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Journal of Materials Processing Technology 214 (2014) 1921–1927

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

jo ur nal ho me page: www.elsev ier .com/ locate / jmatprotec

oint-site structure friction welding method as a tool for drive pinionight weighting in heavy-duty trucks

amareh Mohammadzadeh Polamia,∗, Rudolf Reinhardtb,1,ichael Rethmeierc,2, Alois Schmida,3

Technology Management Daimler Trucks (TG/MFT), Daimler AG, 001/E200, 70546 Stuttgart, GermanyProduction and Materials Technology (PWT/VEP), Daimler AG, H152, 70546 Stuttgart, GermanyHead of Division 9.3 – Welding Technology, BAM Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany

r t i c l e i n f o

rticle history:eceived 19 July 2013eceived in revised form 17 March 2014ccepted 18 March 2014vailable online 26 March 2014

eywords:

a b s t r a c t

To satisfy the applied compressive stresses of friction welded drive pinion fabricated by using the joint-site structure (JSS) method, three different variants were followed: (A) the initial design with two jointswas carried out. Two different burn-off lengths were examined for this variant. (B) The optimum burn-offlength was considered for only one weld zone. (C) The weld zone was moved radially from the initiallocation and two different gap sizes were compared. The smallest gap size for the third variant led tothe largest weld length. The lack of structural welding defects for this variant was assessed by ultrasonic

riction weldingoint-site structurerive pinionightweight design

testing. Hardness of the material after friction welding (FW) was correlated to the Continuous CoolingTransformation (CCT) diagram of the used materials and revealed the phase/microstructure transforma-tion of the material. The simulated applied stresses on the optimized friction welded design of the drivepinion showed suitable results. The new drive pinion friction welded by the JSS method reduced the

by ap

weight of the component

. Introduction

Fuel efficiency is a key profit factor for the long-haul trans-ortation industry. In heavy-duty trucks, lightweight design is very

mportant for achieving further fuel consumption reduction andncreasing the truck payload. Klein (2011) and Grubisic (1986)tated particular requirements of lightweight designs emphasizinghe material properties, geometry and the ability of the materi-ls to withstand high stress. In order to achieve fuel efficiency,ne of many lightweight design approaches is to focus on weight-eduction for truck components such as the drive pinion, i.e. theomponent that is the main focus of this research. The currentndustry-standard drive pinion is produced as a solid forged single-

iece part. Quintenz and Raedt (2009) and Neugebauer et al. (2001)ade use of a hollow shaft to significantly reduce the weight of

omponents. In order to achieve suitable weight reduction and

∗ Corresponding author. Tel.: +49 07 11 17 32123; fax: +49 07 11 17 79074122.E-mail addresses: samareh.mohammadzadeh@daimler.com

S. Mohammadzadeh Polami), rudolf.reinhardt@daimler.com (R. Reinhardt),ichael.rethmeier@bam.de (M. Rethmeier), alois.schmid@daimler.com (A. Schmid).1 Tel.: +49 07 11 17 2 64 79; fax: +49 07 11 17 9026479.2 Tel.: +49 030 8104 1550; fax: +49 030 8104 1557.3 Tel.: +49 07 11 17 5 62 17; fax: +49 07 11 17 79 05 62 17.

ttp://dx.doi.org/10.1016/j.jmatprotec.2014.03.027924-0136/© 2014 Elsevier B.V. All rights reserved.

prox. 14%.© 2014 Elsevier B.V. All rights reserved.

avoid relatively expensive conventional methods (such as drillingthe shafts), a hollow shaft was used in this research. The hollowshaft was joined to the bevel using a FW technique.

