Structure and properties of titanium produced by a new method of chip recycling

Abstract

The paper proposes a new technique of recycling metallic chips and presents the results of examination of the products thus obtained. The aim of the experiments was to recycle commercial purity (cp) titanium Grade 2 chips obtained by turning, and to produce a solid bulk material. The recycling was realized using a plastic working process. The titanium chips were first subjected to preliminary consolidation and then directly extruded using the KOBO process. The properties of the recycling final product were examined and compared with those of the as-received, solid cp-titanium Grade 2 (reference material).

The recycling process yielded a well consolidated solid titanium rod with a diameter of 8 mm. The consolidation effect was high which was confirmed by the fact that only a small number of voids and discontinuities distributed randomly were observed. The final material had a homogeneous grained structure. The equiaxial grains observed on the transverse and longitudinal sections had similar sizes. The mechanical properties of the recycled material (estimated based on the results of microhardness measurements and uniaxial compression tests) were comparable with those of solid cp-titanium Grade 2.

Introduction

The amount of material scrap, such as chips, lumps, or cuttings, left after machining of various metals in the production of semi-finished or final products (especially in series production or in manufacturing complicated shapes). The amount of scrap may be substantial and thus bring about economic losses. In some special cases, such as manufacturing biomedical or aerospace parts made of titanium, the material loss may even amount to 80% (Luo et al., 2010). Therefore, the recycling of material scrap and its repeated use can reduce the production cost. This is especially important with titanium and aluminum, which are used in mass production and are relatively expensive.

The basic and most often used method of scrap recycling is re-melting which is considered to be the reference technology in inventing other potential technologies. In industrial practice, titanium chips are re-melted using the electron beam melting (EB) and vacuum arc remelting (VAR), techniques which permit converting titanium scrap into a useful material (www.timet.com). These technologies are however energy-consuming, and require that the process should be conducted in vacuum or an inert gas. This is especially disadvantageous when dealing with titanium and its alloys since they show high affinity to oxygen and nitrogen. Thus, these technologies are demanding and are limited in use on a wider scale. Therefore, the aim of many research works is now to optimize the existing methods of recycling titanium or to find new techniques of its recycling.

One of the promising ways of recycling metallic scrap, alternative to re-melting, is the technology based on plastic working. Then appropriate high pressure, high temperature and large plastic strain are needed in order to crush the oxides layer (which covers the individual chips) and to activate diffusion as well as the bonding processes (Gronostajski and Matuszak, 1999). Another condition for successful recycling of metallic chips is that their material should be plastically deformable. Xia (2008) reported that, in terms of consolidation, an additional advantage of plastic deformation is the transport of atoms by shape changing and flow of the material.

There are great literature reports concerning the recycling of metallic chips based on the traditional plastic deformation processes and their combinations. In this field most of the numerous achievements in chip recycling described in the literature are dedicated to aluminium and its alloys, which is widely reported in the general reviews by Shamsudin et al. (2016) or Samuel (2003).

For example, Gronostajski and Matuszak (1999) proposed a special direct conversion of aluminium (and some its alloys) chips into compact bearing components, which was done without melting. These processes consisted of: milling or cutting the chips into a granulate (no more than 4 millimeters long), and then mixing with particles of reinforcing-hardening phases, cold pressing-compacting, and, finally, hot extrusion. As a result of these procedures products with density up to 99% and relatively good mechanical properties were obtained. This method was further improved by an additional heat treatment applied to the recycled product with the aim to increase its mechanical properties (Chmura and Gronostajski, 2006).

Tekkaya et al. (2009) presented another technology of recycling aluminium alloy chips realized by plastic working. Various kinds of AA6060 alloy chips were compacted and then hot extruded. The products obtained had the form of bulk profiles and were characterized by relatively good yield strength. In turn Haase and Tekkaya (2015) proposed yet another complex several-stage procedure of chips recycling which included hot extrusion. In their experiments, the aluminium alloy AA6060 chips were first processed by preliminary compaction, then annealed, and finally hot direct extruded (to crush the oxides and produce bonding). It was important that the recycled materials thus obtained were used for manufacturing cans and rods (i.e. the material were successfully subjected to the backward can extrusion and forward rod extrusion). This work gives evidence that the chip recycling procedures based on plastic working can provide a useful engineering material.

Another interesting proposition of the recycling technology was presented by Chiba et al. (2011) who compacted the aluminium alloy machining scrap into billets and then successfully extruded at room temperature into square bars. Just as in our work, the chips were uniaxially compressed in a container to obtain a briquette and then subjected to heating. Although in the experiments conducted by Chiba et al. (2011) the compression and extrusion were “cold”, before the extrusion, they proposed annealing of the briquette so as to avoid some cracks and to reduce the extrusion force. Alternatively, it would be possible to anneal the scrap before compaction, which leads to further softening of the billets. Anyway, as a result, the billets are extruded at a smaller extrusion load.

