6.4 The effect of particle size distribution

6.5.2 SO powder monitoring

Due to the research time limitation, powder supplied by Sandvik Osprey Ltd for powder sustainability monitoring study was processed for over 300 hours.

Samples were examined after every 100 processing hours to check the shapes and particle size distributions. Powders were used for general building work under around 1,000 processing hours in MCP SLM 100 were also examined as a reference in this study. These powders had been found to start to degrade, and were presented in section 3.4.6. Particle shape changes were examined by SEM, shown in Figure 6-13, and particle size distribution changes were tested by Mastersizer 2000, shown in Figure 6-14.

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a) virgin powder; b) processed after 100 hours

c) processed after 200 hours; d) processed after 300 hours

e) processed after 1,000 hours for general building propose in SLM 100 Figure 6-13 SO powder in different stages, examined by SEM

e)

d) c)

b)

a)

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Figure 6-14 Particle size distribution comparison for SO powder 100 hours Virgin

200 hours 100 hours

200 hours

300 hours

300 hours Virgin

Virgin

1,000 hours

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As shown in Figure 6-13 and 6-14, after 300 hours of processing, the particle shape did not show significant change when compared with the virgin SO powder. The majority of particles stayed on spherical shape, with a few large particles began to form un-spherical shapes because of sintering or heat affecting during the process. It can be observed from the SEM images that after 300 processing hours, the amount of large particles with diameters over 80µm started to increase and the fine particles with diameters smaller than 15µm started to disappear. After 1,000 hours processing, the amount of fine particles was much smaller compared with virgin SO powder.

Particle size distribution result showed an observed difference after 300 hours processing time, and a significant difference after 1,000 hours. The trend of fine particles decreasing and large particles increasing can be obtained. After 1,000 hours processing time, the fine particles with diameters smaller than 10µm were nearly gone, and it could be caused by vaporisation and sintering during the SLM process.

Besides the particle shape and size distribution examination, mechanical properties of the parts built by the sample powders were also examined. 3 sets of flat tensile test specimens with a gauge length of 25mm and thickness of 3mm, designed according to ASTM E8-09, were built in parallel with gas flow direction to examine any differences for both tensile strength and surface roughness. They were built on the same position of the substrate under the same building conditions. The main processing parameters include laser power of 50W, lens position of 14.50mm, scanning speed of 200mm/s, solid hatch distance of 0.08mm, layer thickness of 0.05mm, single scan per layer and no pre-heating process. UTS and surface roughness were measured, and the average results were shown in Table 6-8.

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626.33MPa 628.37MPa 623.53MPa 618.79MPa 612.56MPa

Top surface roughness

12.8685µm 12.2576µm 12.6845µm 13.3274µm 14.7117µm

Side surface roughness

6.0758µm 6.3723µm 6.9234µm 7.4417µm 9.2320µm

Table 6-8 Average mechanical properties comparison for SO parts

Due to the changes in particle size distribution of the powder provided by Sandvik Osprey Ltd, the measured tensile strength and surface roughness showed relative changes too. The change in particle size distribution did not generate significant effect on the average tensile strength as well as the top surface roughness; and a decrease trend on the tensile strength and an increase trend on the top surface roughness was obtained. Side surface roughness showed a slight increase after 300 processing hours and an obvious increase after 1,000 processing hours.

6.6 Summary

The results of the second part experimental programme were presented and analysed in this chapter. Two brands of powder were used in the experiments. The powder particle shape, size distribution, flowability and the behaviour on forming the powder bed were examined. Particle size distribution effects on the final part quality were studied. Powders were monitored under a certain period for investigation their sustainability.

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SO powder with wider range of particle size provides higher powder bed density, generates higher density parts under low laser energy intensity, and generates smoother side surface finishing parts. LPW powder with narrower range of particle size provides higher flowability, generates parts with higher UTS and greater hardness under high laser energy intensity.

Main powder degrade phenomenon is the change in powder particle shape and size distribution. LPW powder did not degrade in the monitoring period 800 hours with no significant changes happened in particle shape, size distribution and built part’s quality. SO powder started to degrade after 300 hours with an observed difference in particle size distribution, and this difference was enhanced after 1000 hours of processing. The degradation results the lower tensile strength and higher surface roughness of the built parts.

156 7 Results & Discussions - Model

Inputs Characterisation

7.1 Introduction

This chapter presents the results and analysis from third part of the experimental programme – model inputs characterisation. These inputs include the material properties, loads and boundary conditions, and are essential for the simulation. The overall experiment method for this part was described in section 3.4.

Since the powder bed properties cannot be obtained directly from the literature, relevant measurements were carried out. The results of density, thermal conductivity and specific heat capacity for the powder bed are presented.

Loads and boundary conditions are also discussed in this chapter.

In document Further process understanding and prediction on selective laser melting of stainless steel 316L (Page 173-179)