22 MPR March/April 201122 MPR March/April 2011 0026-0657/11 ©2011 Elsevier Ltd. All rights reserved.

PIM2011

Manufacturersmull prospectsof turbo boostfor gasolineAuto makers have to get more from less: more power from smaller, more efficient engines. Faced with the record of soaring popularity and sales of diesel models, they are looking hard at bringing turbocharging technology to gasoline engines. It will require new thinking about materials and manufacturing methods...

Turbocharging has become the state of the art technology for diesel-powered cars. In Europe, the commonly used past dis-

missals of diesel as “noisy” and “smelly” have given way to widespread acceptance and enthusiasm for a technology that in many ways equals gasoline-powered per-formance. It is something of a point of emphasis that diesel-powered vehicles have several times won against gasoline-powered opposition the prestigious Le Mans endur-ance race staged every year in France.

Turbos have given diesel technology an enormous boost over the last 10 years, and ensure that important purchasing criteria such as the “fun of driving” and “agil-ity” are now also associated with diesel technology, which is already widely known to be economical. Engine manufacturers, faced with ever more stringent emissions regulations and demands for increased efficiency are looking to cut engine weight while boosting performance. Smaller, but turbocharged, gasoline engines are expect-ed to be one of the results.

Even outside the sphere of diesel tech-nology, the turbocharger is experiencing an enormous surge in growth due to the trend towards smaller engines that is being driven by efforts to cut consumption. Combined with direct fuel injection, it is also developing increasingly into a key technology for the gasoline engine [1]. A key difference is that while diesel turbos operate at around 850° C, turbo units in gasoline-powered vehicles have to be able to withstand temperature levels in the order of 1000° C.

Figure 1. MIM example parts applied in diesel-VTG turbochargers: (from left) vanes, adjustment ring, and a roller.

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Latest global automotive production forecasts predict a cumulated annual growth rate (CAGR) of 2.5% for the peri-od 2007 - 2017, but a CAGR of >8% for turbocharged light duty vehicles mainly for gasoline engines [2, 3].

Faced with this market projection, BASF Catamold® is striving to extend its nickel-based superalloy material range for the use in turbochargers and other hot-end components, such as parts in the exhaust gas recirculation (EGR), in order to meet expected market demands.

Making and processing of Ni-base superalloys in MIM is not a new topic, and has already been realised successfully in the past [4]. However, BASF’s work is focusing on material requirements in the high temperature range of combustion engines in automotive.

For such applications MIM is compet-ing once more with conventional manu-facturing processes like investment casting or mainstream powder metallurgy. The paper presented at the European Powder Metallurgy Association’s World Congress in Florence in October outlined some promising results regarding superior mate-rial properties achieved in MIM against cast Ni-base superalloys at elevated

temperatures beside the well known advan-tages which MIM can provide for mass production in the automotive segment.

Metal injection moulding was used relatively early in the production of parts for diesel engine turbochargers. Although MIM offers a reasonable potential for the substitution of conventional manufactur-ing technologies like investment casting, MIM-parts in practical use, up until now, have been limited to a few components in diesel VTG (variable turbine geometry) turbochargers. Some MIM-references are presented in Figure 1.

The reasons for this cautious approach are not easy to identify, especially since the components manufactured by way of MIM have been used in large numbers for the last few years and have proven them-selves in practice.

As a relatively new manufacturing process, MIM is competing with tradi-tional processes in this field, such as invest-ment casting which benefits from many years of experience in the materials used and the tolerances achieved in production. Furthermore, with regard to the develop-ment of new components, it is often not possible for the developer to use a new production technique. The pressures of

time and lack of experience with the proc-ess then lead to MIM frequently not being considered as an alternative [1].

The only way out: Improved or at least the same material properties in MIM for lower overall manufacturing costs com-pared to the conventional process today. This has already been demonstrated by previous work on MIM with Ni-based superalloys which made their way into aeronautic applications [5]

Gas atomised powders of IN713C, MAR M 247, and IN100 were used to prepare MIM-feedstock samples using BASF’s acetal based Catamold binder with a powder loading of 62 to 65 vol%. Small discs (diameter 28 mm, height 5 mm) and MIM tensile bars according to [6] type A2 were moulded. Catalytic debinding with gaseous nitric acid was applied. Sintering trials were performed in a molybdenum-lined furnace using an argon atmosphere.

All sintering trials were performed using a 5 K/min heating rate and a hold-ing time of three hours at the maximum temperature.

