PM processed titanium foam: Influence of morphology and content of space holder on microstructure and mechanical properties

Introduction

Titanium is a unique low density material having tensile strength comparable to that of various grades of steels and iron based super alloys. It has excellent corrosion resistance, which makes it suitable for use in corrosive environments such as the human body. The element is also capable of retaining its high tensile strength at elevated temperatures (∼400°C). ¹ Accordingly, Ti foam is now being utilised in various engineering and biomedical applications.^2–6 Metallic foams possess high strength/weight ratio, high stiffness and good impact energy absorption characteristics in general. This has made them suitable for use in various applications such as lightweight structures and energy and noise absorbers.^2–4 In this context, Ti foam has so far been utilised as sandwich cores in aerospace and naval applications for energy absorption. The excellent heat resistance of Ti foam has made it useful in applications such as heat exchangers and catalyst substrates. ³ Of late, it has also been applied in various biomedical applications due to its excellent biocompatibility and corrosion resistance.^5,6 However, Ti is difficult to be processed through the conventional casting technique due to its high affinity for the atmospheric gases at higher temperatures. ³

It may be worth mentioning that the processing of Ti becomes more effective in partially melted condition through the powder metallurgy (PM) route. Various PM methods have been discussed in the literature for processing titanium foams. One of the basic processing techniques involves partial sintering of loosely compacted Ti powder. ³ However, it becomes difficult to control the size and shape of the pores in the sintered foam samples in this process because the pore morphology depends on the morphology of the Ti powder. Another approach for processing foam is gas entrapment technique^3,7 which involves expansion of entrapped gas bubbles such as argon in the Ti matrix. However, reduced pore connectivity and need of a complicated experimental set-up make this processing route more complicated. ³ Another widely used approach is the space holder technique wherein the space holder material gets evaporated during the process of sintering and leaves behind pores. Accordingly, the pore morphology, content and distribution in this case are dictated by those of the particles of the space holder material. Thus, the pore morphology and level of porosity can be controlled effectively by varying the type, morphology and quantity of the particles of the space holder material. Various space holders like polymers, ⁴ magnesium, ⁸ acicular and spherical urea particles^9,10 have been utilised for making foams. In earlier studies, attention has been focused mainly on the role of macropores (size range, 100–500 μm) in controlling the properties of the samples, wherein the plateau strength has been found to decrease while the compressibility characteristics improved with increasing porosity level.^8,11,12 It is envisaged that morphology and mode of distribution of the pores would also make great contribution towards governing the end properties of the samples and that generation of fine (micro)pores would be beneficial in terms of superior mechanical properties. The dominant presence of micropores (size: <10 μm) is expected to impart higher compressive strength along with other beneficial effects, making the material suitable for possible structural applications. Only limited information appears to exist in this regard, ¹³ thus leaving a wide scope of studies.

In view of the above, Ti foam has been synthesised in the present work by PM route employing urea as the space holder. The influence of the content and morphology of the urea particles was studied on the microstructural features and compressive behaviour of the samples.

