Research progress on refractory composition and deformability of shell molds for TiAl alloy castings
Vo l .
Research progress on refractory composition and deformability of shell molds for TiAl alloy castings
Document code: A
0 , You-wei Zhang
1 1. Titanium Alloy Division, Shenyang Research Institute of Foundry , Shenyang 110022, China 2. School of Materials Science and Engineering, Shenyang University of Technology , Shenyang 110870 , China
2 Chun-ling Bao
At present, most TiAl components are produced by an investment casting process. Environmental and economic pressures have, however, resulted in a need for the industry to improve the current casting quality, reduce manufacturing costs and explore new markets for the process. Currently, the main problems for investment casting of TiAl alloys are cracks, porosities, and surface defects. To solve these problems, many studies have been conducted around the world, and it is found that casting defects can be reduced by improving composition and properties of the shell molds. It is important to make a summary for the related research progress for quality improvement of TiAl castings. So, the development on refractory composition of shell molds for TiAl alloy investment castings was reviewed, and research progress on deformability of shell mold for TiAl alloy castings both at home and abroad in recent years was introduced. The existing methods for deformability characterization and methods for improving the deformability of shell molds were summarized and discussed. The updated advancement in numerical simulation of TiAl alloy investment casting was presented, showing the necessity for considering the deformability of shell mold during simulation. Finally, possible research points for future studies on deformability of shell mold for TiAl alloy investment casting were proposed.
TiAl based alloys; investment casting; shell mold; deformability
-
Nused as high temperature structural materials.
ickel based superalloys are currently widely
However, it is more and more difficult to increase the
service temperature because of the inherent limitation
of superalloys. Due to the co-existence of metallic
and covalent bond, intermetallic materials have higher
specific strength and rigidity than common metals and
better ductility than ceramics, which can be a potential
substitute for conventional superalloys. Among these,
TiAl based alloys are becoming competitive materials
for heat-resistant structural components in aero and
auto engines and have great application potential for
their low density, at only half of nickel based alloys, as
well as good creep resistance and oxidation resistance
[
1
]. In 2007, the US Boeing company declared that TiAl
alloys would be used for low pressure turbine blades in
GEnx engine on the 787 airplane, which could reduce
the weight of the engine by 800 pounds [
2
]. That was a
milestone for the application of TiAl alloys to reduce
engine weight and prompt the application of TiAl
alloys around the world. In recent years, the studies on
TiAl alloys were focused on the relationship among
process, microstructure and mechanical properties,
manufacturing and forming technologies, as well as
the TiAl based composite materials [
3-7
]. Until now,
processes similar to those for nickel based alloys have
been developed for manufacturing and forming of TiAl
alloys, including ingot metallurgical methods (casting
ingot plus thermal mechanical treatment), casting
process and powder metallurgy.
Casting methods such as investment casting process[
8
],
permanent mold casting process[
9
] and centrifugal
casting process[
10
] are still the main forming technology
for TiAl components for their low cost and net shape
advantages over other forming technologies. Among
which, the investment casting method is usually used
to form parts for aero and auto engine blades and high
pressure turbine blade shield plates due to the accurate
casting dimensions and high surface quality.
Although the investment casting process of TiAl
alloys has the advantages of net shape and low cost,
problems including cracks, porosities and surface
defects are easily found in castings [
11, 12
]. The investment
casting process consists of several steps in which one
of the key steps is production of a ceramic shell since
soundness and quality of the ceramic shell has a direct effect
on the soundness of the castings [13]. The residual stress and
subsequent distortion in castings can be related to the difference
between the thermal expansion characteristics of the alloy
and the ceramic shell [
13
]. Mechanical interaction between the
ceramic shell mold and solidifying alloy can cause residual
stress, distortion and hot tearing cracks. According to statistics,
more than 70% of defects and scraps of TiAl alloy castings are
caused by shell mold quality, so the property of the shell mold is
an important issue for TiAl alloy precision casting[
14
]. To solve
these problems, a lot of studies have been carried out on shell
molds for TiAl alloy castings in the aspect of compos (...truncated)