Nanocrystals from Machining Waste Suitable for the Production High Performance Nanostructured Materials

In spite of their interesting properties, a principal barrier to the widespread use of nanostructured materials has been cost. Composed of sub-micron sized grains, or crystals, nanostructured materials are often harder, stronger and more wear resistant than conventional materials, but their manufacture involves time consuming and costly processes. Now, though research engineers at Purdue University have discovered that scrap metal produced in the machining of a material contains nanocrystals that could be used to produce many new and exciting nanostructured materials for applications such as bearings, sensors, and components for computers and electronic hardware, in an inexpensive way.

Machining Chips as a Source of Nanocrystals

Currently, the chips shaved away from metals as they are machined are collected as scrap, melted down and reused. By observing the waste materials produced by the machining process, which is used to make parts in automotive manufacturing and other industries, Srinivasan Chandrasekar and Dale Compton, both professors of industrial engineering at Purdue University, discovered that these chips are either entirely or primarily made up of nanocrystals and could be more valuable weight-for-weight than the material from which the part is being machined. ‘Our contribution has been in developing a process we think can be used to make these materials in large quantities at a very low cost,’ says Chandrasekar.

Benefits of the Process

‘The benefits of this process are high-strength and high wear-resistance with reduced material weight.’ The cost of the new nanostructured materials is expected to be no more than US$2 per kg, which compares favourably with the current cost of making nanocrystals of about US$200 per kg.

The discovery represents a much cheaper way of producing nanocrystals with similar properties to those produced using traditional approaches to manufacturing nanostructured materials.

Applications for the Nanostructured Chips

The Purdue researchers, whose findings appeared in the October 2002 issue of the Journal of Materials Research, envisage two important avenues for the process. ‘The nanostructured chips will enable the large scale production of nanocrystalline particulates (powders), which can be processed into components using conventional powder metallurgy, forming or metal injection moulding methods,’ says Chandrasekar. Alternatively, he believes the chips, or the derived powder may serve as continuous or discontinuous reinforcements for metal and polymer matrices, enabling the creation of new types of advanced composite materials. ‘Possible component applications include gears, diesel fuel systems, bearings, armour and armour penetrators,’ says Chandrasekar. Other potential uses for nanocrystals include reinforcements for concrete in runways and road surfacing.

How the Nanocrystals are Formed

The formation of the nanocrystals is a consequence of the very large-scale plastic deformation introduced during the production process. Traditionally, large-scale deformation is imposed using processes such as rolling, drawing, equal channel angular extrusion (ECAE), or high-pressure torsional straining. With this new process, the very strains caused by the cutting tool produce nanocrystals of about 100 nm in diameter. Shear strains in the range of 2-10, strain rates of up to 10⁶ per second, and shear plane temperatures of up to 0.7 T_m are common features of machining. ‘In our research we knew that there was strain being introduced at the point of the cutting tool,’ says Chandrasekar. The shear strain length, strain rate and shear plane temperature all affect the microstructure of a chip, but to what extend still needs to be understood and is a topic of current work being carried out by the team of researchers.

Materials Suitability and Optimisation of the Process

‘One of the really big advantages of this is that you can do it with almost any material,’ says Compton. ‘You can make nanocrystals of steels, tungsten, titanium alloys and nickel alloys.’ So far the engineers have measured increased hardness in nanocrystals of copper, tool steel, stainless steel, high strength alloys and iron, which have shown to be 100%, 200% and even as much as 300% harder than the same material in bulk form, with grain sizes in the range of 100-300 nm. Table 1 gives a summary of the hardness values measured for the chips and the bulk samples. However, the machining process parameters do have an important influence on the microstructure, i.e. grain size of the chip. ‘While this fact has been established, optimisation of the machining parameters to achieve the ‘perfect’ nanocrystalline chip has not yet been done,’ says Chandrasekar.

Table 1. Hardness and grain size of chip and bulk samples.

Future Work

The researchers understand that further study is needed and currently more materials are being analysed to determine the commercial viability of the process. A number of commercial opportunities are being explored,’ reveals Chandrasekar, ‘particularly in the discrete product sector - ground transportation and aerospace industries.’ In addition, the effect of thermal processing on coarsening of nanostructures is being investigated together with other areas of interest, namely the mechanism of nanostructure formation and optimisation of nanostructure characteristics.

Summary

It is quite likely that the enormous quantities of chips generated in industrial machining operations, which are currently re-melted or disposed of as scrap, are all composed of ultra-fine grained structures. This process offers the metals processing industry a very low cost source of nanostructured metals and alloys, together with the possibility of reusing the chips in high value products.

Source: Materials World, Vol, 10, no. 12, pp. 30-31, December 2002.

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