UNDERSTANDING ADI PART 4 AS PUBLISHED
- 1. TECHNICAL PAPER 36 Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59 No. 5 May 2013 The unique combination of high strength, light weight and ease of manufacturing makes ADI an attractive material for many automotive applications. Several automotive steel forgings and castings have been replaced successfully by ADI castings at substantial weight and cost savings. Mahindra & Mahindra Limited (M&M) was developing a totally new cross- over vehicle XUV-500 - a brand new SUV vehicle on an altogether new platform W-201 in 2006. During the initial conceptual phase itself, they identified ADI as a potential material for differential case. Traditionally, ductile iron castings are used for automotive differential case. But the strength and stiffnessofductileironwasnotadequatetomeettherequiredoutputtorque of the new high engine power vehicle. They needed a high strength material, which was amenable to casting process, relatively cheap, light weight and well proven. ADI had all the desirable properties and was identified as the right choice of material. This paper describes the development of ADI differential case for the 2011 XUV-500 model cross-over vehicle. Paper describes the development frominitialconcept,designingwithADI,simulationandverification,prototype casting and initial trials and production. Paper also describes problems encountered during the development and how they were addressed. PART DESCRIPTION Function and Requirement of a Differential Case Differential forms the part of a drive train (power train). Main purpose of drive train is to transmit power generated by the engine to the wheels. Poweristransmittedthroughaseriesofcomponents–clutch, transmission, drive shaft and differential. Figure 1 shows a simplified schematic drawing of the drive train in a rear wheel drive vehicle. Differential distributes the power (torque) to the wheels. A differential is needed for any two-drive wheels whether it is a Front Wheel Drive (FWD) or a Rear Wheel Drive (RWD) or a Four Wheel Drive/All Wheel Drive (AWD). Whenavehicleismovinginstraightline,speedofallthewheelsremains Understanding Austempered Ductile Iron Process, Production, Properties and Applications – Part IV Case Study – Development of ADI Differential Case S. Gowri1 , Pratap Ghorpade1 , David Prakash2 , A. Pathrabe2 and K. C. Garg3 1 General Manager, Director – Hightemp Furnaces Limited, Bangalore, E-mail : gowri@hightemp-furnaces.com 2 Manager,Deputy Manager, Mahindra & Mahindra Limited, Mumbai 3 Head–QA, Mahindra Hinoday Industries Limited, Pune CASE STUDY the same. But when the vehicle is making a turn, outer wheel has to travel more distance than the inner wheels. If the difference is not compensated, wheels would slip and skid causing excessive tire wear, noise and difficulty in steering. Tocompensatethedifferenceindistance,theouterwheelmusttravel faster than the inner wheel in the same amount of time. Differential provides this mechanism, mechanism that allows the outer wheel to rotate faster during a turn. Thus, main function of differential is to allow the drive wheels to spin at different speeds. In addition to this function, differential also changes the direction of the power being transmitted by 90 degrees (arrow marked in Fig. 1), multiply the torqueviadifferentgearratiosanddistributethetorqueequallytothe right and left wheels. Differential consists of three major components – differential case, ring and pinion gear and the differential gears. The entire assembly is enclosed in a differential carrier. Figure 2 shows the working of a differential. Ring gear (also known as crown wheel) is bolted to the differential case. Inside the differential case are the differential side gears and the differential pinion gears. The propeller shaft is attached Fig. 1:Fig. 1:Fig. 1:Fig. 1:Fig. 1: Schematic drawing of an automotive drive train.
