Feature article: PIM International, Vol.4 No.4 December 2010, pages 22-33, 5444 words

Author: Éric Baril, ing., Ph.D, National Research Council Canada

Industrial Materials Institute, 75 de Mortagne Blvd, Boucherville, Québec, J4B 6Y4, Canada

  

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An essential report that looks at how the correct management of process variables enables Ti-PIM to match the demanding mechanical and chemical property requirements for the all important aerospace and medical markets

Titanium powder injection moulding (Ti-PIM) is coming ever closer to delivering its promise of penetrating high value markets, such as the aerospace and medical sectors.

The biggest remaining challenge is to successfully match the specified mechanical and chemical requirements of final applications.

As Éric Baril from Canada’s National Research Council explains, the challenge of understanding and managing both the starting material and the various processing steps is no small challenge, but is essential to meet customer expectations.

Introduction
Titanium and titanium alloys have several attributes that make them desirable for a large spectrum of applications. Strength-efficient structures and corrosion-resistant service are the two main areas of application where the unique set of characteristics justifies the selection of these materials.

The combination of high strength, stiffness, good toughness, low density and good corrosion resistance provided by various titanium alloys at very low to moderately elevated temperature allows weight savings in aerospace structures and other high-performance applications. The excellent corrosion resistance coupled with good strength make titanium and its alloys useful in chemical and petrochemical applications, and marine environment applications.

For medical applications, the ability to passivate allows titanium based materials to be nontoxic and generally biocompatible with human tissues and bones. This characteristic, coupled with its non-magnetic nature and Young’s modulus close to that of bone, makes titanium the material of choice for biomedical implants among all metallic biomaterials.

Matching the application requirements can be a long journey when several variables are involved in the definition of the material properties. A good understanding of the effect of the key variables on the material attributes is, therefore, necessary in order to keep a firm control on the process and match customer expectations. .......

Further sections of this paper include:

Material requirements
Sources of interstitial contamination
- Analytical techniques
- Contribution of the main process variables to the oxygen pick-up
- Powder contribution
- Binder contribution
- The impact of debinding
- The impact of sintering
Conclusion
Acknowledgements

Figures and Tables:

Fig. 1 Influence of oxygen content on fatigue crack propagation in Ti6Al4V in the annealed condition [5]
Fig.  2 Influence of microstructure on high cycle fatigue behaviour (R=-1) of Ti6Al4V [6]
Fig. 3 Microstructure of Ti6Al4V; (a) Lamellar microstructure and (b) Equiaxe microstructure (Courtesy of Helmholtz-Zentrum Geesthacht – Orley Ferri and Thomas Ebel)
Fig. 4  Yield strength, ultimate tensile strength and elongation as a function of the Oeq from various sources of data
Fig. 5 Prediction of a) yield strength (MPa), b) ultimate tensile strength (MPa), c) elongation (%) for a range of relative density and Oeq from a multivariate model generated from Fig. 4
Fig. 6 Yield strength, ultimate tensile strength and elongation as a function of relative density for Oeq below 0.34wt.%
Fig. 7  Bending fatigue S-N curves of Ti6Al4V-PIM with different post-treatments [12,13,14]
Fig. 8 PIM knee implant parts made by Ti6Al4V-PIM (Courtesy of Maetta Sciences Inc., Canada)
Fig. 9 Sources of interstitial oxygen contamination in typical PIM process [19]
Fig. 10 Absolute contributions of powder, debinding and sintering cycles on the final oxygen content of titanium PIM components [19]
Fig. 11 Pareto graph of the contributions of the main titanium PIM variables to the final oxygen content [19]
Fig. 12  a) Oxygen content of various powders (types and sizes) stored in unsealed steel containers in air under uncontrolled conditions and, b) morphology of HDH and plasma atomised powders [19]
Fig. 13 Oxygen content in titanium foams exposed for 60 minutes at various temperatures in argon containing 20vol%O2 [24]
Fig. 14 Comparison of oxygen content in loose titanium powders and a 2mm rod after exposure for 60 minutes to various temperatures at 10-6 Torr vacuum [19]

Table 1 Selected ASTM standards for titanium products
Table 2 Possible chemical requirements assuming an Oeq of 0.34wt.% compared to the ones of the ASTM standards reported in Table 1
Table 3 Round robin results of oxygen and carbon analyses (wt.%) on hydride/dehydride (HDH) -45µm powder conducted with 4 certified laboratories serving aerospace and medical sectors