5.3 Building conditions

5.3.3 Pre-heating study

A pre-heat of 250°C, according to the maximum value recommended by the machine operating manual [173], can be applied to the bottom of the build substrate during the whole SLM process. A group of 6 tensile test specimens were built using 250°C pre-heating during the whole process, and another group of specimens were built on the same position of building substrate without pre-heating. Stainless steel 316L virgin powder supplied by LPW technology Ltd was used to build samples. The main processing parameters include laser power of 50W, lens position of 14.50mm, scanning speed of 250mm/s, solid hatch distance of 0.08mm, layer thickness of 0.05mm, single scan per layer and no pre-heating process. The results for pre-heating study are shown in Table 5-14. With the limited 250°C on the building substrate, while stainless steel melting temperature is around 1400°C, it is difficult to see any significant improvement compared with non pre-heating process.

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Density Tensile strength Elongation

250⁰C pre-heating 99.4% ±0.23 609.59MPa ±3.35 36.65% ±1.40

Non pre-heating 99.4% ±0.28 605.19MPa ±4.59 36.14% ±1.71

Table 5-14 Average density, tensile strength and elongation for pre-heating study

Some academic works suggested that pre-heating the building substrate can reduce the surface roughness and improve the part accuracy due to reduced thermal gradients and shrinkage, as less heat input is required by the laser to change the powder from a solid to liquid phase [84-86]. Some other research indicated that pre-heating the powder bed does not necessarily improve the part properties when the temperature difference between pre-heating and the material melting temperature is large (>800°C) [87]. The results shown in Table 5-14 have a good agreement with the latter comments, and suggest the pre-heating in this research does not improve the built part’s quality significantly.

5.4 Summary

The results of the first part of the experimental programme were presented and analysed in this chapter. Knowledge on each process parameter’s effect on the SLM process and final part’s physical and mechanical properties was obtained, discussed and used for the process optimisation.

Laser energy density is a key factor which affects the final parts quality. It is controlled by four main process parameters – laser power, beam width on the powder bed (lens position), laser scanning speed and scan hatch distance. The main aim of controlling the energy density is to make sure the

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heat absorbed by the powder is enough for producing dense parts without over-heating. Suitable energy intensity can generate parts with densities very close to 100% (99.93%), as well as high strength. Low energy density resulted in porosity and low strength in the part, while improper high energy densities can cause the surface powder to begin to vaporise and even generate plasma.

Controlling the process parameters needs to consider the wet-ability for avoiding balling phenomenon. The laser scanning speed is the main factor in determining the balling phenomenon during the SLM process. Lower scanning speed increases the risk that the molten powder consolidates more to a spherical structure rather than consolidating into the previous layer.

Building direction did not affect the built part’s quality. The bond between two layers was not weaker than the bond between two exposure points. However, a topping up delay does reduce the bond strength between layers.

The F-theta lens equipped inside MCP SLM-Realizer 100 works well to provide a flat field at the image plane of scan, and deliver the laser energy uniformly to the powder bed throughout the whole building area. The gas flow did not affect laser-powder interaction and cooling process throughout the building area. But it did affect the parts built on the left of the building substrate when evaporation and plasma take place under improper high energy intensity. Re-melting the surface of the part is an efficient way to reduce the top surface roughness.

Oxidation needs to be avoided during the process, as it degrades the powder material, as well as causes porosity and delamination of the parts. A pre-heat of 250°C in this research did not improve the part’s quality significantly, as it is far away from the stainless steel 316L melting temperature.

132 6 Results & Discussions - Raw

Material Characterisation

6.1 Introduction

This chapter presents the results and analysis from the second part of the experimental programme – raw material (powder used in SLM process) characterisation. The overall experimental method for this part was described in section 3.4. The results include the examination and measurement of the powder’s fundamental properties, the effect of particle size distribution on part quality and the metal powder’s sustainability in the SLM process.

Experiments were carried out on stainless steel 316L powder supplied by two suppliers, LPW Technology Ltd and Sandvik Osprey Ltd, with different particle size distributions. Results show the different behaviours of the powder bed density and powder flowability. Effects of the particle size distribution on the final part’s quality are presented, compared and discussed.

Powder material sustainability was studied by monitoring the powder particle shape, size distribution and built part’s physical and mechanical properties after a certain usage time. Results are presented on both LPW and Sandvik Osprey powders.

In document Further process understanding and prediction on selective laser melting of stainless steel 316L (Page 152-155)