- 2. TECHNICAL PAPER Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry Journal 37 Vol 59 No. 5 May 2013 to the drive pinion through UV joint (Fig. 1). As the shaft rotates, drive pinionrotatestheringgear.Astheringgearisattachedtothedifferential case, the differential case also rotates. Differential case supports two planetpiniongears,whichmeshwiththesidegears.Asthepiniongear rotates, the side gears rotate and in turn rotates the drive axles. Differential Case The differential case is the metal frame that encases the pinion gears and side gears. Figure 3 gives a sketch of a differential case. It consists of a flange to which the ring gear is attached, a dome to house the differentialgearsandhousingendsforaxlebearingsupportsoneither ends. Requirements of a Differential Case Differential case changes the direction of power from the input shaft by 90 degrees. The case rotates as the input propeller shaft rotates andhastowithstandtheenginepowerattheratedrpm.Thus,differential case must be able to Supportthegearloads(loadcarryingisfunctionofenginepower and torque generated at peak power) Support differential gears (keep gears in place) Support a locking arrangement for CV joint The differential case must, therefore, be made of a high strength material with adequate stiffness to support and hold the gear loads. PROBLEM AND OBJECTIVES Problem The XUV-500 vehicle, for its class, is designed for a relatively high powerandhightorquetransmission.Thisbeinganewcrossovervehicle, severalchallengeshadtobemetinthedesignanddevelopmentofthe driveline system. Spaceandsizeconstraint–compactpackagingwithinthelimited transaxle space for a given boundary condition. Torque transmission – need of a high strength material; the required output torque was not adequate from strength and stiffnessperspectiveofthenormalductileirongradeusedacross the industry. Weight challenges – the target was to have a weight within set limit. High power to weight ratio – support new monocque concept. Durability-performincrediblyevenintheharshestenvironment. Increase variant extension - flexibility in design to incorporate all variants of the model two wheel (FWD, RWD) and all wheel drive (AWD) model. Keychallengesforthedesignengineerswereoverallweightreduction and packaging the differential case within the transaxle space while maintainingotherrequirementsmentionedabove. Objectives Objective was to use a lightweight high torque capacity and cost- effective material for the differential case to reduce the overall mass ofthedrivelinesystemandtoredesigntheoriginaldifferentialcaseto fit within the existing space. Prime goal was not only design and performance but also cost and manufacturability of the material. Why ADI for Differential Case All other parts in driveline system are made of steel and offered no scope for weight reduction. Differential case was the only and prime candidate for weight reduction. Generally, automotive differential cases are made of ductile iron and the grade used is IS 500/7 with UTS = 500 MPa, YS = 320 MPa and 7% elongation. Sometimes IS 600/3 grade ductile iron is used. The new XUV – 500 vehicle generates a much higher power with a maximum power (140 BHP at 3750 rpm) and maximum torque (330 NM at1600 rpm), and required much higher strength material than standard 500/7 grade ductile Iron. To meet the strength requirement, two options were available Increase the wall thickness of the differential case Use advanced high strength material Fig. 2:Fig. 2:Fig. 2:Fig. 2:Fig. 2: Working of a differential. Fig. 3:Fig. 3:Fig. 3:Fig. 3:Fig. 3: Schematic of a differential case. CASE STUDY
- 3. TECHNICAL PAPER 38 Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59 No. 5 May 2013 Increasingthewallthicknesswouldnotonlyincreasetheweightofthe part but also make it bigger and bulkier and defeat the goal of space constraint. Hence, option of increasing the strength of the material was chosen. Steel is dense and heavy, aluminium has lower stiffness and higher gradesofductileirondonotmeettheelongationandtoughnesscriteria. Design and development of the new vehicle has been an on-going project since early 2006. Around this time, ADI was gaining popularity as the new automotive material in North America and Europe. ADI is known to have twice the strength of ductile iron. A variety of properties can be obtained simply by heat treatment. ADI is the strongest in the cast iron family with excellent