Published ArticleTitanium has fired the imagination of engineers and designers for decades by its 'ideal' combination of high strength, low density and good corrosion resistance. However, its application has unfortunately been limited to those niche markets where performance is more important than cost, such as in the aerospace, military, medical and off-shore oil drilling fields. Extensive efforts have been and still are being expended on ways to make this metal cheaper. There are promising new processes but these have yet to be demonstrated commercially. Nevertheless, there has been a global surge in interest in titanium over the past decade, and in South Africa the government has recently made this a particular focus for research and development funding. With the increased availability of higher quality titanium powder, metal injection moulding offers an attractive method for producing small, intricate components at a reasonable cost. This paper will present an overview of the metal injection moulding process and discuss the particular challenges related to the use of titanium and titanium alloy powders. The state of the global and local industry and markets will also be reviewed

Benson, J.M.

Chikwanda, H.K

English

Central University Of Technology Free State - LibraryCUT, South Africa

Journal for New Generation Sciences: Volume 7 Number 31THE CHALLENGES OF TITANIUM METAL INJECTIONMOULDINGJM BENSON AND HK CHIKWANDAABSTRACTTitanium has fired the imagination of engineers and designers for decades byits 'ideal' combination of high strength, low density and good corrosionresistance. However, its application has unfortunately been limited to thoseniche markets where performance is more important than cost, such as in theaerospace, military, medical and off-shore oil drilling fields. Extensive effortshave been and still are being expended on ways to make this metal cheaper.There are promising new processes but these have yet to be demonstratedcommercially. Nevertheless, there has been a global surge in interest intitanium over the past decade, and in South Africa the government hasrecently made this a particular focus for research and development funding.With the increased availability of higher quality titanium powder, metalinjection moulding offers an attractive method for producing small, intricatecomponents at a reasonable cost. This paper will present an overview of themetal injection moulding process and discuss the particular challengesrelated to the use of titanium and titanium alloy powders. The state of theglobal and local industry and markets will also be reviewed.Keywords: metal injection moulding, titanium, challenges, binders1. OVERVIEW OF METAL INJECTION MOULDING1.1. IntroductionMetal injection moulding (MIM) is essentially a specialised form of injectionmoulding using, instead of plastic, a metal powder mixed with some type ofbinder (usually polymeric) as the feedstock. The initiative to 'plasticise'powdered raw materials with the aid of thermoplastic additives and then to usethinjection moulding to form parts was first developed for ceramics in the 20century. In the late 1970s, this process was adapted to metal powders in theUSA [1] and subsequently MIM has developed into a well established andclearly defined manufacturing technology.1.2. The MIM ProcessMetal injection moulding (MIM) has emerged as a viable method of producingcomplex shaped parts at a competitive cost. The MIM process uses acombination of powder metallurgy and plastic injection mouldingtechnologies to produce net-shape metal parts. The process is comprised offeedstock preparation, injection moulding, de-binding and sintering. The flowdiagram of the process steps in MIM is presented in Figure 1.2The process involves preparation of the feedstock by mixing the metal powder(typically between 50-70vol%) with the binder, pre-heating to melt the latterand obtain sufficient fluidity for injection moulding. The MIM process relies agreat deal on the production of a homogenous powder-binder feedstock mix.The binder is subsequently removed either by heating or chemical extractionand this is followed by sintering to produce the final component.Figure 1: Schematic of the MIM process [2]Some of the challenges of MIM compared with thermoplastic moulding are: the difference in flow and thermal behaviour of the MIM feedstockwhich has to be taken into account for uniform mould filling, and the need to accommodate the considerable shrinkage (up to 20%) ofMIM parts after debinding and sintering in the design of the moulds.1.3. Advantages of MIM processMetal injection moulding is able to successfully compete with traditionalmachining and investment casting in the small parts arena. The economicbenefit of MIM is mainly seen when a large number of small parts of complexconfiguration (> 25 measurable dimensions) is required (Figure 2).3Figure 2: MIM (PIM) is suited to high volumes of complex shaped components[2]. (Low complexity = less than 10 measurable dimensions; high = more than100 dimensions) Furthermore, MIM can achieve savings of up to 50% whencompared