Press and sintering of titanium Robert Frykholma * and Benjamin Brashb Höganäs AB, SE-263 83 Höganäs, Sweden [email protected], [email protected]

Keywords: Compaction, Titanium hydride, Densification keyword index.

Abstract. Titanium based alloys show very high strength to density ratio, and could be the choice of material for a wide range of applications. By practicing press and sintering for production of Ti components, high materials utilization can be assured, and at the same time costly machining operations can be limited. Difficulties in processing and high cost for powders have been limiting factors for progress, but recent development indicates good possibilities on cost effective use of TiH2 powder in the press and sintering segment. In this paper, influence of processing parameters and powder properties on final sintered component properties are discussed.

Introduction Titanium alloys and near net shape processing is a very attractive combination since it allows good material utilization, and difficult and expensive machining operations can be avoided. However, high material cost has been a limiting factor, and applications have mainly been found in aerospace and medical sectors. More and more efforts have during the last years been put into development of processes for production of so called low-cost titanium powder, where the aim is set to reduce powder cost by more than 70% compared to atomized powder. With low-cost powders new application areas, such as automotive where low weight and high performance is of interest, could open up. Powder techniques for component manufacturing where Ti alloys are used today are e.g. metal injection molding (MIM), and additive manufacturing (AM). For both MIM and AM powder properties are very important, and a spherical powder with high tap density is to prefer. This since the packing of the powder before consolidation through heat treatment is important. Most processes for low-cost powder results in rather irregular powder morphology. This is a drawback for these processes, since costly post processing of the powder, such as plasma speroidizing might become necessary. Therefore, it can be difficult to compete with atomized powders for these types of applications. For conventional press and sintering or cold isostatic pressing, the demands on powder properties are different. Spherical morphology might be a disadvantage, since an irregular shape offers better binding between powder particles in green state. This could make some of the low- cost powders more suitable for compaction processes in as-received state. However, the use of Ti powder for press and sintering is at present very limited. Some overviews of the titanium P/M technology and applications can be found in [1-8] Höganäs AB has started to look into the titanium market, and one goal is to produce a Ti or Ti alloy powder possible to use in a press and sintering process similar to that for Fe-based alloys today. For effective press and sintering of metal powders in commercial scale there are a number of factors that need to be considered. To start with, flow properties of the powder are important. If the powder don’t show sufficient flow, transport to and filling of the die might not be satisfactory. A common problem with many of the so called low-cost production routes for Ti powder is that the powder obtained usually has an irregular shape, resulting in rather poor flow properties. They can also have rather small particle size, which also affects flow in a negative