strength- to- weight ratio. For a given elongation ADI has twice the tensile and yield strength of any regular grade ductile iron. The highest grade of that can be produced was 1600 MPa and still with an elongation of 1%. Table-1 compares the properties of 500/7-grade ductile iron grade1 ADI of similar elongation. Stiffness of ADI is only slightly lower than that of ductile iron. Besidesstrength,ADIhasbettertoughness,improvedfatigueproperties and high wear resistance compared to ductile iron. Other benefits considered are: StartingmaterialforADIisductileiron,nomajorchangeinfoundry practice was necessary Strengthisalmostdoubleandpartscouldberedesignedtomake itlighterandthinner There were several examples of ADI applications with supporting data availableinpublicdomain.However,data availableintheliteraturedid nothaveanyinformationonthetorquetransmittingcharacteristicsof ADI. This was one of the functional requirements of the differential case for the new XUV model. So, Mahindra team commissioned an internal study/research to evaluate torque-transmitting capacity of ADI as a material. Rigorous and elaborate material tests conducted in their valley testing lab showed that ADI has ~ 20% higher torque carrying capability than standard ductile iron. So it was clear for the design team, to decide and zero in on ADI as the differential case material. They were convinced beyond any doubt, thatitwaspossibletocreateafamilyofdifferentialcaseswithreduced weight and space with ADI. Futuristic and forward thinking, they were able to identify and conceptualise ADI right at the start of the design stage and introduce ADI as the new technology. APPROACH AND METHODOLOGY OF DEVELOPMENT ADIisanewtechnologytoMahindrateamandalsonewtechnologyto Indian manufacturers. The team had to look into not only design and validationbutalsohadtoconsidermanufacturing,heattreatmentand machining of ADI part. All things had to be accomplished without sacrificing defined goals and objectives, quality and safety and a dead- line of the model release in September 2011. The approach was simple-effective team effort. Internal team was formedtohandlevariousaspectsofproductdevelopmentfromdesign to prototype to full production. The team also identified the partners for casting, heat treatment and machining.ItwasdecidedthatMahindraHinodayLimited(Pune)would make the castings, Applied Process Inc (USA for preliminary development)andsubsequentlyHightempFurnacesLtd.(Pune)would provideADItreatmentandTakshiAutoComponentsPvt.Limited(Pune) would machine the castings. All the partnering facilities are located within reasonable distance of each other and simplified the logistics and flow of materials. Design Design, testing and validation were all done entirely in-house. The team had a basic solid model of a ductile iron differential case used in othermodelsofvehicle.Originaldesignwasmodifiedandredesigned for use in XUV500-W201 platform. FEA was carried out on solid model with load and boundary conditions already established for the new W- 2101 driveline system. After several modifications of the design and evaluation,theteamcreatedanewlightweightandcompactdesignof differentialcaseasshowninFig.4. Duringthedesignphase,differential case was designed for both 2WD and AWD variants. Figure 5 shows the profile,shapeandlengthchangesmadetotheoriginaldesigntoarrive at the new lightweight differential case. TTTTTablablablablable-1: Pre-1: Pre-1: Pre-1: Pre-1: Propopopopopererererertietietietieties of DI vs of DI vs of DI vs of DI vs of DI vsssss. ADI f. ADI f. ADI f. ADI f. ADI for Similaror Similaror Similaror Similaror Similar % Elongation% Elongation% Elongation% Elongation% Elongation Material Grade UTS YS % E BHN (MPa) (MPa) DI 500/7 500 320 7 160-240 ADI Grade 1 900 650 9 269-341 Fig. 4:Fig. 4:Fig. 4:Fig. 4:Fig. 4: Model of the newly designed lightweight ADI differential case. CASE STUDY