with other manufacturing methods [3], especially those involvingextensive machining, as indicated in Figure 3.Figure 3: Cost comparison of MIM with other processing routes [4]Journal for New Generation Sciences: Volume 7 Number 34In addition to these advantages, MIM also confers several other benefits.These are listed in Table1.Table 1: Comparison of MIM with other production methods [5]Together with conventional powder metallurgy, MIM offers the versatility ofblending different powders together to yield composite materials that offerspecial or added qualities.The global MIM market (all metals) has grown annually at a consistent double-digit rate. The actual size in 2006 has been variably estimated by observers tobe between $435 million and close to $1 billion. The top growth areas in 2007are in the medical, automotive and electronic markets [6].1.4. Materials for MIM processMIM components are manufactured from a wide range of materials and moreare being developed. At present, low alloy steel and stainless steel form thebulk of these and smaller quantities are made from tool steels, high-speedsteels and non-ferrous alloys. A strong focus is now being placed on injectionmoulding of titanium, due to its unique property combination of high strength,light weight and good corrosion resistance. In addition, powder processing oftitanium has the potential for significantly reducing the high cost of this metal.Currently MIM parts are being manufactured from alloys such as Ti6Al4V andTi6Al7Nb.52. TITANIUM MIMAs indicated above, the majority of MIM products are made from relativelyeasy-to-handle metal powders such as stainless steel, aluminium and copper.Titanium, however, is a much more difficult metal to work with, especially in thepowder form. This entails a new learning curve with many additionalchallenges to be overcome. The major issues facing the industry arediscussed below.2.1. CostThe usage of titanium in general has been severely limited by the availability ofthe metal in suitable form and quality and this is related to the high cost of itsproduction. The total demand in 2007 is conservatively estimated to beapproximately 80 000 tons (see Figure 4) and growth is envisaged to be slowunless there is a significant price change [3,7].Figure 4: Projected growth (conservative) in global demand for milledtitanium [3,7]To put this into perspective, the cost of titanium at different stages ofproduction is compared with that for steel, aluminium and magnesium (Table2).1Table 2: Cost of titanium compared to other competing metals [8]Journal for New Generation Sciences: Volume 7 Number 31Mg sheet not commonly used. Castings are $2.50-10.00 per pound.The additional processing required to make titanium powder withconventional methods adds considerably to the cost, with prices reaching$30/pound and even higher [3] (Figure 5).Figure 5: Current production costs of the different forms of titanium [3,9]Currently, titanium parts produced by injection moulding are limited to lessthan 30cm in length and the world-wide production is between 3 and 5 tons permonth. For expansion of this market, a powder cost of less than $20/pound isneeded [10].However, there is some hope as several promising new low-cost titaniumprocesses are being developed and, in several cases, the output will be in theform of powder. This has the added advantage of eliminating the costly step ofatomisation or milling, resulting in the projected cost of the powder beingreduced to as low as $10/pound [3]. Nevertheless, the excitement around thishas been diminished somewhat by the slow progress in bringing thesealternative technologies to commercial reality.Avariety of technical difficultieshave been encountered and their resolution is taking longer than expected.Consequently, a significant decrease in the feedstock cost may only berealised in the next 5 years or even more. Despite the current high cost,titanium MIM is making inroads into high value areas, such as the medicalfield, where it now accounts for about 30% of the market for titanium surgicalinstruments [3].2.2. Reactivity of titaniumThe high reactivity of titanium, particularly towards interstitial elements (e.g.carbon, oxygen, hydrogen and nitrogen) is well known and this is exacerbatedwhen the surface area is large, such as with small powder particles.6Ore(Rutile)TiCl4TiO2SpongeIngotForgingPlatePowderSheetCastingFoil7The major effect of this contamination is to reduce the mechanical propertiesof the consolidated component as indicated in Figure 6.Figure 6: Effect of oxygen content on elongation and UTS of Ti-6Al-4V(adapted from [11])This reactivity occurs even at low temperatures and so oxides, for example,are almost impossible to prevent from forming. Additional contamination willalso happen during subsequent processing such as mixing with the binder,during debinding as well as on sintering. Significant absorption of impuritiesoccurs above 260ºC [10] and so any traces of the binder still present afterdebinding will result in a reaction with the metal, usually leading to additionalcarbon build-up.Figure 7 shows an