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The unique combination of high strength, light weight and ease ofmanufacturing makes ADI an attractive material for many automotiveapplications. Several automotive steel forgings and castings have beenreplaced 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 newplatform W-201 in 2006. During the initial conceptual phase itself, theyidentified ADI as a potential material for differential case. Traditionally, ductileiron castings are used for automotive differential case. But the strength andstiffness of ductile iron was not adequate to meet the required output torqueof the new high engine power vehicle. They needed a high strength material,which was amenable to casting process, relatively cheap, light weight andwell proven. ADI had all the desirable properties and was identified as theright choice of material.

This paper describes the development of ADI differential case for the 2011XUV-500 model cross-over vehicle. Paper describes the developmentfrom initial concept, designing with ADI, simulation and verification, prototypecasting and initial trials and production. Paper also describes problemsencountered 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 purposeof drive train is to transmit power generated by the engine to thewheels. Power is transmitted through a series of components – clutch,transmission, drive shaft and differential. Figure 1 shows a simplifiedschematic 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 FrontWheel Drive (FWD) or a Rear Wheel Drive (RWD) or a Four WheelDrive/All Wheel Drive (AWD).

When a vehicle is moving in straight line, speed of all the wheels remains

Understanding Austempered Ductile IronProcess, 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

1General Manager, Director – Hightemp Furnaces Limited, Bangalore, E-mail : [email protected] 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 totravel more distance than the inner wheels. If the difference is notcompensated, wheels would slip and skid causing excessive tire wear,noise and difficulty in steering.

To compensate the difference in distance, the outer wheel must travelfaster than the inner wheel in the same amount of time. Differentialprovides this mechanism, mechanism that allows the outer wheel torotate faster during a turn. Thus, main function of differential is toallow the drive wheels to spin at different speeds. In addition to thisfunction, differential also changes the direction of the power beingtransmitted by 90 degrees (arrow marked in Fig. 1), multiply thetorque via different gear ratios and distribute the torque equally to theright and left wheels.

Differential consists of three major components – differential case,ring and pinion gear and the differential gears. The entire assembly isenclosed in a differential carrier. Figure 2 shows the working of adifferential. Ring gear (also known as crown wheel) is bolted to thedifferential case. Inside the differential case are the differential sidegears 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.

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to the drive pinion through UV joint (Fig. 1). As the shaft rotates, drivepinion rotates the ring gear. As the ring gear is attached to the differentialcase, the differential case also rotates. Differential case supports twoplanet pinion gears, which mesh with the side gears. As the pinion gearrotates, the side gears rotate and in turn rotates the drive axles.

Differential Case

The differential case is the metal frame that encases the pinion gearsand side gears. Figure 3 gives a sketch of a differential case. It consistsof a flange to which the ring gear is attached, a dome to house thedifferential gears and housing ends for axle bearing supports on eitherends.

Requirements of a Differential Case

Differential case changes the direction of power from the input shaftby 90 degrees. The case rotates as the input propeller shaft rotatesand has to withstand the engine power at the rated rpm. Thus, differentialcase must be able to

Support the gear loads (load carrying is function of engine powerand 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 strengthmaterial 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 highpower and high torque transmission. This being a new cross over vehicle,several challenges had to be met in the design and development of thedriveline system.

Space and size constraint – compact packaging within the limitedtransaxle space for a given boundary condition.

Torque transmission – need of a high strength material; therequired output torque was not adequate from strength andstiffness perspective of the normal ductile iron grade used acrossthe industry.

Weight challenges – the target was to have a weight within setlimit.

High power to weight ratio – support new monocque concept.

Durability - perform incredibly even in the harshest environment.

Increase variant extension - flexibility in design to incorporate allvariants of the model two wheel (FWD, RWD) and all wheel drive(AWD) model.

Key challenges for the design engineers were overall weight reductionand packaging the differential case within the transaxle space whilemaintaining other requirements mentioned above.

ObjectivesObjective was to use a lightweight high torque capacity and cost-effective material for the differential case to reduce the overall massof the driveline system and to redesign the original differential case tofit within the existing space. Prime goal was not only design andperformance but also cost and manufacturability of the material.

Why ADI for Differential CaseAll other parts in driveline system are made of steel and offered noscope for weight reduction. Differential case was the only and primecandidate for weight reduction.

Generally, automotive differential cases are made of ductile iron andthe grade used is IS 500/7 with UTS = 500 MPa, YS = 320 MPa and7% elongation. Sometimes IS 600/3 grade ductile iron is used.

The new XUV – 500 vehicle generates a much higher power with amaximum power (140 BHP at 3750 rpm) and maximum torque (330NM at1600 rpm), and required much higher strength material thanstandard 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.

