79 ]. Focusing on AlSi10Mg samples, the gas pores can also be caused by the H 2 O reduction during the L-PBF process due to the thermal cycles induced by the printing methodology [ 100 ].…”

Mitigation of Gas Porosity in Additive Manufacturing Using Experimental Data Analysis and Mechanistic Modeling

“…First, a mechanistic model [42,43] of the LPBF process was tested, calibrated using experimental results, and used to compute temperature fields and molten pool geometry. Then, experimental data [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] on porosity during LPBF of stainless steel 316, Ti-6Al-4V, Inconel 718, and AlSi10Mg were collected from the literature. The results from the well-tested model were used to derive and calculate a gas porosity index corresponding to all experimental cases.…”

“…A total of 93 sets of data on gas porosity formation for four alloys at various processing conditions were collected from the literature [44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60]. Among the 93 sets of data, 60 cases had gas pores and 33 cases were without experimentally detected gas pores.…”

“…Refs. [2,3,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59]67,84] are cited in the Supplementary Materials.…”