Although IN-713C is normally just used in its “as cast” condition, an addi-tional heat treatment process was car-ried out for this material. Metallographic

GMR-235 (investment casting) IN713C (MIM)

Figure 2. Typical microstructures of an investment cast superalloy (GMR-235), and in MIM (IN713C) [7].

Table 1. Typical composition of Inconel 713 C, MAR-M 247, and IN100.

Ni Cr Co Mo Nb W Ta Al Ti C

IN713C 74 12.5 - 4.2 2 - - 6.1 0.8 0.12

MAR-M 247

60,7 8,6 10 0,7 - 10 3,1 5,5 1,2 0,15

IN100 55 12.5 18.5 3.2 - - - 5 5 0.07

24 MPR March/April 2011 metal-powder.net

characterisation and tensile testing up to 1050° C was performed on selected sam-ples and compared with literature data of corresponding cast material.

There are some disadvantageous char-acteristics associated with the investment casting of Ni-based superalloys. The

consequences for turbocharger compo-nents made of these alloys are well known. High hardness is generally a feature of Ni-based superalloys, which makes them difficult to machine. Special equipment and experience for the machining itself is required. Therefore, any additional

finishing step especially for this class of material should be avoided as part costs do rise significantly. In contrast, MIM offers an enhanced near net shape performance avoiding additional processing in most cases.

Thermodynamics limits desired melt compositions as undesired phase trans-formations can occur in Ni-base superal-loy’s liquid state, but can be realised by the powder metallurgical route. However, PM-Ni-base superalloy material needs to be hot isostatic pressed (HIP) in order to reach sufficiently high densities to achieve the normal Ni-base superalloy material properties. This is not compulsory for

MIM – as sintered investment casting

Figure 3. Yield strengths of investment cast and MIM (as sintered) IN713C up to 1050° C, measured on air (ASTM E21 - 09), and their corre-sponding microstructure analysis of γ′ – phase.

Table 2. Sinter conditions and resulting densities for IN713C, MAR-M 247, and

IN100.

Tsint [° C] Atmosphere Density[g/cm³]

IN713C 1270 Ar 7,79 (99,3%)

MAR-M 247 1335 Ar 8,38 (98,4%)

IN100 1265 Ar 7.84 (100%)

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MIM, as very high densities are already present at the as-sintered state.

Further, the high viscosities and sur-face tensions of diverse Ni-base superalloy melts make them difficult to use in cast-ing. IN100, for example, which possesses promising data regarding creep and heat resistance at moderate material costs tends to show hot crack formations during cool-ing when cast. In summary, MIM provides the possibility for making materials that may be difficult or not possible to cast.

Owing to the finely dispersed powders used, MIM components have a very homo-geneous microstructure differing consider-ably from the microstructure of an invest-ment-cast component. Figure 2 illustrates this. The microstructure of investment cast Ni-base superalloy GMR-235 displays local carbide formation, segregation, and pro-nounced dendrite structures. These defects altogether weaken the material’s properties.

A “skin” can appear on the surface area of cast components. This outer layer

doesn’t display the chemical composition and therefore desired properties of the material and has to be removed, typi-cally by grinding, if a welding step follows. Oxide enriched areas due to segregation as mentioned earlier disturb (friction) welding.

The following Ni-base superalloys have been developed as MIM feedstock samples in response to the rising interest expressed by turbocharger manufacturers for these grades:

investment castingMIM – as sintered

MIM – heat treated

Figure 4. Yield strengths of investment cast and MIM (as sintered, and heat treated) IN713C up to 1050° C, measured in air (ASTM E21 - 09), and their corresponding microstructure analysis of γ′ – phase.

26 MPR March/April 2011 metal-powder.net

IN713C, MAR-M 247, and IN100. While the first two are already in use as cast alloys (IN713C for diesel-, MAR-M 247 for gasoline turbocharger turbine wheels), IN100 has not yet been pro-duced satisfactorily in cast form. This offers a good chance for MIM, as IN100 illustrates very good high temperature properties at reduced cost compared to MAR-M 247 (see Table 1 for their chemi-cal composition).

Because of the relatively high propor-tions of aluminium and titanium, sinter-ing of such a material is impossible under the gases normally used for sintering (H2, N2) resulting in hydrides and nitrides. Nonetheless, it has been demonstrated that, with the use of Ar as the sinter-ing gas, the formation of those chemical compounds can be avoided, and outstand-ing densification achieved. Sinter tempera-tures applied and resulting densities are given in Table 2.