Designing bimetallic parts using expensive materials only whenthey are essential creates significant cost-saving opportunities. Dis-similar materials applied for the bevel gear and the shaft parts,for instance, substitute the current homogenous material of thedrive pinion. Bevel gears are usually highly stressed, so that high-quality demands on the material are fulfilled. In contrast, the shaftmaterial can be chosen to be less expensive. According to the Amer-ican Welding Society (AWS (1989)), FW is the ideal method forjoining metals that are not necessarily similar. Donohue (2008)mentioned the cost advantages of using composite rather thanone piece shaft in FW of the pump. Machedon and Machedon(2007) demonstrated the application of FW for some automobilecomponents in which FW of the drive shafts replaced electric arcwelding technology. Using this method, the end component fittingprocess was simplified and the costs were reduced too. Grünauer(1987) discussed possible weld-zone geometries for different com-ponents. In a patent registered by Fett and Colbert (2000), the axleportion was friction welded to the flange portion of the motor vehi-

cle axle shaft. Kong et al. (2010) tested the replacement of theas-forged automobile reverse idle gear shaft with friction-weldeddissimilar joints. They found the optimal FW parameters for thistest.

1922 S. Mohammadzadeh Polami et al. / Journal of Material

iabcigofioasTtapoid

absitm

Fig. 1. JSS of friction welded crankshaft.

FW was introduced as a joining method for circular profiles, usedn most cases for butt welding. However, Nied et al. (1995) invented

method for FW a shaft to a disk having the tapered wedge onoth parts, resulting in a weld interface in an area of lower stressompared to conventional butt welding. Steinmetz et al. (2011)ntroduced a new JSS FW method, in which a shaft was joined to theear of a crankshaft. They discussed the advantages of this methodf shaft/crankshaft joining for preventing the interruption of theber direction of the shaft and the consequences for the strengthf its gear teeth part. Fig. 1 shows the JSS of the crankshaft and itsdvantage. Bernhard et al. (2007) registered a patent applying sitetructure for joining a shaft to a disk hub connection flange by FW.he shaft and the hub are joined by overlap FW in which, in con-rast to the end-face conventional FW, the surfaces to be joined arerranged at the circumference of the component. Two defined gapsrevent spread of the softening material during FW. The benefitf this method has been clearly recognized and experimentations underway to apply this method for other components, such asrive pinion.

Four different geometries for FW of a bevel to a hollow shaftnd reducing the component weight were tested and comparedy Mohammadzadeh Polami et al. (2012). In this investigation, the

imilar JSS shown in Fig. 1 was adapted for the truck drive pin-on. However, it was demonstrated that this joint cannot withstandhe applied stresses on the drive pinion gear part. Therefore, this

ethod needed to be optimized.

Fig. 2. Initial design of JSS FW for drive pinion

s Processing Technology 214 (2014) 1921–1927

Sahin (2005) discussed the most important parameters affect-ing the FW process, such as friction time, friction pressure, forgingtime, forging pressure and rotation speed. Sathiya et al. (2005)demonstrated that the burn-off length, defined as the differencein specimen length before and after friction welding, tends toincrease with increasing friction time. Sathiya et al. (2007) dis-cussed the optimum friction time and forging pressure at whichhigher strength of ferritic stainless steel joints was obtained. Theyalso concluded that the burn-off rate played an important role asregards the metallographic structure and mechanical properties ofthis joint. Bennett et al. (2011) described that the time to the tran-sition to gross plastic flow at inertia FW shortens as the frictionpressure increases. This is due to the higher friction pressure caus-ing an increase in heating rate around the interface region, whichinitiates plastic flow at lower temperature. In any case, the mate-rial needs to become pasty enough during the process. Accordingto DIN EN ISO 15620, a sufficient burn-off period is required forgeneration of heat to permit consolidation during forging. The fric-tion machine is usually adjusted to a specific burn-off length, untilwhich the friction continues. Nevertheless, information regardingthe JSS FW is limited only for few experiments indicating this expla-nation is insufficient. Therefore, in this study it is tried to find theappropriate friction pressure and time, for the material to becomepasty enough and get the required upset length.

This investigation aims to design the JSS for a bevel/shaft com-posite workpiece in order to reduce the drive pinion weight,minimize defects and maintain a compliant fatigue strength. Fromthe FEM simulation results demonstrating the stress requirementsfor the drive pinion, the optimal number of joints and locations,were obtained. For optimal joints, two different gap sizes wereanalyzed and compared. Micrograph analyses in conjunction withnon-destructive ultrasonic testing proved good weld quality of theoptimized design. HV10 hardness measured in the middle of theweld zone and its influence on the CCT diagrams of the joined mate-rials showed the thermal capacity during the test for the last twovariants.

