Layered fabrication of tool steel and functionally graded materials with a Nd:YAG pulsed laser

2010-11-01T14:06:41Z (GMT) by Wei-Nien Su
Rapid Prototyping technologies have been developed to transform three-dimensional computer models into physical prototypes within a compressed period of time. The problem of many existing laser and metal powder based techniques is the insufficient strength of parts to meet the practical requirements due to incomplete sintering or melting of powders. One of the research objectives was to melt tool steel powder completely and form fully dense fused structures with a 550 W Nd: YAG pulsed laser. The other objective was to produce material structures with graded composition, so-called Functionally Graded Materials (FGM). It is believed that this process could eventually produce preforms with complex material structures. Tungsten carbide was selected to be mixed with tool steel powder for possible wear resistance applications. The investigation on laser fusing tool steel was first carried out. The optimal process settings were concluded by measuring the contact angle, the surface roughness, bead height and variance of bead width of each single bead produced under various conditions. Fusing overlapped beads and multiple layers was then followed by studying the effects of scan spacings, scanning patterns and layer thickness. A scanning pattern was developed to effectively reduce porosity. Dense cubes of tool steel were then successfully produced with porosity of less than 1 %. Based on the findings from processing tool steel powder, different ratios of WC and tool steel were mixed and processed under the same processing conditions to produce FGMs. Various analysis techniques, including scanning electron microscopy, energy and wavelength dispersive X-ray were applied to examine the microstructures. WC was found partially dissolved in the matrix and evidence of liquid phase sintering was found in powder densification. Hardness, microhardness and nano-indentation testing were performed to show the hardness values in accordance with compositional changes in macro, micro, and nano-scales. The FGM of 80wt% H 10 and 20wt%WC showed an increase in hardness of at least 5-10% from the samples of H10.