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 1 way. One way to increase flowability is to mechanically alter the particle shape. This can be done by milling, but then there is also a risk of introducing more oxygen into the powder in the milling process. Also, this will result in even finer particle size. Another method is plasma spheroidization. The powder particles are then introduced into a plasma torch, where they are melted into spherical shape. This, however, significantly increases production cost. Also, as mentioned, spherical particle shape is usually not an advantage when it comes to compaction and the resulting green strength of the compacts. It is a method more suitable for powder to be used in additive manufacturing applications where high tap density is important. An alternative to altering the shape of the particles is to modify surface properties with additives or perform agglomeration. [9,10]. A powder with small particle size can be difficult to handle in compaction. Fine particles might fall between punch and die, and cause the tool to seize. This can be avoided by minimizing the die-punch clearance, but this in turn will lead to difficulties when using high compaction pressure. Powders with small particle size are therefore easiest to run with low compaction pressure. This will of course be reflected in the green density, where high levels will be difficult to reach. But even at high compaction pressure high green density might be difficult to reach with Ti. The powder particles will deform during compaction, the oxide scale will break up and expose metal surfaces susceptible to cold welding, resulting in strong bonds between powder particles, and a porous metal matrix difficult to densify further. An alternative to use metal Ti powder in press and sintering operations, is to use a hydride powder. The choice of base powder will highly influence the green strength and compaction behavior. TiH2 is a hard and brittle phase, and compaction of TiH2 powders can give quite high green densities. The reason is that the problem with friction and cold welding, which can be pronounced for Ti, is reduced. The hydride particles rearrange and break during compaction, and a high green density can be obtained. Sintering of TiH2 powders is described in [11], where trials resulted in relative sintered density greater than 99%. In [12] press and sintering of Ti and TiH2 powder is compared, and higher green densities were reached with TiH2. Since no strong bonds are formed between powder particles during compaction of TiH2, very low green strength is obtained. For this powder material it can therefore be necessary to use some sort of additive to enhance green strength and make the green components possible to handle without breaking. For effective compaction, a lubricant also has to be added, and the best solution to the green strength problem is to use a lubricant which can also provide green strength to the component. Introduction of an organic lubricant to a Ti powder mix has to be done with caution; Ti has very high affinity for both C and O, and all residues of the lubricant present at high enough temperature will be dissolved in the titanium matrix and affect mechanical performance of the sintered part. A demand on a lubrication system for Ti is therefore that it effectively can be removed at sufficiently low temperature, without leaving any residues in the powder compact. For this reason admixed lubricants are usually avoided for titanium and instead die wall lubrication is applied. This is however not an effective solution for large scale commercial production of sintered components. Comparison between internal and die wall lubrication when compacting CP Ti can be found in [13]. In this study highest densities were obtained with internal lubrication. In traditional press and sintering it is usually desired to have as low as possible dimensional change between green and sintered state. High compressibility of the powder in order to reach as high green density as possible is an advantage. During sintering bonds are formed between powder particles, but shrinkage or swelling is kept at a minimum. Density levels around 90% of theoretical are common. Sintering to high density would mean loss of dimensional accuracy, and costly secondary operations would be necessary. This means that performance might not be on the same level as for a fully dense material, but steel press and sintering is to a large extent cost oriented, not striving for maximum performance, and for many applications press and sintering is a cost effective solution which provides the performance necessary.

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 2

Titanium P/M is more high performance oriented due to the high cost for the powder in combination with more complicated processing. To obtain properties usually desired, close to full density is a requirement. Therefore one has to compact to close to full density, or accept shrinkage during sintering. High green density levels have been reached with TiH2. However, with TiH2 shrinkage will nevertheless take place due to the change in molar volume associated with the removal of hydrogen to obtain metal Ti. Consequently, shrinkage has to be accepted when working with Ti alloys in press and sintering operations if aim is set for highest performance.

Experimental The purpose of this work was to study the influence of processing parameters on green strength, green density, dimensional change, and sintered density. The powder used in this study was of type TiH2. The material was produced by Höganäs AB. For comparison, also a milled hydride powder was included. Figure 1 shows scanning electron micrographs of the powders. As can be seen, the morphology is quite different between the powders, with a more rounded irregular shape of the Höganäs powder, and a more angular appearance of the milled TiH2. Both powders have relatively small particle sizes, but some larger particles are present in the milled powder. Physical properties are presented in Table 1.

Figure 1. SEM micrographs of the TiH2 powders used in the study; Höganäs solid state reduced TiH2 (left), and milled TiH2 (right).

Table 1. Physical properties of the powder. Höganäs Milled TiH2 TiH2 X50 6-10 µm 16.7 µm Apparent density 1.29 g/cm3 1.05 g/cm3 Tap density 1.7 g/cm3 -

To obtain flow of the powder, a pre-treatment with a polymeric lubricant was performed. In the treatment process, a lubricant addition of 1-2% by weight was used. This treatment increased flow of the fine powders, and made it possible to effectively handle in a press and sintering operation. Table 3 shows flow values for the powders measured with a Gustavsson flowmeter. The measurement was not performed according to standard. Due to the low density of the powder, only 25 g of powder was used instead of the required 50 g. The time was then doubled for comparison reasons.

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 3

For compaction, no further addition of lubricant to the system was necessary.

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 4

Table 2. Flow properties for lubricant treated powder with 2% of lubricant.