- 4. TECHNICAL PAPER Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry Journal 39 Vol 59 No. 5 May 2013 Byredesigningthedifferentialcase and taking advantage of the high strength of ADI, engineers were able to achieve 33% weight savings, reducing the wall thickness from 8 mm to 3.5 mm and reducing overall size of the differential case by 30%. Production of Castings OncethedesignofADIdifferential case was finalised, based on the FEA simulation and verification focus was on the successful production of castings. As the volume requirement was huge, parts needed to be mass- produced and heat-treated. Mahindra approached Hinoday Foundry, a captive foundry of Mahindra, to supply differential case as finished ADI part. Foundry was already making ductile iron differentialcaseforvariousmodels of Mahindra vehicles. They were familiar with the specification, requirementandproductionofthe differentialcase.But,ADIwasnew to them but realising the enormous potential and impact, foundry readily accepted the challenge. Thefoundryteamconsultedwith Applied Process Inc, USA, on the special foundry requirements for ADI. Quite contrary to common myththatADIrequiresspecialiron withalotofalloyingelementsand isverydifficulttomake,thereality was quite contrary. The requirements were simple, consistent chemistry, consistent process and consistent microstructure. Key factor is consistency and this was already in place in the foundry and no additionalspecialrequirementwas needed for making ADI. As with any new part development, the cross- functional team used the solid Fig. 5:Fig. 5:Fig. 5:Fig. 5:Fig. 5: Original and modified design according to shape, profile and length. Original & Modified Design of Differential Case CASE STUDY
- 5. TECHNICAL PAPER 40 Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59 No. 5 May 2013 model and drawing supplied for the design of the new mould and gating system. Advance computer techniques and software of solidificationandflowsimulationwereemployedtoestablishthegating system for a sound casting. Preliminary Trials at Applied Process Inc, USA Whilecross-functionalteamwasoptimisingthemoulddesignandgating system, foundry had started preliminary investigation on test bars. Eight test bars were sent to Applied Process for ADI treatment and microstructureanalysis.Fourofthetestbarswereofnormalchemistry whiletheotherfourbarshadmolybdenumaddition.Nodulecountand nodularity of all test bars exceeded the recommended value of 100 nodules per mm2 and nodularity was above 90%. After ADI heat treatment, two test bars were tested at an external lab. Test bars passed the UTS and YS requirement but the elongation was very low. A second set of test bars also showed similar results. Suspecting poor quality of test bar, examination of the fracture surface was carried out. Fracture surface and vicinity showed a lot of micro-porosity. Second trial was done with actual castings. 15 numbers of castings alongwithfivetestbarsweresenttoAppliedProcessforADItreatment. Castings were run with the same cycle as trial 1. After ADI heat treatment,asectionwascutfromtheflangeofthecastingandsentto an outside lab for mechanical properties evaluation. Samples taken directlyfromthecastingexhibitedmechanicalpropertiesspecifiedfor ADI grade 1. However,themicrostructureatthethickestsectionofthecastingwas notsatisfactory. Macro-etchingofthethickestsectionshowedquitea bit of pearlite (dark etch) as shown in Fig. 6. Presence of pearlite in the microstructure is an indication of inadequate alloying elements in the metal. Chemical analysis was performed on the sample and found out that casting had less than recommended alloying elements and high level of tramp elements. Insufficient or inadequate or under-alloyed part would not thorough-harden and would have mixed structure. Thefoundrywasgiventheminimumrequiredchemistrytothorough- harden and advised to keep tramp elements to a low level and to improve the test bar quality. Production of a Lot of Castings Greensandwasmixedinamullerwithautomaticsandcontrolsystem. Moulding was done in a high-pressure horizontal Disamatic machine; with two cavities in a mould. The base metal was melted in a coreless induction furnace. Sandwich typemagnesiumtreatmentwasfollowedforspheroidisation.Autopour system was used for mould filling and maximum hold time was set at 16 minutes. For every batch, all process parameters including pouring temperature were recorded and the castings were marked with identificationofpartnumber/month/day/shift/cavityandotherdetails. As per recommendations, input raw materials were controlled for tramp elements by adjusting the charge material. Foundry has an in- house lab and testing facilities. After verifying quality of soft castings, chemistry,micro,mechanicalproperties andlevel2NDT,200castings weresenttoAppliedProcessforprocessingalongwithmachinedtest bars poured in a standard Y Block mould. This lot of castings considered the production lot was sent to Applied Process for processing. After