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|Title: ||Layered fabrication of tool steel and functionally graded materials with a Nd:YAG pulsed laser|
|Authors: ||Su, Wei-Nien|
|Keywords: ||Rapid prototyping technologies|
|Issue Date: ||2002|
|Publisher: ||© Wei-Nien Su|
|Abstract: ||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.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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