AD Flow [g/cm3] [s/50 g] Höganäs TiH2 0.76 105

Milled TiH2 0.99 88

Compaction of cylindrical specimens with a diameter of 25 mm was performed using a single action tool, at compaction levels between 200-800 MPa. 20 g of material was used for each specimen. Compaction was performed at room temperature, and with the die heated to 90°C. Green density was obtained by measurement of dimensions and weight. Green strength was calculated from the force required to break an unsintered specimen supported as a simple beam while subjected to a uniformly increasing three-point transverse load. Sintering was performed at 1150, 1200, and 1300°C for 2h. Delubrication and sintering trials were performed in a laboratory furnace in H2 and Ar, where Ar was used during the last 30 min at sintering temperature and during cooling for removal of hydrogen from the sintered compacts. After sintering density was obtained by using weight in air and weight in water method.

RESULTS AND DISCUSSION Compaction of lubricant treated powders was performed at different compaction levels, with different amount of lubricant, and with both cold and warm die. For the warm compaction trials, the die was heated to 90°C, while the powder was kept at room temperature. The results for green strength are presented in Figure 2. Both compaction pressure and amount of lubricant has a positive effect on green strength. It can also be seen that warm die compaction increases strength. Above 5 MPa in green strength, the specimens are quite possible to handle without too much caution, and most of the points in the diagram are found above this limit. As was mentioned in the introduction there might be benefits with low compaction pressure, and by using warm die compaction and an addition of 1.5 % lubricant, it is possible to go as low as 200 MPa. The green strength is almost doubled for warm die at this pressure. At higher compaction level, the positive effect drops, and at 800 MPa the green strength increases with approx. 30%. Also for 1% lubricant there is a positive effect from warm die compaction, but not as pronounced as for 1.5%; increase in strength is approximately 20-30% over the whole compaction range.

25.0 1%

20.0 1,5% 2% 15.0 1% warm die (MPa)

1.5% warm die 10.0 Strength 5.0 Green 0.0 200 400 600 800 Compaction pressure (MPa)

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 5

Figure 2. Green strength for different compaction pressure, lubricant addition, and tool conditions. Both the Höganäs and the milled powders were found to be quite difficult to compact to high green density. Compaction curves for 2% lubricant are found in Figure 3. The morphology of the Höganäs powder is a disadvantage when it comes to compaction, and the milled powder reaches approximately 8-10 % units higher in green density for all compaction pressures. The curves show that for both powders there is a response for increased pressure all the way up to 800 MPa. There is a tendency to flattening, but based on the shape of the curve it is reasonable to assume that higher densities could be possible to reach with higher pressure. Compaction trials for green density measurements were performed with 1% and with 2% lubricant. The densities obtained were almost identical for the two cases. In the compaction range tried, and with a lubricant addition of 1-2%, it is therefore powder properties that limit densification. To visualize the densification necessary to achieve full density, relative density curves are presented in Figure 4. The solid lines show density relative to what is actually possible to achieve in green state, where the volume of the lubricant (2% by weight) also has been included. For the milled powder it can be seen that there is still approximately 6 % of porosity at the highest compaction pressure. For the Höganäs powder the porosity level for the same pressure is approximately 13%. If the lubricant is considered as porosity and is removed from the calculation, the green density is related to pure TiH2 and the relative density values drop with ~4-4.5 % units at 800 MPa, and with somewhat less at lower pressures. The gap represents the volume of the lubricant. During compaction the lubricant redistributes and fills out the porosity in the green component. With high compaction pressure, and porosity levels moving towards zero, lowering amount of lubricant becomes important for densification. At low compaction levels reduction of amount of lubricant content will have little effect on final green density. If possible, the amount of lubricant should nevertheless be reduced, in order to limit the amount of impurities necessary to remove during delubrication. The third curve shows density related to pure Ti, i.e. here also hydrogen has been removed from the calculation. What the curve represents is density related to full density obtained after sintering. It is clear from the curve that even if high green density is reached for the TiH2+lubricant system, there is still substantial densification needed during sintering in order to achieve full density of the final components. For the milled powder, the density related to Ti at the highest compaction level is 71%, and for Höganäs powder only 66%. This corresponds to a linear shrinkage of approximately 11 and 13% needed for full density after sintering for milled and Höganäs TiH2, respectively. In the lower compaction pressure end, the corresponding shrinkage is 17 and 19%, respectively.

