dc.description.abstract |
Titanium (Ti) has attracted wide attention for decades since it exhibits outstanding properties
for various applications. However, due to expensive raw Ti and costly processing, its
applications are still limited – only in those applications where the performance can justify
the costs. Therefore, Metal Injection Moulding (MIM) has received increasing attention in
recent decades to prepare Ti products as a cost-effective and competitive processing route.
Nevertheless, impurity control is still a great challenge to Ti Metal Injection Moulding (Ti-
MIM). Binder system design and the corresponding debinding techniques are crucial for the
impurity control of Ti-MIM. Hence, developing new binder systems specifically for Ti-MIM
is of great importance.
Water-soluble binder systems are environmentally friendly. Much attention has been paid to
binder systems based on polyethylene glycol (PEG) due to the commercial availability, high
solubility in water and non-toxicity. Although PEG-based binder systems have been reported
in the literature for years, their applications in the Ti-MIM industry are rarely seen.
Furthermore, few studies on specific interactions among the particular binder system
components and the effects of such interaction on Ti-MIM feedstocks are available.
Therefore, this work aims to develop an easy-to-decompose PEG-based binder system for Ti-
MIM and evaluate polymeric interactions between different binder components to promote
the applications of water-soluble binder systems in the Ti-MIM industry.
In this project, an extensive study of water-soluble PEG-based binder systems was carried
out. Rheological analysis, thermal analysis (thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC), mechanical tests, impurity measurements (oxygen
(O), carbon (C) and nitrogen (N)), and microstructural analysis (optical microscopy and
scanning electron microscopy (SEM)) have been extensively used to evaluate binder systems.
Importantly, the macromolecular interactions between different binder components were
investigated with Fourier-transform infrared (FTIR) spectroscopy. Furthermore, an in situ
polarised optical microscope was employed to observe the PEG crystallisation process and
reveal the mechanism of void formation in the PEG/polymethyl methacrylate (PMMA)
binder systems and how to eliminate these voids. This study may benefit the development of
environment-friendly and clean binder systems suitable for Ti-MIM. The main findings are
summarised below:
1) The biodegradable polymer, polypropylene carbonate (PPC), was proven a promising
backbone polymer for the PEG-based binder systems. Although PPC cannot act as the
backbone component alone, incorporating a small amount of PMMA into the
PEG/PPC binder system can effectively improve the green strength and provide
adequate dimensional stability for the subsequent thermal processing. Therefore, a
PEG/PPC/PMMA binder system was proposed and investigated for Ti-MIM. In this
binder system, the development of H-bonding interactions through the C=O group
from PMMA and PPC in the ternary binder blend resulted in enhanced green strength
and improved thermal stability. Ti feedstock made from this binder system obtained
good rheological properties and feedstock homogeneity. Furthermore, the resulting
sintered samples showed low impurity (O, C and N) contents and good mechanical
properties fulfilling ASTM F-2989 Grade 3 requirements.
2) The promising PEG/PPC/PMMA binder system was further improved by the adding a
small amount of polyvinyl acetate (PVAc). The small amount of PVAc effectively
improved the rheological properties and homogeneity of Ti feedstock and the green
strength of the injection moulded samples. The resulting sintered samples made from
this improved binder system showed excellent mechanical properties (98% of relative
density, 549 MPa of UTS and 14.7% of elongation rate) and low impurity (O, C and
N) contents. The optimised binder system D (76wt% PEG + 17wt% PPC + 3wt%
PMMA + 2wt% stearic acid (SA) + 2wt% PVAc) provided good mechanical
properties while maintaining the environment-friendly and clean nature of the
PEG/PPC based binder system.
3) PVAc addition was proven a simple yet effective way to solve the issue of voids
formation in MIM green samples associated with the PEG/PMMA binder systems.
The direct evidence of the void formation of the PEG/PMMA binder system was
found for the first time using in situ polarizing optical microscopy. The mechanisms
of the PEG/PMMA binder system void formation and the PEG/PMMA/PVAc binder
system void elimination were investigated. The voids were successfully eliminated by
incorporating PVAc into the binder system. PVAc addition into the PEG/PMMA
binder system leads to complex interactions between the three components. The three
components interact in a way that the voids created due to PEG/PMMA interactions
are filled up by molecular chains of PVAc. Titanium feedstock made from this novel
binder system had good rheological properties, which resulted in a defect-free
injection moulding process. The green samples had excellent shape retention during
the subsequent water debinding process. As a result, the sintered articles showed good
mechanical properties. The designed binder system maintains the environmentfriendly
nature of the PEG-based systems without compromising quality.
In short, PPC might be a promising option for Ti-MIM due to its low decomposition
temperature and no decomposition residues. Although PPC cannot act as a backbone
component alone, with a small amount of PMMA and PVAc, a promising easy-to-decompose
and clean PEG/PPC based binder system was developed. Furthermore, PVAc addition was
effective to solve the issue of voids formation in MIM green samples associated with the
PEG/PMMA binder system. Hence, another easy-to-decompose and feasible PEG/PMMA
binder system was developed with the incorporation of PVAc. |
|