processing, a few castings were tested by Applied Process and also by the foundry. Table-2 shows results obtained from actual sample from the casting tested by the foundry. Mechanicalandmicrostructurewerefoundtosurpasstherequirement of grade1 ADI properties. After receiving the lab report, the foundry was more confident on the process and development and convinced on the process of making good ADI. After discussion and approval from Mahindra, 100 castings from the batch were finish machined and sent for field-testing and validation. Durability test was done over several kilometer run. After validationandapprovalofthepart,chemistry,processparametersand ADI cycle were frozen for production run. ThefoundryhasbeenproducingADIdifferentialcasefortheXUV-500 since 2010 without any problems. ADI Heat Treatment ADI refers to heat treated ductile iron. Figure 7 shows the process stepsinADIheattreatment.Castingareheatedtoaustenitisingregion, Fig. 6:Fig. 6:Fig. 6:Fig. 6:Fig. 6: Pearlite ball seen on macro-etching of sample. CASE STUDY TTTTTablablablablable - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Propopopopopererererertietietietieties of ADI Prs of ADI Prs of ADI Prs of ADI Prs of ADI Procococococeeeeesssssssssseeeeeddddd CastingsCastingsCastingsCastingsCastings UTS YS ELONGATION (MPa) (MPa) (%) ADI GRADE1 900 650 9 CASTING 1 1055 945 16.25 CASTING 2 1020 786 13.5 CASTING 3 1000 821 11.2
- 6. TECHNICAL PAPER Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry Journal 41 Vol 59 No. 5 May 2013 typically 850 to 920 ºC, held at the temperature under protective atmosphere and then quenched in molten salt bath. Temperature of the salt is typically in 260-400 ºC range. Time is generally in the range of 1.5 hours to 4 hours depending on the chemistry and properties required. Key to successful austempering is the rapid transfer step from austenitisation temperature to isothermal transformation temperature – cooling rate fast enough to avoid pearlite formation and temperature high enough to avoid martensite formation. For differential case, properties required are that of Grade1 ADI (ASTM A897/A897M) with YTS/YS/%E of 900 MPa/650 MPa/9 %. During the prototype casting development, the foundry was given the chemistry required for through hardening. Initial ADI development trials (three trials) were done at the Applied Process Inc. austempering facility in USA. Further production castings wereheattreatedbyHightempFurnacesintheirGurgaonfacilityuntil theendoftheyear2011.PPAPwasdoneoncastingsfromthefirstfive batchesofthecastings.Thecastingswerecheckedformicrostructure, hardness and mechanical properties by an external lab for Hightemp. Thefoundryalsodidacrosscheckintheirin-housetestingfacility.The castings were checked at the locations marked on the part shown in Fig. 8. The casting shown in the figure is as-cast differential case. Since January of 2013, castings are austempered at Hightemp Furnaces facility in Pune. As the parts were transferred from GGN to Pune plant, PPAP was done again for first fifteen batches. Table-3 gives the test results of four consecutive batches. From each batch, four castings were tested. Results show the consistency and repeatability of the process. Fig. 7:Fig. 7:Fig. 7:Fig. 7:Fig. 7: Typical ADI Heat Treatment Cycle. Fig. 8:Fig. 8:Fig. 8:Fig. 8:Fig. 8: Locations where micro, hardness and tensile samples were taken. TTTTTablablablablable -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Propopopopopererererertietietietieties of Fs of Fs of Fs of Fs of Four Consour Consour Consour Consour Consecutivecutivecutivecutivecutive Bae Bae Bae Bae Batttttchechechecheches of ADI Hes of ADI Hes of ADI Hes of ADI Hes of ADI Heaaaaattttt-----TTTTTrrrrreeeeeaaaaattttteeeeed Cad Cad Cad Cad Castingsstingsstingsstingsstings CHARGE UTS (MPa) YS (MPa) ELONGATION (%) BHN NO. 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 TH -16 1013 1006 978 1020 943 889 798 851 10.6 10.5 11.6 11.80 293 285 293 285 302 302 302 302 302 293 302 293 302 302 302 311 TH - 17 986 1033 1022 1017 876 937 848 833 10.2 13.00 13.1 12.6 302 293 285 285 293 285 285 293 302 302 293 293 293 285 293 293 TH-18 977 1058 1031 997 837 951 888 893 9.32 12.4 11.80 11.6 293 293 285 293 293 302 293 285 302 302 293 302 302 311 293 285 TH-19 1018 1022 997 1043 903 894 924 886 10.9 12.5 11.2 12 293 293 285 293 293 293 293 293 293 302 293 293 302 302 285 302 AUsferrite=Mixture of Acicular Ferrite AndHighCarbonAustenite Fig. 9:Fig. 9:Fig. 9:Fig. 9:Fig. 9: Microstructure after ADI heat treatment. CASE STUDY