Experimental

Ti powder (average particle size of ∼45 μm and purity of 99·5%) supplied by Alfa Aesar, USA, urea acicular crystals (average length and width of urea particles, 1176±468 μm and 223·9±80·6 μm respectively; purity, 99·5%) and urea particles (average particle size, 51·1±22·3 μm; purity, 99·5%) supplied by Rankem/RFCL Ltd, India, were used as the starting materials. The Ti powder was mixed with varying quantities of urea. The content of urea in the case of acicular and fine particles was decided with an objective to obtain foams with a comparable level of porosity but with different pore morphologies. Accordingly, in the case of acicular crystals, the added volume fractions were 40, 50 and 60%, and those of fine particles were 50, 60 and 70%. The higher volume fraction of fine particles was added in order to compensate for a faster rate of shrinking of fine pores so created than those of coarser acicular pores during sintering. After uniform mixing, the powder mix was cold compacted using a single action hydraulic press supplied by PES Hydraulics (India) Pvt. Ltd, India. Cold compaction was performed in a 10 mm diameter cylindrical die at an applied pressure of 100 MPa. The cold compacted samples were heated at 300°C for 2 h in a tubular furnace with a view to realise complete removal of urea; the latter was confirmed through reweighing the compacts after preheating. It may be mentioned that the treatment leading to the vaporisation of urea is termed as preheating, and samples so treated are called preheated samples. The preheated powder compacts were sintered in a vacuum furnace supplied by Vacuum Techniques Pvt. Ltd, India, at 1100°C for 1 h at a vacuum level of 10⁻⁵ mbar. The rates of heating and cooling during sintering were maintained at 10°C min⁻¹. The pore morphology was examined using a JEOL 5600 scanning electron microscope. The pore size distribution in the samples was calculated using ImageJ analysis software. The density of the sintered samples was calculated with respect to the theoretical density of Ti. The cold compacted and sintered compacts were cut normal to the compacted surface to examine the pore morphology. Three samples in each case were used for the compression tests using an Instron 8801 universal testing machine at a strain rate of 10⁻³ s⁻¹.

Results and discussion

Figure 1 shows the frequency distribution of length and length/width ratio of the acicular urea particles. The length of maximum number of particles lied in the range of 1000–1500 μm, while the length/width ratio ranged from 2 to 11. The frequency distribution of the finer urea particles showed that the maximum number of particles was in the size range of 20–50 μm (Fig. 2). Figure 3 represents SEM images of the cold compacted mass normal to the compacted surface. White regions present in the microstructures correspond to acicular (coarse) and spherical (fine) particles of urea distributed in the titanium matrix (Fig. 3a and b respectively).

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1

Particle size distribution showing length and length/width ratio of acicular urea particles

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2

Particle size distributions of finer urea particles

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3

Images (SEM) showing a acicular and b fine particles of urea (white regions) in compacted samples

Figure 4 displays microstructural features of the preheated samples indicating pore morphology and their mode of distribution. The formation of acicular pores (Fig. 4a) in the case of using acicular particles of urea was noted. The pores were observed to be fine and uniformly distributed (Fig. 4b) when fine urea particles were used during processing. The uniform distribution of pores obtained in this case was due to the finer size of the urea particles, which caused them to mix much easily with the titanium powder. On the contrary, the mixing was not much effective in the case of using coarse acicular particles, thus causing a relative difference in the uniformity of pore distribution after preheating.

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4

Images (SEM) showing a acicular and b fine pore morphology (dark regions) in matrix obtained after preheating

The preheated compacts using acicular particles of urea are shown in Fig. 5. The shape and size of the preheated compacts remained undistorted until the total acicular urea content in the mixture was ∼60%. However, the compacts began to get slightly distorted during preheating and became fragile and difficult to handle when acicular urea content was increased >60%. In the case of fine urea particles, the preheated compacts were observed to be much stable and easier to handle (Fig. 6) as compared to those of acicular particles. The shape and size of the preheated compacts remained undistorted in this case even after adding (fine) urea as high as 70%. However, a further increase in the urea content led the preheated structure to collapse. Inferior stability of the preheated compacts containing acicular urea to that of the ones with fine urea particles (Fig. 5 versus Fig. 6) could be attributed to the fact that the (coarser) acicular pores (Fig. 4a) combined together to make the preheated structure weaker and distorted. On the contrary, formation of finer and more uniformly distributed pores (Fig. 4b) resulting from the use of fine urea particles imparted the samples relatively higher stability (Fig. 6) as well as ease of handling. It may also be mentioned that acicular pores having sharp corners/edges give rise to fast rate of crack nucleation and propagation in view of sharp curvature and resulting high stress concentration therein.