2. Materials and methods

2.1. Materials

The bevel part was produced from a higher strength material,i.e. 18CrNiMo7-6, since, the gear teeth, machined later on this

(left side before FW, right side after FW).

S. Mohammadzadeh Polami et al. / Journal of Materials Processing Technology 214 (2014) 1921–1927 1923

Table 1Chemical composition of 18CrNiMo7-8 (1.6587, in accordance with DIN EN 10084), (wt%).

Element C Si Mn P S Cr Mo Ni Fe

Content 0.15–0.21 0.40 0.50–0.90 0.025 <0.035 1.50–1.80 0.25–0.35 1.40–1.70 Balance

Table 2Chemical composition of16MnCr5 (1.7131, in accordance with DIN EN 10084), (wt%).

Element C Si Mn P S Cr Mo Ni Fe

Content 0.14–0.19 0.40 1.00–1.30 0.025 <0.035 0.80–1.10 – – Balance

p1p

b

2

iAb

ipo(rnzc

afctspt

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Fig. 3. FEM simulation of initial design of JSS FW for drive pinion.

art, required higher strength. The hollow shaft was formed from6MnCr5 material. Case-hardened steel was used in each case. FWrovided the dissimilar bevel to the shaft joint.

The chemical compositions of the two materials used for theevel and the shaft are shown in Tables 1 and 2 respectively.

.2. Test procedure

The new JSS FW technique for the drive pinion component wasnitially designed to have two joints welded in a conical area (Fig. 2).s can be seen, the two weld zones did not fill the provided areaetween them, shown as non-fusion area.

When the drive pinion is fastened inside the rear axle of the truckn working condition, compressive principal stresses on the com-onent are applied. These applied stresses with the initial designf the JSS FW were described by Mohammadzadeh Polami et al.2012) using FEM simulation and are shown in Fig. 3. Initial resultsevealed that the front welded joint of the bevel was placed undero stress and the other weld zone is under high stress. The criticalone of the drive pinion that supported the applied stresses wasoncentrated at the edge of this weld zone.

For simplification purposes, the weld samples were scaled downnd the rotated bevel part was considered to have a cylindricalorm. These changes were essential to allow the experimental pro-edure with the required iteration steps in the laboratory. Initially,

he two single pieces of the sample – the cylindrical head and thehaft – were positioned in the FW machine. The cylindrical headart was clamped in the spindle chuck and the spindle was broughto a predetermined rotation speed. The shaft was clamped in a

able 3W parameters for variants A1, A2, B, C1 and C2.

Processing parameters Variant A1 Variant A2

Friction rotation speed (rpm) 1500 1500

Friction pressure (MPa) 167 184–202

Upsetting pressure (MPa) 219 219

Burn-off length (mm) 8 10

Friction time(s) 10 15

Gap size (mm) 1.5 1.5

Radial overlap (mm) 2 + 2 2 + 2

Fig. 4. JSS FW design before welding for two weld zones with a shaft diameter of65 mm.

fixture mounted to a hydraulically actuated tailstock slide and theaxial pressure (Table 3) was applied from this side (Lyman, 1983).

2.3. Design optimization

The objective of this investigation was to find a suitable jointwith a greater weld zone providing higher joint strength. To achievethis, the weld zone geometry was improved in different stages anddivided into three categories: Variant A – two weld zones, wheretwo different burn-off lengths were tried out; variant B – one weldzone; and variant C – radial relocation of the weld zone, for whichtwo different gap sizes were compared.

Fig. 4 shows the initial design before FW of two weld zones in theJSS for the hollow shaft and the cylindrical head part. Detail X is amagnification showing the 2 mm distance between the connectedshaft and the head part, called the radial overlap. The weld zoneafter FW is shown in Fig. 5(a) and (b). Here, two different burn-offlengths for A1 = 8 mm and A2 = 10 mm were specified.

According to the FEM-simulation (Fig. 3), only the one weld zoneof the JSS friction-welded drive pinion was under stress. There-fore, the weld zones can be limited to one zone where the stress isapplied. The empty spaces between the two weld zones critical forthe notch effect were avoided. The geometry design with 10 mmburn-off length and 3 mm radial overlap for type (B) is shown in

Fig. 5(c).

