Page 37 - MSAM-3-2

Materials Science in Additive Manufacturing LPBF of Ti-Al-graded multi-materials

5101 micro-sclerometer (Bang Yi Precision measuring roughness, thereby influencing the subsequent LPBF
Instrument (Shanghai) Co., LTD, China), applying a load process. At a scanning speed of 2400 mm/s, a rough surface
of 200 g and a dwell time of 10 s. Fourteen points, with with a Ra value up to 33.35 μm was observed, with cracks,
a step size of 0.2 mm symmetrically positioned about pores, and protrusions evident in Figure 3A. During the
the interface, were measured. Compression tests at room LPBF process, excessive laser energy input led to significant
temperature were conducted using a CMT5205 testing heat accumulation in the molten pool, resulting in excessive
machine (MTS, America). The ultimate compressive residual thermal stress. Consequently, the concentration
strength and strain of Ti6Al4V/AlMgScZr-graded multi- of residual thermal stress induced crack formation at the
material samples were determined from the engineering interface. The protrusions might be caused by partially
stress-strain curves. Fracture morphologies of samples melted powder particles or local disturbances in the
were characterized using SEM. molten pool. Increasing the scanning speed to 2600 mm/s
still resulted in the presence of pores and protrusions
3. Results and discussion (Figure 3B), albeit with a reduced Ra value of 29.78 μm.
3.1. Surface morphology and densification behavior The higher scanning speed helped decrease residual
thermal stress in the graded layer. Samples processed at
Figure 3 demonstrates the surface morphology and 2800 mm/s exhibited a further reduction in Ra value to
roughness of the graded layer of LPBF-processed Ti6Al4V/ 24.18 μm, although some protrusions were still observed.
AlMgScZr-graded multi-material parts at different This reduction in surface roughness may be attributed to
scanning speeds (Figure 2C-1). The samples were oriented decreased cracking and warping, which were beneficial for
vertically, with the Ti6Al4V layer at the bottom and the subsequent LPBF processes. However, when the scanning
graded layer at the top. speed reached 3000 mm/s, the insufficient laser energy input
Various issues, such as overlapping of the molten pool, prevented the complete melting of the powder in the molten
pores, balling, protrusions, and cracks, can impact surface pool. As a result, a roughened-graded layer with a Ra value

Table 1. The process parameters applied in the fabrication of Ti6Al4V/AlMgScZr-graded multi-material parts using laser powder
bed fusion process

Parameters Materials
Ti6Al4V AlMgScZr Graded 1 Graded 2 Graded 3 Graded 4
Laser power (W) 175 200 200 200 200 200
Scanning speed (mm/s) 950 1500 2400 2600 2800 3000
Laser thickness (mm) 0.05 0.03 0.03 0.03 0.03 0.03
Hatch distance (mm) 0.05 0.06 0.06 0.06 0.06 0.06

A B C D

Figure 3. The surface morphology and surface roughness of Ti6Al4V/AlMgScZr-graded multi-material-graded layer at different scanning speeds.
(A) 2400 mm/s. (B) 2600 mm/s. (C) 2800 mm/s. (D) 3000 mm/s. Scale bars: 200 μm, magnification ×250.

Volume 3 Issue 2 (2024) 5 doi: 10.36922/msam.3088