example of the increase in oxygen pick-up through the MIMprocess for Ti-6Al-4V. It can be seen that the initial content of the powder is keyto being able to meet the ASTM specification of 2000ppm in the finalcomponent. The temptation to use lower quality and less expensive titaniumpowders is often counter-productive as these tend to have levels of oxygenthat are too high [12].Journal for New Generation Sciences: Volume 7 Number 3Elongation(%)UTS(ksi)Figure 7: Increase in oxygen content during processing of Ti-6Al-4V powder system [13]8Carbon contamination can be reduced to a certain extent by using a higherdebinding temperature but this has the drawback of raising the oxygen levelas well (Figure 8).Figure 8: Effect of debinding temperature on carbon and oxygen levels in Ti-6Al-4V [14]3. BINDERSFrom the previous discussion, it is clear that the binder plays a critical part inensuring the final quality of the component and failures/poor results canlargely be attributed to the use of the wrong one.The requirements of a binder are multiple:- must 'wet' the powder particles- must melt at a suitably low temperature for injection moulding;- must be easily removed by using chemical or thermal means withoutleaving residues; if thermal, it must decompose at temperatures lowenough to minimise chemical reaction with the metal powder particles(ideally <260ºC);- must contain no other deleterious elements that could degrade thetitanium metal;- must provide sufficient green strength;- must provide lubricity;- must be environmentally friendly.In general, binders are composed of several constituents:Primary binderPowder surfactant/lubricantAdditive to improve green strength(ºC)9Examples of binder systems that are reported to be compatible with Ti-6Al-4V[2]: Polypropylene, ethel vinyl acetate, paraffin wax, carnauba wax,dioctyl phthalate; Polyethylene, paraffin, stearic acid; Polypropylene, polymethyl methacrylate, paraffin wax, stearic acid; Naphthalene, stearic acid, ethylene vinyl acetate; Paraffin wax, polyethylglycol, polyethylene, stearic acid; Paraffin wax, copolymer, stearic acid; Atactic polypropylene, carnauba wax, paraffin wax, stearic acid; Atactic polypropylene, ethylene vinyl acetate, paraffin wax, carnaubawax, di-n-butyl phthalate.The perfect binder has yet to be discovered and much research is beingconducted in finding suitable combinations and as well as procedures fordebinding. Unfortunately this is mostly of a proprietary nature and is notdisclosed. Nevertheless there are some indications of what is being used andthese are summarised in Table 3.Journal for New Generation Sciences: Volume 7 Number 310Table 3: Summary of binders and other details of commercial/research Ti MIMactivities* gas atomised ** hydride dehydride # ethylene vinyl acetate ## hydroxyapatite11According to [14], the optimum powder loading is 72vol% as this gives themaximum green density. Shrinkage after sintering is dependent upon thepowder loading as well as the particle size distribution. This can range from 20to 10% for powder loadings of 50 to 70vol% respectively [24].4. The MIM MarketThe titanium MIM product needs to be tailored to the actual needs of aparticular market. The previous discussions around minimising impurities areonly of importance for components requiring mechanical integrity but there isa lower end of the market where aesthetic value is a more dominant need e.g.jewellery, non-load-bearing items etc. Here lower quality powders couldpotentially be used to help reduce costs. The market can be divided into fourbroad categories, depending on the level of quality required [25], as indicatedin Table 4.Table 4: Ti MIM market segmentation [25]Although certain standards are mentioned, these are not specific to MIM ascurrently there are no specifications (apart from proprietary guidelines)relating to either the feedstock or final products. The need for internationallyaccredited standards is recognised as an urgent requirement and theirabsence is seen as a hindrance to more widespread acceptance of thistechnology [25].5. CONCLUSIONSTitanium has always appealed to engineers and designers as it has an almostideal combination of properties: lower density, high strength and excellentcorrosion and fatigue resistance. However the very high cost in producingtitanium products has effectively limited its use to niche markets wherefunction is of greater concern than cost, such as in the aerospace, military,medical and off-shore drilling fields.Journal for New Generation Sciences: Volume 7 Number 312Powder metallurgy however is seen as a possible saviour for titanium as manyof the energy-intensive and time-consuming steps can be eliminated and thusthe final cost can be substantially reduced. In addition, new primary titaniumproduction technologies are being developed that promise to lower the costseven further, especially when the output is in the form of powder. This latterbenefit would further eliminate the need for atomisation or milling and the priceof the metal powder is estimated to fall from about $30/pound to as low as$10/pound.MIM is a very attractive