Sintering metal powders in outer space - which is roughly what microgravity implies - sounds no more difficult than banging nails into a plank, but as useful as nailing jelly to a wall. In fact it's more like the other way around. PM could be an important process for astronauts but turns out to be much more difficult than expected.

Presented by John L Johnson of Pennsylvania State's Center for Innovative Sintered Products, this fascinating paper sheltered under the mundane academic title of Critical learning from microgravity sintering of tungsten alloys: implications for extraterrestrial fabrication and repair.

Why should we want to sinter metals in outer space? And why not simple low-temperature metals rather than difficult refractory materials, such as tungsten-based alloys? As I write this, I'm watching space-walking astronauts repairing the thermal shields on the Shuttle Discovery, before it returns to Earth. If available, could refractory metals have been utilised to patch or strengthen the damaged heat-shield? Refractory materials are already important in extraterrestrial propulsion, radiation and thermal systems, and the future could offer substantial opportunities for repair and construction techniques based on free-form fabrication from powders. Not surprisingly, however, little data exists from which sintering behaviour and properties can be predicted.

Let's first get the terminology out of the way. When we really mean "zero gravity," why do we talk of "microgravity" or "µg"? In fact, zero gravity is merely a concept, like absolute zero. We can approach it, but we can't reach it. Just as the act of measuring "absolute zero" - if there was such a thing - would raise its temperature, so any conceivable device for measuring gravity would exert a gravitational field of its own. In practice, the field in a coasting Space Shuttle is about as low as you'll get. Typical values are 10-6g in space, about 1/6g on the moon and 3/8g on Mars, where 1g indicates the value of gravity on the Earth's surface.

The researchers set out to fill some of the technological gaps in microgravitational PM, persuading NASA to carry a series of liquid-phase sintering (LPS) experiments on Space Shuttle Columbia's flights STS-65, STS-83 and STS-94. These took place in the mid-a990s, but have now become very topical. Indeed, the paper was being published at this time because some of the results had been reanalysed in the context of the need for in-space fabrication. NASA had been funding groundwork in preparation for additional micro-g LPS experiments on board the International Space Station, but the researchers were told earlier in 2005 that these had been cancelled. Focus has been shifted to accommodate NASA's new priorities.

In space, as on terra firma, there are well-established alternatives for manufacture and repair of low-temperature materials such as steel and aluminium, but few for exotic metals like tungsten. For very practical reasons, the investigation followed the effects of gravity - and its near absence - on densification, distortion and solid-liquid separation of commercial-type sintered heavy alloys with 35 to 93 wt% tungsten and a matrix of Ni and Fe in 7:3 ratio.

Although not described in the paper itself, several ground-based processing conditions were used to produce samples for microgravity liquid-phase sintering. These included: cold isostatic pressing (CIPing) and argon presintering (1400ºC), die pressing then CIPing, CIPing and H2 presintering (1400ºC), die pressing, CIPing and vacuum presintering (1400ºC), CIPing and liquid-phase sintering (1500ºC), and HIPing. After ground-based processing, the samples were machined to form cylinders 10mm in diameter and 10mm high. Each sample was placed in its own alumina crucible, then into a tantalum cartridge, evacuated and triple-sealed to ensure containment. Final sintering was carried out in the Large Isothermal Furnace on board Columbia (Figure 1).

Figure 1. Cartridge sintering on the Space Shuttle Columbia.

Densification

Pores in microgravity sintered compacts proved unexpectedly stable. Buoyancy effects were all but absent, but nonetheless the pores were highly mobile and rapidly coalesced, producing enormous holes, up to 200µm across, from original porosity of the order of 1µm (Figure 2). Pore coarsening was estimated to have taken place at 1000 times the grain-growth rate.
There was a notable difference, however, between high-W grades, with a strong solid-state skeleton, and low-W alloys which relied completely on liquid-phase sintering. In the first case, almost theoretical density was achieved in microgravity, but in the second there was virtually no densification at all (Figure 3).

Figure 2. Coalescence to form large pores in a liquid phase sintered tungsten heavy alloy processed in microgravity.

Figure 3. Effect of sintering time and gravitational level on the densification of 78W and 93W

Solid-liquid separation

In W-base heavy alloys, the binder (typically some combination of Ni, Fe and Co) has roughly half the density of the tungsten, a difference of about 9g/cm3. On Earth, gravity causes W grains to settle, resulting in a microstructural gradient (Figure 4). With low-W alloys, the top layer may contain no tungsten at all.

Figure 4. Gravity induces grain settling, producing higher W volume fractions at the bottom than at the top of the settled region. The difference increases with lower W contents. The samples were sintered for 120 minutes at 1500°C. All of the samples were first presintered to full density in 1g.

