Technical Paper: PIM International, Vol.4 No.4 December 2010, pages 54-62, 3825 words

Authors: A. T. Sidambe[1], I. A. Figueroa[1],[3], H. Hamilton[2], I. Todd[1]

[1] University of Sheffield, Materials Science and Engineering Department, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, United Kingdom
[2] Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading, Berkshire, RG4 9NH, United Kingdom
[3] Instituto de Investigaciones en Materiales, UNAM, 04510 México D. F., México

Abstract

Ti-6Al-4V powders with powder size of sub 45µm were mixed with a binder with a water soluble component and subjected to metal injection moulding studies. The binder system consisted of a major fraction of water soluble Polyethylene Glycol (PEG), a minor fraction of polymethylmethacrylate (PMMA) and some stearic acid used as a surfactant. A critical powder loading of 69 vol % was obtained which exhibited a pseudo-plastic flow.

The injection moulding, debinding and sintering processes were studied. A low-cost, two stage and rapid debinding process which involved solvent debinding in distilled water at 55°C for 6 hours and then removing the remaining PMMA via thermal pyrolysis by heating in flowing argon was selected. Sintering was also carried out in flowing argon.

Carbon and oxygen contents achieved were within ISO 5832 standard specifications for titanium, as were the mechanical properties of as-sintered specimens.

Introduction

Titanium and titanium alloys exhibit a high specific strength and stiffness, outstanding corrosion resistance and biocompatibility. This combination of properties makes titanium and its alloys an excellent choice for applications in watch parts and sports goods [1], and also provides a great potential for biomedical and aerospace applications. However, the processing of titanium is limited by costly, multi-step processes of fabrication and associated geometry design constraints [2]. Metal injection moulding (MIM) is a technique that can provide minimisation of such problems.

MIM is a well-established, cost-effective method of fabricating small-to-moderate size metal components. In this process metallic powders are injected into a mould. Plasticity and fluidity of the powder is essential for this to take place and this is achieved by the use of binder material. All binder systems are based on two important major groups of ingredients, polymers and waxes with minor additions of lubricants, surfactants and coupling agents. After injection moulding the binders are then removed in a process known as debinding and the remaining “brown” part is then sintered at elevated temperatures to achieve a densified part.........

Further sections of this paper include:

• Experimental
• Results
  - Rheology and Feedstock
  - Solvent Debinding
  - Thermal Debinding
  - Sintering
• Discussion
  - Solvent Debinding
  - Thermal pyrolysis
• Conclusion
• Acknowledgements
• References

Figures and Tables:

Fig. 1 Scanning electron micrograph showing the morphology of the Ti6Al4V
Fig. 2 Particle size distribution for Ti6Al4V
Fig. 3 Chemical Structures of PEG and PMMA
Fig. 4 Apparent viscosity vs apparent shear rate of 69 vol % Ti-64 feedstock at different temperatures
Fig. 5 Scanning electron micrograph of an as moulded component surface
Fig. 6 Scanning electron micrographs showing the development of pores during the removal of PEG after 1 hour and after 4 hours at 55°C
Fig. 7 Hg porosimetry result after leaching of MIM sample
Fig. 8 Amount of PEG removed from the moulding versus the leaching time at 55°C
Fig. 9 DSC showing scans of an as moulded Ti6Al4V green specimen and that of a leached part after solvent debinding. The scans were carried out at 5°C/min from 30 to 70°C
Fig. 10 Scanning electron micrograph which shows the brown part after the removal of the PMMA during thermal pyrolysis
Fig. 11 Weight loss curves for PMMA at a heating rate of 5°C/min for Ti/binder mix in Ar and air before and after thermal pyrolysis
Fig. 12 Photograph illustrating the linear shrinkage undergone by a MIM sintered Ti6Al4V part
Fig. 13 The pore structures and microstructure of the Ti6Al4V MIM components
Fig. 14  Fracture surface of a Ti6Al4V MIM-tensile test sample after mechanical test showing a honeycomb structure which signifies a typical sign for a ductile break
Fig. 15 ln(1/F) with leaching time at 40, 55 and 75°C. F is the remaining fraction of PEG

Table 1 Ti6Al4V chemical properties
Table 2 Ti6Al4V injection moulded component mechanical and chemical results

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