manufacturing method, akin to plastic injectionmoulding, and has been demonstrated to be particularly cost-effective whencomponents are small (typically 30g), complex shaped and where highvolumes are needed. Although the set-up costs are high, the large volumesand elimination of significant post-processing steps such as machining,means the component prices are reduced substantially. A well-establishedtrack record has been developed over the past 30 years using aluminium,stainless steel and copper powders.However, titanium powders are a much more exacting proposition, due mainlyto the very reactive nature of this metal, and significant challenges need to beovercome before the full potential of titanium PM, and in particular MIM, canbe realised. The main issues are the following: the high metal price is unlikely to drop substantially within the next 5years or so as the new primary metal technologies are still facingnumerous technical difficulties; the need for higher quality powders, containing low levels of oxygenand other contaminants, to maximise mechanical performance forcritical applications; better binder systems are required to prevent interstitialcontamination; improvements in processing to fully remove the binder and minimisethe pick-up of oxygen; and the development of international standards specific to MIM titaniumproducts.Although South Africa is a significant titanium pigment producer, there hasbeen very little down-stream titanium metal processing/manufacturingactivity. Titanium metal has to be imported, usually as the final product.At bestthere are some limited secondary processing (i.e. shaping, machining andjoining etc.) activities being undertaken.The Department of Science and Technology, together with several localinstitutions, has prepared a national strategy to develop a titanium metalindustry and there are now plans to have a primary production facility in placeby 2020.13Furthermore a Titanium Centre of Competence is currently being establishedto coordinate and expand existing research within South Africa institutions inregard to the above mentioned challenges and also to address other issuesthat will face the broader titanium industry in the future.6. REFERENCES[1.] www.pim-international.com/aboutpim, accessed on 11 September2007.[2.] FH Froes. Metal Powder Report, Volume 61, Issue11, December2006, 20-23.[3.] The Titanium Age: Supply Constraints and New Markets, Thintri Inc.,2005 Edition.[4.] http://meditech.cardiff.ac.uk/29March-2007/presentations/Martin%20Kearns.pdf, accessed on 7August 2007.[5.] Adapted from www.injectamax.com/mimchart.htm.[6.] Metal Powder Industries Federation News, May 16, 2007,www.mpif.org/newsroom/2007-5.pdf, last accessed on 14 August2007.st[7.] MS Kamon. Titanium 2005, proceedings of 21 Annual Conference,25-27 September, 2005, Scottsdale, Arizona. International TitaniumAssociation.[8.] FH Froes, H Friedrich, J Kiese and D Bergoint. JOM, Volume 56, No.2, 2004, 40-44.[9.] FH Froes, MN. Gungor and MA Imam. JOM, Volume 59, No. 6, 2007,28-31.[10.] Froes FH, Developments in Titanium P/M, http://www.webs1.uidaho.edu/imap/MPR%20Paper.pdf, last accessed on 14 August2007.[11.] Adapted from FH Froes. Metal Powder Report, Volume 61, Issue 11,December 2006, 20-23.[12.] Meta l Powde r Repo r t , Oc tobe r 2005 , www.me ta l -powder.net/newsview/ archive/newsview_oct05.html, last accessedon 15August 2007.Journal for New Generation Sciences: Volume 7 Number 314[13.] B Williams. Metal Powder Report, Volume 58, Issue 10, October2003, 30.[14.] G Shibo, Q Xuanhui, H Xinbo, Z Ting, D Bohua. Journal of MaterialsProcessing Technology, 173, (2006), 310-314.[15.] R Gerling, E Aust, W Limberg, M Pfuff, FP Schimansky. MaterialsScience and EngineeringA, 423, (2006), 262-268.[16.] Meta l Powde r Repo r t , Oc tobe r 2005 , www.me ta l -powder.net/newsview/ archive/newsview_oct05.html.[17.] E Nyberg, M Miller, K Simmons and KS Weil. Materials Science andEngineering, C, 25, (2005), 336-342.[18.] Metal Powder Report, Volume 61, Issue 10, November 2006, 22-27.[19.] H Tokura, K Morobayashi, K Ai, Y, Noro, Y Ishii. Journal of JapanSociety of Precision Engineering, Volume 67, Number 2, February2001, 322-326.[20.] A Fanelli, M Behi, CP Ballard Jr, JV Burlew. United States Patent,Number 5,746,957, May 5, 1998.[21.] ES Thian, NH Loh, KA Khor, SB Tor. Advanced Powder Technology,Volume 12, Number 3, (2001), 361-370.[22.] W Limberg, E Aust, T Ebel, R Gerling, B Oger. Metal Powder Report,Volume 60, Issue 3, March 2005, 22-27.[23.] KS Weil, E Nyberg, K Simmons. Journal of Materials ProcessingTechnology, 176, (2006), 205-209.[24.] RM German and A Bose. Injection Molding of Metals and Ceramics,Metal Powder Industries Federation, 1997, 221-222.[25.] D Whittaker. Powder Injection Moulding International, Volume 1, No.1, March 2007, 27-32.

The challenges of titanium metal injection moulding

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The challenges of titanium metal injection moulding

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Central University Of Technology Free State - LibraryCUT, South Africa