Metallographic characterisation and tensile testing up to 1050° C was performed on the three materials and compared with literature data of cor-responding cast material.

The diagram in Figure 3 shows yield strength measured over temperature for both investment cast and metal injection moulded IN713C. Also on display the corresponding SEM-images of micro-structures showing the - precipitates.

What can be seen here is a coarser -structure for the MIM-material com-

pared to the fine -structure of the cast alloy.

For temperatures up to 700° C the result in yield strength of the IN713C specimen made by MIM gives better values com-pared to the cast material thanks to the overall more uniform structure of MIM as outlined earlier in Figure 2.

Starting at about 750° C the well known decrease in yield strength for IN713C is visible. Here the drop for MIM is more distinct. The reason for that is the coarser structure of the – phase. The – pre-cipitates embedded into the – matrix of Ni-base superalloys allow high tem-perature applications above 500° C where austenitic, heat resistant steels reach their limits.

In order to improve yield strength a heat treatment process was applied to the as-sintered MIM material. A much finer

– phase could be achieved, very similar in size of the – precipitates found in the cast IN713C microstructure.

This results in even higher yield strength of the MIM-material. At temperatures above 800 C it is converging with the curve of cast IN713C and continues congruent with it for the higher temperatures.

There is more work to be done. Hot ten-sile test measurements will be performed on MAR-M 247, and IN100 specimen,

too, to be published in due time. Another comparison between cast and MIM super-alloy MAR-M 247 is planned, and beyond that the potential for the application of IN100 not yet applied in automotive tur-bocharging will be further investigated.

A closer look at creep and low-cycle fatigue (LCF) will be essential to establish the requirements on a range of turbo-charger parts. Coarse grain size as found in cast material should show good creep behaviour whereas the fine grain sizes in MIM not only give higher yield strength, but should also provide better LCF - an important issue for movable parts inside turbochargers.

Turbocharging is already almost univer-sal on diesel engines and will become even more common in gasoline applications as engineers improve fuel consumption and emissions by designing downsized engines having higher specific outputs. Due to exhaust gas temperature reaching 1000° C and more in gasoline engines the material requirements on turbocharger and other hot end components is fierce. Costs - espe-cially in automotive - for such Ni-base superalloy components are very crucial and therefore ask for a cost-effective man-ufacturing process.

Metal Injection Moulding has already proved to be competitive in the field of tur-bocharging of diesel systems, and thanks to the latest developments shown in this work MIM provides superior mechanical properties with regards to yield strength compared to investment cast Ni-base superalloys at very high temperatures making MIM an excellent choice for the production of gasoline turbocharger parts.

The AuthorsThis feature is based on MIM Superalloys for Automotive Applications, a paper by Andreas Kern, Martin Blömacher, Johan ter Maat, and Arnd Thom who all work for BASF SE, G-CA/MI Powder Injection Moulding, D-67056 Ludwigshafen, Germany. It was given at the EPMA’s PM World Congress in Florence.

References[1] New manufacturing opportunities in the turbocharger due to Metal Injection

Molding (MIM); M. Blömacher, A. Kern, J. H. H. ter Maat, A. Thom PIM Int. 3 (2009) p. 37 – 42

[2] About Publishing Group: The global market for automotive superchargers and turbochargers; Jeff Daniels, 2007, p. 10 - 12

[3] J D Power Automotive Production Forecast 2009 – database, https://cluster.jdpa.com/servlet/dcs

[4] Processing of Superalloys via Powder Injection Molding; K. F. Hens, J. A. Grohowski, R. M. German, J. J. Valencia, T. McCabe, Advances in Powder Metallurgy and Particulate Materials, vol. 4, Metal Powder Industries Federation, Princeton, NJ, 1994, pp. 137-148.

[5] Powder Injection Molding of Inconel 718 Alloy; A. Bose, J.J. Valencia, J. Spirko, R. Schmess

Advances in Powder Metallurgy & Particulate Materials, MPIF, NJ, 1997, Vol. 3, pp. 18.99 – 18.112.

[6] ISO 2740: 1999 (E) [7] Metal Injection Moulding of Ni-Base Superalloys; H. Wohlfromm, A. Ribbens, J.

H. H. ter Maat, M. Blömacher EPMA; EuroPM 2003, Valencia[8] Superalloys – A Technical Guide; M. J. Donachie, S. J. Donachie, ASM

International, 2002