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 6

Höganäs TiH2 Milled TiH2

3.6

3.4

3.2 ) 3 3 (g/cm 2.8

2.6 density

2.4

Green 2.2

2 200 300 400 500 600 700 800 Compaction pressure [MPa]

Figure 3. Compaction curves for Höganäs TiH2 and milled TiH2.

Milled TiH2+binder TiH2 Ti Höganäs TiH2+binder TiH2 Ti 100

95

90

85 (%)

80 density

75

70

Relative 65

60

55

50 200 300 400 500 600 700 800 Compaction pressure [MPa]

Figure 4. Relative density for the powders, relate to the TiH2+lubricant system, to pure TiH2, and to pure Ti.

When sintered at 1300°C, the fine powders show very high sintering activity, and the shrinkage is substantial, Figure 5. The Höganäs powder shows densities above 99% of theoretical for all compaction pressures, with close to full density for 800 MPa compactions. The high densification obtained is positive from compaction point of view. It shows on the possibility to compact Höganäs powder at low pressures, which means easier compaction with less tool wear and possibility to use tools with small die-punch clearance. The milled TiH2 shows lower density levels, and a stronger dependence on compaction pressure, where 99% of theoretical density is reached after 800 MPa compaction, and above 95% after 200 MPa. The sintered density is strictly increasing with compaction pressure, and no

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 7 flattening can be determined. The reason for the higher porosity is the larger powder particle size and smaller specific surface, which leads to lower sintering activity. The linear dimensional change, green to sintered, can be found in Figure 6. Change in diameter and height was measured. The Höganäs powder shows a smooth variation in DC with compaction pressure, while the milled powder is a bit more unpredictable, especially in the height DC. This becomes very evident when looking at the variation between the curves for height and diameter DC. The larger variations for the milled powder could be a result of wider particle size distribution. A predictable behavior regarding dimensional change is an important factor when it comes to production of component and tolerances needs to be kept tight.

Höganäs TiH2 Milled TiH2

100 4.49 99 ) (%) 4.44 3

98 (g/cm

density 4.39 97 density 4.34

sintered 96

95 4.29 Sintered Relative

94 4.24 200 300 400 500 600 700 800 Compaction pressure [MPa]

Figure 5. Relative and absolute density after sintering.

Höganäs TiH2, diameter Höganäs TiH2, height Milled TiH2, diameter Milled TiH2, height ‐5 (%) ‐10 change ‐15

‐20 Dimensional

‐25 200/29 300/44 400/58 500/73 600/87 700/102 800/116 Compaction pressure [MPa]

Figure 6. Dimensional change, green to sintered for Höganäs TiH2 and the milled TiH2.

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 8

With the high densification obtained at 1300°C, trials were performed to investigate the effect of lower sintering temperature. In figure 7, relative density curves are found for 1300, 1200, and 1150°C. The curves are denoted H and M, for Höganäs and milled powder, respectively. Lower sintering temperature results in lower densities and a more clear dependence on green density, but densification of Höganäs powder is very high already at 1150°C, and values between 96.5 and 99% are obtained.

100 1300°C, H 99 1200°C, H 1150°C, H 98 1300°C, M [%]

97 1200°C, M

density 96

95

Relative 94

93 200 300 400 500 600 700 800 Compaction pressure (MPa)

Figure 7. Relative density for different compaction pressure and sintering temperature. Höganäs powder denoted H, and milled powder denoted M.

In the micrographs of Figure 8, examples of pore structure after sintering can be found. The specimens were compacted at 600 MPa and sintered at 1300°C. For both types of TiH2 a fine and evenly distributed porosity is observed. For the milled TiH2, with lower sintered density, the pores are larger and more frequently occurring. At lower sintering temperature and/or compaction pressure, the same type of porosity is found, only with increased amount.