The FEM simulation showed that the highest stress for JSS FW isin the transition radius between the bearing support (on the shaft)and the gear (pinion part). Therefore, an attempt was made to move

Variant B Variant C1 Variant C2

1500 1500 1500216–260 300 185285 420 25010 14 1010 14.4 321.5 2 0.63 3 3

1924 S. Mohammadzadeh Polami et al. / Journal of Materials Processing Technology 214 (2014) 1921–1927

Fig. 5. Different variants of weld zone design for category A to C: (a) A1 two weldzones, with burn-off 8 mm (b) A2 with burn-off 10 mm, (c) B with one weld zone,(d) C1 one radially moved weld zone with a gap size of 2 mm and (e) C2 one radiallym

ttsad((

at

the fatigue strength of the component diminishes due to the notcheffect. In the improved design in accordance with the FEM simula-

oved weld zone with a gap size of 0.6 mm.

he weld joint in category B as far radially outwards as possible fromhe most stressed area. It can be seen in Fig. 5(d) and (e) that thehaft diameter in C has been increased from 65 mm in variant And B (Fig. 5(a–c)) to 77 mm (Fig. 5(d and e)). For this design, twoifferent new geometries varying in their gap sizes were comparedC1 and C2). For the design, the gap heights in C1 (2 mm) and C20.6 mm) are shown in Fig. 5(d) and (e).

The variants C1 and C2 provide a similar radial overlap of 3 mm

nd burn-off lengths of C1 = 14 mm and C2 = 10 mm. Table 3 showshe FW parameters for different weld zone design variants.

Fig. 6. Macrostructure of variant A1.

The design was further evaluated with the help of macrosectionsetched with nitric acid (HNO3) together with ultrasonic testing.A larger weld surface area was obtained in drive pinion design.Finally, FEM- simulation demonstrated stress concentration in thisnew geometry design.

3. Results and discussion

3.1. Two weld zones design and effect of burn-off length on weldzone length

Weld zone properties and length were obtained by macrostruc-tural examination. In the first step, the influence of the burn-offlength in the initial situation (two weld zones) was analyzed. Fig. 6shows the macrostructure of variant A1 (burn-off 8 mm), and themacrostructure of variant A2 (burn-off 10 mm) is shown in Fig. 7.

Varying the burn-off length requires different friction pressures.This effect was examined by the A1 variant (8 mm burn-off length);friction pressure was adjusted to 167 MPa. For A2 (10 mm burn-off),the friction pressure ranged between 184 and 202 MPa. Macro-graphs did not show any significant differences between A1 and A2.The macrographs showed that the weld zones of both variants werelonger on the right (approx. 20 mm) than on left (approx. 10 mm).According to the FEM simulation, the highest stresses are appliedin the corner where the bevel connects to the shaft, i.e. in this casein the weld zone on the left (10 mm). Due to the small size of theconnected area subjected to the applied stresses of the drive pin-ion component, the weld zone had to be improved. Increasing theburn-off length from 8 mm to 10 mm did not show any substantialdifferences between the connected area in variants A1 and A2.

3.2. One-weld zone design

The macrostructure of the one-weld zone design with 3 mmradial overlap is shown in Fig. 8. The macrostructural imagesshowed a larger weld zone (20 mm) in the most stressed area ofthe drive pinion component. The friction pressure for this variantstarted at 216 MPa and ended at 260 MPa. Joining through one weldzone avoided the non-fusion area between the two joints (Fig. 2).

3.3. Radial relocation of weld zone and gap size influence

Although joining the two parts through one zone produces alonger contact length in the critical area, the contact still introducesthe risk for the component failing to withstand the high stress fromthe bearings. Since, the flash was still located in the critical area;

tion, Fig. 5(d) and (e), the weld zone was radially moved outwardsfrom the highest-stressed area. As a result, the flash-generating

S. Mohammadzadeh Polami et al. / Journal of Materials Processing Technology 214 (2014) 1921–1927 1925

Fig. 7. Macrostructur

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size variant) in both CCT diagrams (presented by Reese (2013))

