Feature article: PIM International, Vol.3 No. 4 December 2009, pages 21-37, 11553 words

Author: Professor Randall M. German, San Diego State University, USA

Associate Dean of Engineering, College of Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1326, USA

The powder injection moulding (PIM) of titanium, its alloys and compounds is a key area of current R&D and a spectrum of commercial opportunities arise from the marriage of titanium’s attributes with the shaping capabilities of PIM. In this specially commissioned feature, Prof. Randall German, San Diego State University, USA, reviews the current status of each aspect of Ti-PIM, from materials through to processing, properties and applications.

Titanium is always listed as a material of great promise. Once titanium production was enabled by reducing the tetrachloride with reactive metals (for example the Kroll and Hunter processes and now the Armstrong process), titanium became a wonder metal, at least for the marketing department. Titanium-rich minerals are abundant, but the conversion of the mineral to pure titanium is difficult.

Several attributes make titanium desirable, including oxidation and corrosion resistance, biocompatibility, and its lack of magnetism. When compared to stainless steel, titanium has a lower density, generally higher strength, and an ability to operate at higher temperatures and in more aggressive environments. Global consumption of titanium ranges from 50 to 60 million kg per year with final costs upward of $100 per kg for mill products. For PIM, new powder production technologies provide hope for small powders with low oxygen content, as would be suitable for
injection moulding. As knowledge is gained, Ti-PIM will be paced by economics since many of the technical issues have been addressed by considerable research over the past 20 years.

The technology

Titanium was first detected in compounds in the 1790s, but extraction of the metal using Ca, Na or Mg is a recent occurrence [1]. Below 882°C titanium is a hexagonal close-packed crystal structure (alpha) with less ductility when compared to the high temperature body-centered cubic phase (beta). However, the higher atomic packing density of the alpha phase is useful for high temperature applications. Thus, several titanium alloys form mixtures of the two phases to customise properties. Aerospace interest arises from the high strength to density ratio, useful in air up to about 600°C, while medical interest is from the high corrosion resistance and excellent biocompatibility.......

Further sections of this article include:

Markets for Ti-PIM
Applications - Evolution of titanium PIM 
 - Early demonstrations 
 - Powders 
 - Binders 
 - Mixing and feedstock 
 - Moulding 
 - Debinding 
 - Sintering 
 - Finishing 
 - Tolerances and design factors 
 - Impurity Control 
 - Microstructure control 
 - Properties 
 - Novel processes Examples of Ti-PIM products
Research and development activities
Summary
Acknowledgements
References

Figures and Tables:

Fig. 1 Scanning electron micrograph of sponge fines sieved to -325 mesh

Fig. 2 Gas atomised titanium alloy powder with the desirable size, shape and low impurity content useful in PIM

Fig. 3 Rotating electrode titanium alloy powder

Fig. 4 Angular titanium powder formed by hydriding titanium, milling the hydride, and then vacuum dehydriding the milled material (HDH)

Fig. 5 Tumbled sponge fines with a round particle shape to improve rheology and packing

Fig. 6 The sintered oxygen content versus composition for mixtures of gas and hydride-dehydride titanium powders [9]. The gas atomised powder had a median particle size of 23 µm with 0.16 wt. % oxygen and the HDH powder had a median particle size of 17 µm with 0.23 wt. % oxygen. After mixing, moulding, debinding, and sintering at 1250°C for 3 h, the pure gas atomised powder delivered 97% density, 550 MPa tensile strength, and 23 % elongation, while the pure HDH powder delivered 99% density, 710 MPa and 8% elongation respectively. The intermediate mixtures showed values between these two end points

Fig. 7 Oxygen content versus peak temperature. These data are relevant to sweepgas vacuum debinding. Plotted here is the oxygen accumulation with exposure to increasing temperature, showing rapid gains over approximately 400°C. These experiments were performed by Lefebvre and Baril [63] using porous titanium selectively exposed for 1 h in a flowing argon-oxygen atmosphere

Fig. 8 A plot comparing final oxygen and carbon levels from thermal debinding using vacuum and argon, illustrating the oxygen detriment from argon as the debinding atmosphere [55]

Fig. 9 Carbon and oxygen contents versus peak hold temperature during thermal vacuum debinding of a mixture of gas atomised and hydride-dehydride powders (Ti-6Al-4V) at a pressure of 10-3 Pa [55]. All runs were for 1 h at the peak temperature with starting carbon level of 0.056 wt. % and oxygen level of 0.192 wt. %