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Increasing the wall thickness would not only increase the weight of thepart but also make it bigger and bulkier and defeat the goal of spaceconstraint.

Hence, option of increasing the strength of the material was chosen.Steel is dense and heavy, aluminium has lower stiffness and highergrades of ductile iron do not meet the elongation and toughness criteria.

Design and development of the new vehicle has been an on-goingproject since early 2006.

Around this time, ADI was gaining popularity as the new automotivematerial in North America and Europe. ADI is known to have twice thestrength of ductile iron. A variety of properties can be obtained simplyby 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 yieldstrength of any regular grade ductile iron. The highest grade of thatcan be produced was 1600 MPa and still with an elongation of 1%.Table-1 compares the properties of 500/7-grade ductile iron grade1ADI of similar elongation. Stiffness of ADI is only slightly lower thanthat of ductile iron.

Besides strength, ADI has better toughness, improved fatigue propertiesand high wear resistance compared to ductile iron. Other benefitsconsidered are:

Starting material for ADI is ductile iron, no major change in foundrypractice was necessary

Strength is almost double and parts could be redesigned to makeit lighter and thinner

There were several examples of ADI applications with supporting dataavailable in public domain. However, data available in the literature didnot have any information on the torque transmitting characteristics ofADI. This was one of the functional requirements of the differentialcase for the new XUV model. So, Mahindra team commissioned aninternal study/research to evaluate torque-transmitting capacity ofADI as a material. Rigorous and elaborate material tests conducted intheir valley testing lab showed that ADI has ~ 20% higher torquecarrying capability than standard ductile iron.

So it was clear for the design team, to decide and zero in on ADI as thedifferential case material. They were convinced beyond any doubt,that it was possible to create a family of differential cases with reducedweight and space with ADI. Futuristic and forward thinking, they wereable to identify and conceptualise ADI right at the start of the designstage and introduce ADI as the new technology.

APPROACH AND METHODOLOGY OF DEVELOPMENT

ADI is a new technology to Mahindra team and also new technology toIndian manufacturers. The team had to look into not only design andvalidation but also had to consider manufacturing, heat treatment andmachining of ADI part. All things had to be accomplished withoutsacrificing 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 wasformed to handle various aspects of product development from designto prototype to full production.

The team also identified the partners for casting, heat treatment andmachining. It was decided that Mahindra Hinoday Limited (Pune) wouldmake the castings, Applied Process Inc (USA for preliminarydevelopment) and subsequently Hightemp Furnaces Ltd. (Pune) wouldprovide ADI treatment and Takshi Auto Components Pvt. Limited (Pune)would machine the castings.

All the partnering facilities are located within reasonable distance ofeach other and simplified the logistics and flow of materials.

Design

Design, testing and validation were all done entirely in-house. Theteam had a basic solid model of a ductile iron differential case used inother models of vehicle. Original design was modified and redesignedfor use in XUV500-W201 platform. FEA was carried out on solid modelwith load and boundary conditions already established for the new W-2101 driveline system. After several modifications of the design andevaluation, the team created a new lightweight and compact design ofdifferential case as shown in Fig. 4. During the design phase, differentialcase was designed for both 2WD and AWD variants. Figure 5 shows theprofile, shape and length changes made to the original design to arriveat 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 ADIdifferential case.

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By redesigning the differential caseand taking advantage of the highstrength of ADI, engineers wereable to achieve 33% weightsavings, reducing the wallthickness from 8 mm to 3.5 mmand reducing overall size of thedifferential case by 30%.

Production of Castings

Once the design of ADI differentialcase was finalised, based on theFEA simulation and verificationfocus was on the successfulproduction of castings. As thevolume requirement was huge,parts needed to be mass-produced and heat-treated.

Mahindra approached HinodayFoundry, a captive foundry ofMahindra, to supply differentialcase as finished ADI part. Foundrywas already making ductile irondifferential case for various modelsof Mahindra vehicles. They werefamiliar with the specification,requirement and production of thedifferential case. But, ADI was newto them but realising theenormous potential and impact,foundry readily accepted thechallenge.

The foundry team consulted withApplied Process Inc, USA, on thespecial foundry requirements forADI. Quite contrary to commonmyth that ADI requires special ironwith a lot of alloying elements andis very difficult to make, the realitywas quite contrary.

The requirements were simple,consistent chemistry, consistentprocess and consistentmicrostructure. Key factor isconsistency and this was alreadyin place in the foundry and noadditional special requirement wasneeded for making ADI.