- 7. TECHNICAL PAPER 42 Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59 No. 5 May 2013 Figure 10 shows the loading of differential case in the austempering furnace in Pune faciltiy. Furnace is a pusher type furnace with a gross loading capacity of 1000 kg. Load in the furnace chamber is heated under a protective atmosphere of endo-gas and LPG with a carbon potential of 1.1. After the heat treatment, parts are cleaned in a three-tank washer system. The castings are certified based on hardness at the specified location. PROCESS FLOW Differential case castings undergo fair amount of machining before finalassemblyofthedifferential.Flangeareaismachined;holesdrilled Fig. 10:Fig. 10:Fig. 10:Fig. 10:Fig. 10: ADI furnace showing castings being loaded into the furnace. forboltingringgearandtheinternalboresaremachinedaccuratelyto house the pinion and side gears. Figure 11 shows the process flow of the part. As-cast castings from the foundry are sent to the machine shop for roughmachining. Rough machined castings are ADI heat-treated and sent back to the machine shop for finish machining. Finished castings are checked for dimensionalaccuracyandsenttoMahindrashopforfurtherassembly. Castings can also be finish machined prior to ADI heat treatment. In order to take full advantage of excellent machinability of ductile iron, castings can be machined before the heat treatment. This would save a lot of time, money and simplify logistic planning. However, it should be noted that during ADI heat treatment, some amount of growth occurs and must be accounted for in the machining allowance. Plans are underway to fully finish machine before ADI treatment. MACHINING Machining of differential case is not one step but requires many set- ups. Rough machining is done in a milling machine. Finish machining goes through a different set ups milling, turning, boring and hobbing. Machining needs to be done with utmost precision as the required tolerance for differential case is very tight. Dimensions have to be checked at critical locations for every casting. Initially, the machine shop faced a lot of problems during machining. Fig. 11:Fig. 11:Fig. 11:Fig. 11:Fig. 11: Process flow of castings. CASE STUDY
- 8. TECHNICAL PAPER Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry Journal 43 Vol 59 No. 5 May 2013 ADI does not machine like ductile iron and same tool set-up does not work well for ADI. They found difficulty in machining and complained about frequent tool failure. ADI, particularly the differential case grade of ADI is not difficult to machine. Understanding ADI microstructure and the effect of various operationsonmicrostructureandtransformationoccurringatthetool contact is very important for successful machining of ADI. With time, experimenting and experience gained, the machine shop modified their tool set-up and machining practice to machine ADI differential case. Today, with appropriate tool material, feed rate, tool speed and depth of cut, they are able to machine ADI castings successfully. Problems encountered during machining of ADI differential castings and how they were overcome is in itself a separate project and would be elaborated in the next part of the Understanding ADI series. SUMMARY Differentialcaseisacriticalpartofavehicledrivelinesystem.Differential cases are traditionally made of ductile iron. For the new cross-over vehiclebuiltonanewplatformwithahigherenginepowerandtorque, traditional ductile iron properties were inadequate to meet the functional requirement of strength and torque transmission. Working as a team, Mahindra engineers designed a new lightweight compact ADI differential case meeting the programme objectives of weight reductionandspaceconstraintandhavingallfunctionalrequirements. ADI part was lighter by 33% and smaller by 30% and had 20% more torque transmitting capability than traditional ductile iron part. The ADI differential case has passed all the validation tests and has proven itselfoverandaboveexpectation.Itisbelievedtobethefirstapplication of ADI in India in automotive applications. Acknowledgements The authors are grateful to the management of Mahindra & Mahindra Limited for their unwavering support and to all the individuals who contributed to the development of ADI differential case for XUV-500 cross-overvehicle. CASE STUDY