Things are strikingly different in outer space. In microgravity there is no settling, though some clustering occurs. More interestingly, away from Earth's gravity it is possible to make very dilute alloys that are not normally possible. Figure 5, for example, shows microstructures of 35W alloys sintered for 600 min in (a) 1g and (b) µg. In the first case, the tungsten particles settle into a thin layer, giving a local solid volume fraction of about 0.6. By contrast, W particles in the microgravity samples remain dispersed, apart from a tendency to cluster and agglomerate.

Figure 5. Micrographs of 35W alloy sintered for 600 minutes at 1500°C in (a) 1g and (b) mg. Both samples were presintered to full density in 1g.

Though grain settling is absent in microgravity, gradients can still occur. One 78W sample sintered for 180 minutes in microgravity showed a distinctive surface-to-core microstructural gradient (Figure 6), with a liquid binder pool - plus localised macroporosity - at the centre. The reason for this anomalous effect was unknown, and it could not be followed up in the limited time available.

Figure 6. Cross-sectional views of 78W sintered for 180 minutes at 1500°C in microgravity showing surface-to-core microstructural gradients.

Distortion

On Earth, rapid sintering leads to a loss of structural rigidity. Weakness occurs when the pores are closed by rapid densification because the solid skeleton is dissolved by newly formed liquid, especially with high liquid content or when pores rapidly coalesce. When this semisolid system densifies, the capillary forces associated with open pores are also lost, leaving saturated pores and no secondary strength from a solid skeleton. Distortion is thus delayed until densification is practically complete.

Surprisingly, gravity helps suppress distortion. Stress induces grain-grain contact, which provides increased structural rigidity. By contrast, without grain settling, sintering in microgravity results in significantly more distortion. This is illustrated in Figure 7A for 83W compacts sintered from identical green compacts in the same furnace for the same time-temperature combination, but the one on the right was processed in microgravity. The left-most image shows the nearly fully dense W-Ni-Fe compact after pre-sintering at 1400°C and machining into a cylinder. The centre compact, with its well-established "elephant's foot" slumped shape, followed 1g sintering at 1500°C for 2 hours, while that in the far right image was processed in µg. All other parameters were identical.

The presence of pores in µg samples seemed to increase distortion unless there were sufficient grain contacts to form a rigid skeleton. For instance, the 93W samples did not distort in either 1g or µg. However, the 78W sample, which lacked a rigid skeleton, gave more distortion and less densification in µg than in 1g. Since there was no buoyancy force on the pore space in µg, the compacts distorted freely as the pores agglomerated. There was also extra liquid since it did not fill the pore space. Most surprising to the investigators was the evolution toward hollow spheres, where all or most of the pores coalesced into a single "jumbo-sized" central pore and the whole body took on a near-spherical form, as shown in Figure 7B. This was contrary to expectations that sintering in space would enable a wider range of compositions to be liquid-phase sintered to full density and offer greater precision. Instead, it was now seen that microgravity sintering led to lower performance, inability to eliminate pores and more distortion.

Much of the remaining parts of the paper is devoted to a mathematical model of the sintering reactions, simulations of sintered alloy shapes under varying gravity (Figure 8) and to comparisons between calculated and measured sintered component profiles. These are best studied in the original document. It was concluded that solid volume profile and dihedral angle were the dominant factors, with a critical contiguity value of 0.39 to minimise distortion of W/Ni/Fe alloys. The same critical contiguity was thought necessary to permit densification in microgravity.

Perhaps the key finding is that alloys, processes and products cannot be qualified for space fabrication and repair by ground-based experiments. Microgravity compacts will be weaker with more distortion and less densification.

Figure 7A. 83W compacts (left prior to sintering, centre Earth-sintered, right microgravity sintered).

Figure 7B. Cross-section of 78W sintered for 120 minutes in mg. This sample had a green density of 62 per cent and was not presintered to full density in 1g.

Where do we go from here?

What of the future? The experiments described in this fascinating paper were very much along the lines of "Let's try the conventional techniques and see what happens." What happened was that the product was very inferior indeed. So perhaps it would be a good idea to look at the possible advantages of micro-g sintering, without preconceived ideas of what the techniques should be. In space we have a near-vacuum and high-strength sunlight that can easily be concentrated into a small space. So why not expose the compact to this vacuum before sintering, thoroughly removing any contained gas whilst all pores are still connected, but without the elaboration and expense of triple-sealing in evacuated tantalum cartridges? And why not think "outside the box?" One could, for example, imagine some kind of solar-powered zone sintering as an alternative process, where the directional solidification forces might effectively replace gravity. It would be interesting to know what other possibilities are already under investigation by this enterprising research group.

Figure 8. Shape simulations - gravity effect.