20µm

Figure 8. Porosity of specimens compacted at 600 MPa and sintered at 1300°C in vacuum. Höganäs TiH2 (left) and milled TiH2 (right).

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 9

CONCLUSIONS

Comparison of compaction and sintering behavior of a TiH2 powder produced by Höganäs AB and a milled “conventional” TiH2 powder was performed. The Höganäs powder has a small particle size and irregular shape. The milled powder is coarser and more angular in shape. For lubrication and enhanced green strength, the powders were treated with a lubricant. Green strength measurements of Höganäs powder results in values 2.5-18 MPa in the compaction range 200-800 MPa. By utilizing warm die compaction green strength was increased, and acceptable results were obtained already at 200 MPa. The highest green density was reached with the milled TiH2; 89% of theoretical at 800 MPa, for the Höganäs powder 83% was reached. Compared to a milled TiH2, with more angular powder particle shape, the sintering activity of the Höganäs powder is higher, leading to higher sintered density. Density levels, after sintering at 1300°C for 2h, are relatively unaffected by green density for the Höganäs powder, which shows density levels above 99% of theoretical for all compaction pressures tried. The sintered density of the milled powder shows a dependence on green density, with levels 95-98% for 200-800 MPa compaction. For lower sintering temperature, the Höganäs powder is more sensitive to green density. At 1150°C relative density between 96.5-99% is obtained.

REFERENCES [1] F. H. Froes, D. Eylon, Powder metallurgy of titanium alloys, International Materials Review, vol 35, no 3 (1990) 162-182 [2] C. G. McCracken, C. Motchenbacher, D. P. Barbis, Review of titanium powder production methods, Powder Metallurgy, vol 46, no 5 (2010) 19-26 [3] F. H. Froes, S. J. Mashl, V. S. Moxson, J. C. Heibeisen, V. A. Duz, The technologies of titanium powder metallurgy, JOM, vol 56, no 11 (2004) 46-48 [4] F. H. Froes, M. Ashraf Imam, Cost Affordable Developments in titanium technology and applications, Key Engineering Materials, vol 436 (2010) 1-11 [5] R. M. German, Status of metal injection molding of titanium, Powder Metallurgy, vol 46, no 5 (2010) 11-17 [6] I. M. Robertson, G. B. Schaffer, Review of densification of titanium based powder systems in press and sinter processing, Powder Metallurgy, vol 53, no2 (2010) 146-162 [7] M. Qian, Cold compaction and sintering of titanium and its alloys for near-net-shape or preform fabrication, Int J of Powder Metallurgy, vol 46, no 5 (2010) 29-44 [8] D. Whittaker, Powder processing, consolidation and metallurgy of titanium, Powder Metallurgy, Vol. 55, No. 1 (2012) 6-10 [9] R. Frykholm, H. Vidarsson, Ti alloys in PM applications, proceedings World Congress on Powder Metallurgy & Particulate Materials, Orlando, Florida, USA, Volume 09 (2014) [10] R. G. Neves, B. Ferrari, A. J. Sanchez-Herencia, E. Gordo, Improvement of sintering behaviour of titanium by colloidal techniques, Powder Metallurgy, Vol. 56, No. 4 (2013) 258- 262 [11] H. Wang, M. Lefler, Z. Z. Fang, T. Lei, S. Fang, J. Zhang, Q. Zhao, Titanium and titanium alloy via sintering of TiH2, Key Engineering Materials, vol 436 (2010) 157-163

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 10

[12] I. M. Robertson, G. B. Schaffer, Comparison of sintering of titanium and titanium hydride powders, Powder Metallurgy, vol 53, no 1 (2010) 12-19 [13] T. Zwitter, X. Xu, P. Nash, Energy efficient press and sinter of titanium powder for low-cost components in vehicle applications, Proc. Cong. On “Advances in Powder Metallurgy and Particulate Materials”, Las Vegas, USA, June 2009

Presented at Press and Sintering of Titanium 2015 in Lüneburg, Aug.31—Sep.3 Page 11