In the context of our work, a similar and interesting method of chip recycling was presented by Tang and Reynolds (2010). They reported on consolidating aluminum alloy machining chips by the friction back extrusion process. In this way they obtained a fully dense product in the form of wires. Just like in our work, that extrusion was expanded by rotation of the die, which delivered additional strain. Tang and Reynolds (2010) reported that the surface quality of the product depends on the rotational speed of the die. The results of their work have proved that the combined movement of the tools used for consolidation is the right idea.

Another process of chip recycling, similar to our work, was described by Hu et al. (2012) who also used cold pressing, heating and direct hot extrusion. As a result of preliminary consolidation they also obtained a briquette. The final product was a bulk polycrystalline rod with a diameter of 8 mm. Even though they did not recycled titanium chips (but magnesium alloy chips), the procedure of recycling was similar and consisted of the same stages. This may suggest that this kind of procedure is correct and effective.

Some literature data also inform on attempts at recycling magnesium scrap. Watanabe et al. (2001) reported that the AZ31 magnesium alloy chips were cold pressed and consolidated by hot extrusion. This work confirms that using the procedure as described above it is possible to recycle successfully also magnesium chips.

Generally, based on the literature described above, it should be concluded that the processes of chips recycling are mainly conducted via several-stage procedures. These methods (containing compacting-pressing, heating and plastic working) lead to the successful recycling process.

Recently, special methods of the plastic working (i.e. severe plastic deformation methods − SPD) have been extensively developed (Estrin and Vinogradov, 2013). At the present, these methods are successfully used for producing nano-metals (Zhu et al., 2004). In view of their specificity, they seem to be promising also in the field of metallic scrap recycling.

Compared to the remelting technique, the advantages of the SPD methods lie in the high energy involved in the process, which permits new metallic bonds to be formed. Moreover, there are also the high compressive stresses induced in the material, which favor its densification and consolidation. Another advantage of these methods, especially important in view of the high reactivity of titanium, is the relatively low temperature of the process. The SPD consolidation techniques may be conducted at much lower temperatures and thus they are suitable for treating particles with highly metastable structures (Luo et al., 2010). At the present, some new recycling methods based on the SPD technique are proposed. The newest reports confirm that this is the promising and evolutionary research line, which now, in the laboratory scale, permits recycling metallic scrap into a useful engineering material. In other words the SPD methods may have a great potential in the field of scrap recycling.

Publications devoted to recycling chips of pure titanium are scarce. There are only few literature data, which are concerned with recycling chips of the Ti6Al4V alloy. In general, in the case of titanium and its alloys, the recycling is conducted using the SPD methods where the plastic deformation is realized at an elevated temperature (just as is the case in our experiments). For example, Luo et al., (2010) and Luo et al. (2013) used the ECAP method (Equal Channel Angular Pressing − one of the SPD techniques) with back pressure to recycle titanium Grade 2 chips after milling. The chips were too small and the authors did not obtain a preliminarily consolidated briquette. Their ECAP process was performed at temperatures between 400 and 600 °C and the fully dense bulk product with grained structure was produced, possessing strength comparable to that of commercially pure Ti. In turn, Lui et al. (2016) consolidated various types of Ti6Al4V chips using ECAP with back pressure, where the chips were preliminarily compacted in the ECAP channel. The recycling process was conducted at 600 °C (i.e. much higher than the temperature applied in our work) and the recycled billets were subjected to a complex heat treatment with hot forging to produce the desired final microstructure. They reported that the mechanical properties of the recycled material were comparable to or better than those of the commercial Ti6Al4V reference alloy. Shi et al. (2016) also processed the Ti6Al4V chips by ECAP with back pressure. The chips were preliminarily compressed into a solid charge. Then the ECAP was performed at temperatures ranging from 400 to 500 °C and the final density was 98–100%. Sachdev et al. (2012), on the other hand, subjected Ti6Al4V titanium alloy chips to recycling using a complex procedure which included grinding, washing, magnetic separation, vacuum annealing, briquetting, encapsulation, and finally hot extrusion. This approach differs from those described above and is chiefly devoted to achieve maximum chemically pure material.

It should be noted that the processes of recycling titanium and its alloys, reported in the literature, are conducted at the elevated temperatures, higher than those used in our study. In general we can state that, based on plastic working, the recycling of the Ti chips or chips of Ti alloys is possible and effective.

The present paper proposes a new technique of recycling titanium chips and presents the results of examination of the final product of recycling. The aim of the experiments was to produce a solid bulk product of high mechanical strength whose properties should be comparable with those of commercially available pure titanium. The proposed technology is based on plastic deformation processes, such as upsetting and extrusion. The main stage of the recycling process is direct extrusion synchronized with the cyclically rotating die (i.e. KOBO method − Korbel and Bochniak, 2004). It should be noted that the processes of recycling of pure titanium reported in the literature are scarce. Moreover, the use of KOBO method for consolidating and recycling titanium chips is new and has not been reported. Therefore, the presented article may be an interesting contribution in the field of titanium chips recycling.