As with any new partdevelopment, 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

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model and drawing supplied for the design of the new mould andgating system. Advance computer techniques and software ofsolidification and flow simulation were employed to establish the gatingsystem for a sound casting.

Preliminary Trials at Applied Process Inc, USA

While cross-functional team was optimising the mould design and gatingsystem, foundry had started preliminary investigation on test bars.Eight test bars were sent to Applied Process for ADI treatment andmicrostructure analysis. Four of the test bars were of normal chemistrywhile the other four bars had molybdenum addition. Nodule count andnodularity of all test bars exceeded the recommended value of 100nodules per mm2 and nodularity was above 90%. After ADI heattreatment, two test bars were tested at an external lab. Test barspassed the UTS and YS requirement but the elongation was very low.A second set of test bars also showed similar results. Suspecting poorquality of test bar, examination of the fracture surface was carriedout. Fracture surface and vicinity showed a lot of micro-porosity.

Second trial was done with actual castings. 15 numbers of castingsalong with five test bars were sent to Applied Process for ADI treatment.Castings were run with the same cycle as trial 1. After ADI heattreatment, a section was cut from the flange of the casting and sent toan outside lab for mechanical properties evaluation. Samples takendirectly from the casting exhibited mechanical properties specified forADI grade 1.

However, the microstructure at the thickest section of the casting wasnot satisfactory. Macro-etching of the thickest section showed quite abit of pearlite (dark etch) as shown in Fig. 6. Presence of pearlite in themicrostructure is an indication of inadequate alloying elements in themetal. Chemical analysis was performed on the sample and found outthat casting had less than recommended alloying elements and highlevel of tramp elements. Insufficient or inadequate or under-alloyedpart would not thorough-harden and would have mixed structure.

The foundry was given the minimum required chemistry to thorough-harden and advised to keep tramp elements to a low level and toimprove the test bar quality.

Production of a Lot of Castings

Green sand was mixed in a muller with automatic sand control system.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. Sandwichtype magnesium treatment was followed for spheroidisation. Auto poursystem was used for mould filling and maximum hold time was set at16 minutes. For every batch, all process parameters including pouringtemperature were recorded and the castings were marked withidentification of part number/month/day/shift/cavity and other details.

As per recommendations, input raw materials were controlled fortramp elements by adjusting the charge material. Foundry has an in-house lab and testing facilities. After verifying quality of soft castings,chemistry, micro, mechanical properties and level 2 NDT, 200 castingswere sent to Applied Process for processing along with machined testbars poured in a standard Y Block mould.

This lot of castings considered the production lot was sent to AppliedProcess for processing. After processing, a few castings were testedby Applied Process and also by the foundry. Table-2 shows resultsobtained from actual sample from the casting tested by the foundry.Mechanical and microstructure were found to surpass the requirementof grade1 ADI properties.

After receiving the lab report, the foundry was more confident on theprocess and development and convinced on the process of makinggood ADI. After discussion and approval from Mahindra, 100 castingsfrom the batch were finish machined and sent for field-testing andvalidation. Durability test was done over several kilometer run. Aftervalidation and approval of the part, chemistry, process parameters andADI cycle were frozen for production run.

The foundry has been producing ADI differential case for the XUV-500since 2010 without any problems.

ADI Heat TreatmentADI refers to heat treated ductile iron. Figure 7 shows the processsteps in ADI heat treatment. Casting are heated to austenitising region,

Fig. 6: Fig. 6: Fig. 6: Fig. 6: Fig. 6: Pearlite ball seen on macro-etching of sample.

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TTTTTablablablablable - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Pre - 2: Mechanical Propopopopopererererer tietietietieties of ADI Prs of ADI Prs of ADI Prs of ADI Prs of ADI ProcococococeeeeesssssssssseeeeedddddCastingsCastingsCastingsCastingsCastings

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

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typically 850 to 920 ºC, held at the temperature under protectiveatmosphere and then quenched in molten salt bath. Temperature ofthe salt is typically in 260-400 ºC range. Time is generally in the rangeof 1.5 hours to 4 hours depending on the chemistry and propertiesrequired. Key to successful austempering is the rapid transfer stepfrom austenitisation temperature to isothermal transformationtemperature – cooling rate fast enough to avoid pearlite formationand temperature high enough to avoid martensite formation.

For differential case, properties required are that of Grade1 ADI (ASTMA897/A897M) with YTS/YS/%E of 900 MPa/650 MPa/9 %. Duringthe prototype casting development, the foundry was given thechemistry required for through hardening.