Fig. 10 Carbon and oxygen contents versus debinding hold time at 600°C in 10-3 Pa vacuum [55]. The powder corresponded to Ti-6Al-4V from a mixture of 90% gas atomised and 10% hydride-dehydride with an initial (prior to mixing, moulding, or debinding) carbon level of 0.056 wt. % and oxygen level of 0.192 wt. %

Fig. 11 Example time-temperature effects on sintered density for a HDH powder formed by PIM [106]

Fig. 12 Sintered density after 2 h hold for various peak sintering temperatures for a gas atomised Ti mixed with elemental Nb and milled TiAl to give Ti-6Al-7Nb [36]. A temperature of about 1300°C would be required to sinter this powder to a closed pore condition suitable for containerless HIP. If the hold time is doubled, then the required temperature drops to about 1250°C

Fig. 13 Plot of sintered tensile strength for Ti-6Al-7Nb versus sintering temperature with a 2 h hold to show how the progressive density increase in Fig. 12 translates into a strength increase [36]. The linear fit has a correlation coefficient of 0.999

Fig. 14 Tensile elongation to fracture for Ti-6Al-7Nb processed by PIM with final sintering for 2 h in the same study used for Figs. 12 and 13 [36]

Fig. 15 A scatter pot of 33 reports on the sintering time and sintering temperature used to obtain at least 95 % dense Ti-PIM body. Although there are differences in particle characteristics and alloys, the median condition is a temperature slightly over 1250°C held for 3 h

Fig. 16 Substrate effects on sintered -325 mesh Ti-6Al-4V formed from gas atomised Ti and prealloyed Al-V, where sintering was at 1200°C for 4h after debinding [45]. This study considered both zirconia and yttria as the substrates and further compared how a vacuum “baked” treatment at 1250°C for 2 h impacts the sintered strength and oxygen level. The lower axis is strength in MPa and the upper axis is oxygen in wt. % (strength is red with white hashing, oxygen is green)

Fig. 17 An example of the near surface contamination obtained in PIM titanium, in this case the microhardness trace is plotted versus depth from the free surface for a 98 % dense HDH powder compact vacuum sintered at 1250°C for 2 h [106]

Fig. 18 Optical micrograph taken from a cross-sectioned eyeglass hinge fabricated by Ti-PIM, showing considerable residual porosity

Fig. 19 Plot of the CP Ti sintered tensile strength versus oxygen content for sintered mixtures of gas atomised (0.16 wt. % oxygen initially) and HDH (0.23 wt. % oxygen initially) powders, vacuum sintered at 1250° for 3 h [9]. The straight line reflects a 0.97 correlation between strength and oxygen content

Fig. 20 Elongation to fracture plotted versus oxygen content after sintering, using data from the samples for Fig. 19 [9]. The solid line is a regression to the data, giving a 0.90 correlation

Fig. 21 Density and tensile strength for Ti-12Mo compacts sintered for 5 h at each temperature [64]. The compacts were formed from mixed powders with the titanium being -400 mesh HDH powder with 0.35 wt. % oxygen and 0.01 wt. % carbon, moulded with a wax-polymer binder and debound in vacuum at 400°C for 5 h prior to vacuum sintering (10-3 Pa) at various temperatures. At temperatures above 1150°C the sintered microstructures showed considerable grain coarsening that degraded strength in spite of less porosity

Fig. 22 Scatter plot of tensile strength and fracture elongation for CP Ti fabricated by PIM, based on a variety of reports. This scatter illustrates the high variation possible depending on the many adjustable processing parameters. A similar plot was given by Whittaker for Ti-6Al-4V by PIM [41]

Fig. 23 Corrosion comparison based on immersion mass loss after one week in 5% HCl solution shown versus the residual porosity for a Ti-5Al-2.5Fe composition. The porosity variation was achieved by sintering in vacuum at 1100°C for times up to 24 h [58]

Fig. 24 Microstructure for a vacuum sintered 99.5 % dense Ti-TiC composite densified at 1400°C for 60 min, giving a hardness of 54.5 HRC

Table 1 Parameters that influence the success of Ti-PIM

Table 2 Measured characteristics of different titanium or titanium alloy powders

Table 3 Examples of property combinations attained using PIM processing for titanium.