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5

Preheated compacts obtained after adding a 40 vol.-%, b 50 vol.-% and c 60 vol.-% of acicular crystals of urea

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6

Preheated compacts obtained after adding a 50 vol.-%, b 60 vol.-% and c 70 vol.-% of fine particles of urea

Table 1 depicts the effect of urea content on the final porosity generated in the foam samples after sintering. The porosity level showed a rising trend with the increasing content of urea in both the cases. Interestingly, the porosity level obtained in the sintered samples using acicular urea particles was somewhat greater than that of the added quantity of urea, while the trend reversed in the case of using fine particles of urea. It may be mentioned that a higher quantity of finer urea particles (compared to that of the acicular particles) had to be added to obtain a similar porosity level (Table 1). This is in view of the fact that fine pores generated in the former case have a tendency to shrink/collapse rapidly during sintering, while an acicular pore due to its larger size forms acicular porous regions with fine microporosity. The formation of fine microporosity in the acicular porous regions could be attributed to the high driving force available for pore shrinkage at the regions with sharp curvature in the acicular shaped pores.

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Table 1

Variation of porosity with changing content and morphology of urea in sintered samples and comparison of properties

Morphology of urea used (present study)	Quantity of urea added/vol.-% (present study)	Final porosity after sintering/vol.-%		Properties of the plateau regions
		Present study	Earlier study 8	Approximate plateau stress/MPa		Approximate failure strain/%
				Present study	Earlier study 8	Present study	Earlier study 8
Acicular particles	40	44	46	250	116*	55	38
	50	53	53	90	75*	58	42
	60	64	62	55	40	58	55
Fine particles	50	46	46	110*	116*	54	38
	60	54	53	70*	75*	55	42
	70	62	62	75	40	55	55

*Distinct plateau region not observed; stress value shown corresponds to yield stress only.

Figure 7 shows the microstructure of the sintered foam samples at the cross-section normal to the compacted surface processed using acicular urea particles. The formation of acicular porous regions with varying shape and size containing fine micropores may be noted in the figure, the size range of the pores being 1–10 μm. The acicular (porous) regions obtained using 40% urea got densified nearly completely and contained a smaller number of micropores within (Fig. 7a). It can also be observed that the micropores were present only within these acicular (porous) regions, while the rest of the titanium matrix underwent complete densification. An increase in the number and size of these acicular porous regions (Fig. 7b and c) and also the level of microporosity therein with the rising content of urea can also be observed. The size of these acicular porous regions increased up to 1200 μm while using 60% of acicular urea particles. This increase in their size was due to a larger number of acicular pores present before sintering (Fig. 4a), which inhibited the rate of filling and closure of pores. This slower filling up of the large sized acicular pores created large sized acicular porous regions with an increased level of microporosity.

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7

Images (SEM) of samples processed using acicular crystals of urea with a 44 vol.-%, b 53 vol.-% and c 64 vol.-% porosity

Figure 8 shows the microstructure of the sintered foam samples in the case of using finer particles of urea. The figure depicts an increase in porosity with the increasing content of urea. The pores were isolated in nature (Fig. 8a), a feature similar to that of closed cellular foam, when the overall porosity was <50%. With an increase in the amount of porosity, some coarse pores in the size range of 200–500 μm were also observed. This feature was essentially more prominent when the overall porosity increased beyond 60% (Fig. 8c). These larger pores could be an indication of the formation of interconnected network of pores. This observation can further be strengthened with the observations made in a previous study, ⁸ which depicts that a transition from closed/isolated to interconnected type pore morphology begins once the overall porosity increases beyond 55%. The pore size distribution of the samples processed using the fine particles of urea is shown in Fig. 9. There was a fairly large amount of fine pores (1–10 μm) in this case. The amount of the micropores increased with a rise in the overall porosity level of the samples.