Initial ADI development trials (three trials) were done at the AppliedProcess Inc. austempering facility in USA. Further production castingswere heat treated by Hightemp Furnaces in their Gurgaon facility untilthe end of the year 2011. PPAP was done on castings from the first fivebatches of the castings. The castings were checked for microstructure,hardness and mechanical properties by an external lab for Hightemp.The foundry also did a cross check in their in-house testing facility. Thecastings were checked at the locations marked on the part shown inFig. 8. The casting shown in the figure is as-cast differential case.

Since January of 2013, castings are austempered at HightempFurnaces facility in Pune. As the parts were transferred from GGN toPune plant, PPAP was done again for first fifteen batches. Table-3gives the test results of four consecutive batches. From each batch,four castings were tested. Results show the consistency andrepeatability 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 tensilesamples were taken.

TTTTTablablablablable -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Pre -3 : Mechanical Propopopopopererererer tietietietieties 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 (%) BHNNO. 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

TH -16 1013 1 0 0 6 978 1020 943 889 798 851 10 .6 10.5 11 .6 11.80 293 285 293 285 302 302 302 302302 293 302 293 302 302 302 311

TH - 17 9 8 6 1033 1022 1017 8 7 6 937 848 833 10.2 13.00 13.1 12 .6 302 293 285 285 293 285 285 293302 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 285302 302 293 302 302 311 293 285

TH-19 1018 1022 997 1043 903 894 924 8 8 6 10.9 12.5 11.2 1 2 293 293 285 293 293 293 293 293293 302 293 293 302 302 285 302

AUsferrite=Mixture of Acicular FerriteAnd High Carbon Austenite

Fig. 9: Fig. 9: Fig. 9: Fig. 9: Fig. 9: Microstructure after ADI heat treatment.

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Figure 10 shows the loading of differential case in the austemperingfurnace in Pune faciltiy.

Furnace is a pusher type furnace with a gross loading capacity of1000 kg. Load in the furnace chamber is heated under a protectiveatmosphere of endo-gas and LPG with a carbon potential of 1.1. Afterthe 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 beforefinal assembly of the differential. Flange area is machined; holes drilled

Fig. 10: Fig. 10: Fig. 10: Fig. 10: Fig. 10: ADI furnace showing castings being loaded into thefurnace.

for bolting ring gear and the internal bores are machined accurately tohouse the pinion and side gears. Figure 11 shows the process flow ofthe part.

As-cast castings from the foundry are sent to the machine shop forrough machining.

Rough machined castings are ADI heat-treated and sent back to themachine shop for finish machining. Finished castings are checked fordimensional accuracy and sent to Mahindra shop for further assembly.

Castings can also be finish machined prior to ADI heat treatment. Inorder to take full advantage of excellent machinability of ductile iron,castings can be machined before the heat treatment. This would savea lot of time, money and simplify logistic planning. However, it shouldbe noted that during ADI heat treatment, some amount of growthoccurs and must be accounted for in the machining allowance. Plansare 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 goesthrough a different set ups milling, turning, boring and hobbing.Machining needs to be done with utmost precision as the requiredtolerance for differential case is very tight. Dimensions have to bechecked 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.

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ADI does not machine like ductile iron and same tool set-up does notwork well for ADI. They found difficulty in machining and complainedabout frequent tool failure.

ADI, particularly the differential case grade of ADI is not difficult tomachine. Understanding ADI microstructure and the effect of variousoperations on microstructure and transformation occurring at the toolcontact is very important for successful machining of ADI.

With time, experimenting and experience gained, the machine shopmodified their tool set-up and machining practice to machine ADIdifferential case. Today, with appropriate tool material, feed rate, toolspeed and depth of cut, they are able to machine ADI castingssuccessfully.

Problems encountered during machining of ADI differential castingsand how they were overcome is in itself a separate project and wouldbe elaborated in the next part of the Understanding ADI series.

SUMMARY

Differential case is a critical part of a vehicle driveline system. Differential

cases are traditionally made of ductile iron. For the new cross-overvehicle built on a new platform with a higher engine power and torque,traditional ductile iron properties were inadequate to meet thefunctional requirement of strength and torque transmission. Workingas a team, Mahindra engineers designed a new lightweight compactADI differential case meeting the programme objectives of weightreduction and space constraint and having all functional requirements.ADI part was lighter by 33% and smaller by 30% and had 20% moretorque transmitting capability than traditional ductile iron part. TheADI differential case has passed all the validation tests and has provenitself over and above expectation. It is believed to be the first applicationof ADI in India in automotive applications.

Acknowledgements

The authors are grateful to the management of Mahindra & MahindraLimited for their unwavering support and to all the individuals whocontributed to the development of ADI differential case for XUV-500cross-over vehicle.

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UNDERSTANDING ADI PART 4 AS PUBLISHED