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Our overall objective was to achieve material properties in our samples that were similar to that of standard bronze and steel by improving the printing, debinding, and sintering processes. More specifically, we hoped to
fully characterize shrinkage of parts
maintain part geometry
achieve densities that were within 80-100% of casted bronze and steel
reduce pore size and irregularity
achieve hardness and tensile strength values within 80% of bronze and steel.
The first part of our design solution focused on optimizing printer settings, such as nozzle temperature and flow rate, to produce high quality prints. Then our project shifted to focus on optimizing printing settings, debinding, and sintering parameters to improve the debinding and sintering of parts. The main parameters we focused on were infill density and compression which involved the creation of a compression mechanism.
Compression Mechanism
Compression Mechanism
Made entirely of machinable alumina as it had to be chemically inert and thermally stable in temperatures up to 1260 C and had to be easily machinable. Pressure load is applied via an alumina screw that is threaded through the top plate.
We found that applying compression during the debinding and sintering process was the most successful parameter for improving geometric retention and shrinkage.
Shrinkage
Through analysis of measurements of "green" parts and post-sintered parts we found that measurements in all directions averaged ~8% shrinkage. Higher infills led to less shrinkage.
Geometric retention
Uncompressed (left) vs. compressed via compression mechanism (right) bronze benchy. Reduced shrinkage in compressed benchy as well as straighter columns and a more angled roof are visible. Compressed benchy maintains the shape of original benchy print better than uncompressed.
Density
Infill had little effect on density. Our average bronze density was 7.64 g/cm3 (87% of bulk density). Our average stainless steel 316L density was 5.56 g/cm3 (70% of bulk density).
Tensile
Bronze
E ~ 61 GPa
Bulk: 111 GPa
Sut ~ 171 MPa
Bulk: 468 MPa
SS 316L
Too soft for use.
Hardness
Bronze
HRB < 30
Bulk: 26-133
SS 316L
HRB: 37.42 (BASF)
HRB: 56.35 (TVF)
Bulk: 80
Microstructure
Microstructure
Image processing and SEM on a stainless steel 316L sample revealed an average void size of 0.0056 mm2 and a porosity of 18%. It also showed significant oxidation and incomplete sintering.
SEM secondary electron image of incomplete sintering inside of pore in steel sample.
EDS oxygen map corresponding to the image on the left, indicating high levels of oxidation in grain boundaries and pores.
SEM secondary electron image of silicon dioxide glass inside of pore in steel sample. Silicon dioxide likely diffused throughout the samples at high temperatures, expedited by high levels of porosity.
EDS oxygen map corresponding to the image on the left.
EDS silicon map corresponding to the image on the left.
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