4. PROCESS PARAMETER DETERMINATION
4.4. Discussion
It was found that different combinations of process parameters can be used to fabricate near fully dense parts. Due to the different material properties of the materials considered, it is expected that the process parameters required for this would vary.
Material thermal properties and beam absorption rate of the metal powder are some of the important properties that affect the melt pool formation [157], and therefore the overall complete 3D build. Even though the melt range and powder absorption of the laser beam are approximately similar for both materials studied (Ti-6Al-4V ELI and 316L-SS), the thermal conductivity is dramatically different which leads to distinct behaviours (Table 4-10).
Table 4-10: Generic data wrought material Ti-6Al-4V ELI and 316L-SS
Characteristic Ti-6Al-4V ELI 316L-SS
Thermal conductivity (W/mK) at 20 ºC 6.7 [154] 16.2 [155]
Melting range (°C ) 1604-1660 [154] 1371-1399 [155]
Measured powder absorbability 0.36-0.44 ([49]; [158]; [43])
0.3-0.4 ([49]; [111]; [128];
[159]; [65])
The width of the melt pool for Ti-6Al-4V ELI is 63% greater in comparison to the width of 316L-SS melt pool, for the same exposure time of 220µs and at a LT of 60µm.
The variation of the melt pool width between Ti-6Al-4V ELI and 316L-SS was approximately 28% at a low exposure time of 70µs and 80µs respectively. However, when the exposure time was increased to 220µs for both materials while maintaining the same LT, the variation between the two increased by 35% to 63% (Figure 4-15). From the investigation of the melt pool formation, it was noticed that the melt pool geometry of the Ti-6Al-4V ELI and 316L-SS was formed differently for the same energy density.
The thermal conductivity of a material plays a major role in the geometry of the melt pool formed. A metal with a low thermal conductivity tends to retain the heat in a limited area for longer while a metal with a high thermal conductivity dissipates the heat away from the melt pool more rapidly. The size of the melt pool for Ti-6Al-4V increased more with increased energy density as it has lower thermal conductivity compared to 316L-SS). In addition the thermal conductivity of the inert gas was found to affect the process as it can increase the thermal conductivity of a material powder [125]. A gas such as Helium can increase the thermal conductivity of the powder by as much as 300% compared to Argon and Nitrogen. This was not considered to be a reason for different melt pool size in this study as both materials were processed under the same inert gas (Argon). Laser beam absorption was not considered as a factor in producing a different melt pool size. Both alloys (Ti-6Al-4V and 316L-SS) were found to have similar range of measured powder absorbability.
Figure 4-15: Change in melt pool width vs the ET for Ti-6Al-4V ELI and 316L-SS at LT of 60µm
In general, there are two main mechanisms that lead to the development of pores.
a) Lack-of-fusion; which may be caused when the overlapping distance of individual points or melt tracks is insufficient [127], when the applied energy is inadequate to melt the powder, or when the powder layer is too thick for all the material to be melted during the set exposure time. In pulse laser PBF systems, PD can play a role in creating voids when the distance between two consecutive points is greater than the optimum distance.
b) When the applied energy is excessive in comparison to the required energy. This results in evaporation or keyholing [128]. This is when the fusion process passes the thermal conduction region and enters into the keyhole region. Exaggerated overlapping in HD or/and PD, long ET and high laser power can contribute to the development of keyholes in PBF parts.
It is clear that using the value of the VED to calculate the proper applied energy for a certain level of density/porosity, is not universally correct. The VED should be used more as a guideline for indication purposes to narrow down into the region of desired operating conditions, with then finer tuning of the operating parameters for the identification of optimal conditions. The value of VED and SS do not provide enough information to describe the effect of process parameters, therefore individual process
100 150 200 250 300 350
10 50 90 130 170 210 250
Width (µm)
Exposure Time (µs)
Ti-6Al-4V 316L-SS
parameters should be carefully selected for a specific combination value of VED or SS.
However, the VED can be used to restrict the delivered energy to be within acceptable levels. Going below or above a specific VED value can impact the build quality. In this study, a VED below 40J/mm3 or above 60J/mm3 was found to be unsuitable for the selected particle size of 316L-SS alloy.
The distribution of the pores is generally uniform in all the samples investigated regardless of the frequency observed. However, the frequency of pores around the edge of the samples was observed to be generally constant and appeared to be independent of the pores distribution in the bulk area. Because the value of melt parameters along the borders of the samples was fixed for all fabricated parts, the shape and size of the pores at the edges were the same for all samples. The porosity at the edge can be caused by high temperature due to the turning point of the melt tracks, particularly at the joining point between the border and scan area of the layer.
The work described in this chapter provides foundation for the next chapter, which develops process models. It also provides an important contribution towards the consideration of process parameters optimisation. The work emphasises the importance and necessity of taking into account the relationships between process parameters.
These relationships significantly influence the stability of the process by providing an important understanding of the interaction between how and how much a process parameter will behave in conjunction with other process parameters.