Direct laser sintering of stainless steel : thermal experiments and numerical modelling

SLS is a commercial solid freeform fabrication process. Layers of powder material are bonded by a laser beam to rapidly manufacture three dimensional freeform models. In this work, the direct SLS of room temperature single-phase
steel powder beds is researched.
Two approaches are adopted:
- Experimental analysis of heat transfer in the process;
- Numerical modelling of the process.

Experimental work involves the use of analytical equations to calculate the thermal conductivity and laser energy absorptance of the SLS powder bed. Experiments take place in a range of representative situations. Two temperature
measurement systems are used, requiring some custom-designed elements. Conductivity values in the range 0.07 to 0.25 W/(m. K) are found, dependent on atmospheric gas and powder particle size. Absorptance varies from 0.08 to 0.21,
dependent on atmosphere and material type. Measurements are made to learn more about temperature variation in the bed with position and time. It is found that processed material melts and solidifies in under 4 seconds in studied cases.

Numerical modelling involves developing and testing an existing Fortran model of the SLS process. A method is devised to visualise modelled parts in 3D. Preprocessing
is simplified, and more status information is communicated during execution. The effect on modelled parts of changes made to the program are tested. The stability of part depth is improved. The nature of parts is categorised
against input parameters. The area, relative density and morphology of manufactured and modelled single layer parts are compared. Manufactured scans are found to have a variable cross-sectional shape, whereas the cross-sectional
shape of modelled scans does not change. Modelled parts are found to be significantly smaller than manufactured parts. Reasons for these two differences are suggested.