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8

Images (SEM) of foam samples processed using fine particles of urea with a 46 vol.-%, b 54 vol.-% and c 62 vol.-% porosity (arrows indicate interconnected network of pores)

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9

Pore size distribution in samples processed using fine particles of urea

The true stress–strain diagram of the sintered foam samples is shown in Fig. 10. Typical plateau/yield strength and failure strain values corresponding to different porosity levels generated in the samples using acicular and fine urea particles are also shown in Table 1. The plateau/yield strength reduced with the increasing porosity level, while the failure strain followed a reverse trend. The samples processed using acicular particles of urea delineated elastic and plateau regions clearly irrespective of the porosity level (Fig. 10). The formation of distinct plateau regions with a larger plateau area at the porosity levels of 53 and 64% (Fig. 10) indicates an increased energy absorption capacity of the samples in this case. On the contrary, no plateau formation could be observed up to the porosity level of 54% in case of the samples processed using fine urea particles; plateau formation took place in this case at the porosity level of 62% only (Fig. 10). Moreover, the samples with acicular porous regions (Fig. 7) displayed higher plateau/yield strength and failure strain (Fig. 10 and Table 1) than the ones with uniformly distributed fine pores (Fig. 8) at porosity levels up to ∼54%. This could be attributed to: higher energy absorption capacity before getting crushed and greater requirement of stress to cause yielding of large size (100–500 μm) acicular porous regions (with fine pores therein) distributed in a fully densified matrix in the samples (Fig. 7a and b); and the uniformly distributed tiny pores and cells in the latter case deformed by the formation of crush bands, ¹⁴ and hence, no plateau was obtained (Fig. 8a and b). On the contrary, a higher porosity level of 62% caused the plateau strength to become greater for the samples with evenly distributed fine pores than the ones containing acicular porous regions, the failure strain following a reverse trend. Factors responsible for this behaviour could be: the higher aspect ratio of acicular shaped porous regions (Fig. 7c) needing less stress to get crushed; ¹⁵ and better deformability characteristics of the evenly distributed interconnected fine pores in the samples (Fig. 8c).

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10

Stress–strain diagram of samples having acicular porous regions and fine pores (—: acicular porous regions; …: uniformly distributed fine pores)

A comparison of properties of the currently studied samples with those of an earlier investigation ⁸ at comparable porosity levels (Table 1) indicates that the samples with acicular porous regions (Fig. 7) attain higher plateau/yield strength and failure strain. As far as the samples with uniformly distributed pores (Fig. 8) are concerned, they possessed higher failure strain, while the yield strength was somewhat less than that of the earlier study ⁸ up to the porosity level of ∼54% (Table 1). However, the plateau stress became higher, while the failure strain remained unchanged compared to the ones with the earlier investigation^8,12 at a higher porosity level of 62%. This could be attributed to the presence of (homogeneously distributed) interconnected (finer) pores in the present study capable of more effectively delaying the process of fracture in the material. ⁸ Microstructural features of the samples developed in this study suggest the basic difference to be the mode of distribution, e.g. coarse (200–1200 μm) acicular porous regions containing fine micro pores distributed in a fully densified matrix versus uniformly distributed fine pores with size range of 1–10 μm ( 7 Figs. 7 and 8 respectively). On the contrary, the mode of pore distribution in the samples processed with fine urea particles and the ones in the earlier studies^8,11,12 was identical (evenly distributed pores in the matrix), while pores were much coarser in the earlier investigations^8,11,12 than those in the present study (100–500 versus 1–10 μm). Accordingly, an identical trend observed in terms of porosity level leading to the formation of plateau and comparable yield strength in the two cases at porosity levels up to ∼54% (Table 1) suggests the contribution of the pore size towards controlling the properties to become marginal. However, the factor affected the failure strain substantially, with finer pore size displaying superior failure strain characteristics at the mentioned porosity levels. On the contrary, the finer pore size greatly improved the plateau stress with the failure strain remaining unaffected at a higher porosity level (62%). Moreover, superior properties of the samples with acicular porous regions among all (Table 1) strongly indicate the greater influence of the mode of pore distribution than that of the pore size. Moreover, the attainment of plateau at a much lower porosity level (44 and 53% versus >60%) and higher plateau stress indicates higher yield/plateau strength and hence enhanced energy absorption capacity in the case of the samples with acicular porous regions developed in the present study compared to the ones containing uniformly distributed coarser pores.^8,11,12

Conclusions