Fig. 8. Macrostructure of variant B with one weld zone.

otch effect was placed under less stress. Besides reducing themount of stress in the weld area, the optimized design createsuitable heat distribution between the head and the shaft. Clearly,n the previous variants A and B, the shaft in the weld zone was thin-er than the head (Fig. 5). Therefore, the shaft at the end weld zoneas formed without a suitable joint to the head. In variant C, the

hicker hollow shaft with more material was connected to the headart, resulting in better heat distribution compared to the previousariants with less material in the shaft. For the radial displacementf the weld zone variant, two different gap sizes were consideredFig. 5(d) and (e)). The macrostructures of these variants C1 and C2re shown in Fig. 9.

As can be seen in Fig. 9, variant C1 resulted in a slightlyarger weld zone (25 mm) compared to variants A and B (approx.0 mm). The experiment revealed that the smaller gap size leadso an obviously larger contacted area. In variant C1 with 2 mmap size, <26 mm weld length was obtained. However, variant C2ith 0.6 mm gap size was joined with almost double weld length

approx. 42 mm) compared to the size of the previous variantC1). The reason for this is that during the FW process, the plas-

icized material flows into the small gap and fills it, generating aide weld zone. Bernhard et al. (2007) explained that during FW

he softened displaced material from the joint region fills the gap

Fig. 9. Macrostructure of variants C1 (gap si

e of variant A2.

and prevents the softened materials from spreading. The displacedmaterial solidifies as a result of the lower ambient temperature; itobtains a pasty consistency in the process and prevents the spreadof the material subsequently being displaced due to its higherviscosity. This constitutes a fundamental difference from the con-ventional overlap FW in which the pasty bead material can readilyescape from the joint site.

3.4. Weld quality of optimized weld zone designs

Varying the gap size affected the corresponding friction time inthe test. The smaller gap in C2 compared to that of C1 almost dou-bled the time required to join the two parts (together). The frictiontime for C1 with 14 mm burn-off length took 14 s and for C2 with10 mm burn-off length 32 s (Table 3). Apparently in C2, longer timewas required for its friction phase despite of the smaller burn-offlength (10 mm). It can be noted that in C2, with the longer frictiontime, more heat is induced during the friction phase.

Fig. 9 shows the existence of different microstructural zones,particularly the weld and the heat-affected zone. Furthermore,hardness measurements reveal the heat distribution arising fromheating and plastic deformation. Significant correlation betweenthe hardness in two different joint geometries with different gapsizes is shown. Due to the small gap in variant C2, the friction timerequired for C2 was longer than C1 (Table 3). Thus, more heat isintroduced which heats up a larger area. By superimposing thehardness results of the two joints on the CCT diagrams for eachmaterial, the heat treatment between the two variants was com-pared. The hardness of the two variants C1 and C2 is plotted in thediagrams in Fig. 10. C1 reveals maximum 470 HV and C2 maximum330 HV in the weld zone. It follows that the weld with the biggergap size (C1) is harder than the weld with a smaller gap.

The weld quality was investigated by ultrasonic testing. Theresulting graph shows the existence of cracks or other non-fusionareas within the weld zone.

The CCT diagram of the two applied materials (18CrNiMo7-6and 16MnCr5) reveals the phase transformation during the coolingprocess for the related hardnesses. For example, the phase trans-formation for 470 HV (i.e. maximum hardness of the bigger gap

shows the formation of the martensite. However, with the smallergap size and maximum hardness of 330 HV, both materials showminimal martensite transformation. Clearly, a more desirable weld

ze = 2 mm) and C2 (gap size = 0.6 mm).

1926 S. Mohammadzadeh Polami et al. / Journal of Materials Processing Technology 214 (2014) 1921–1927

qc

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Fig. 12. FEM simulation of optimized JSS FW for drive pinion.

Fig. 10. Hardness variation diagram in the weld zone of variants C1 and C2.

uality was obtained in variant C2 with the smaller gap, whererack initiation was hindered due to the bainite structure.

As can be seen in Fig. 11, the green line 2 shows the partiallyoined area and the blue line (region 1) defines the fusion part.ariant C2 with the larger weld zone results in more blue areas

region 1) detected (48%) compared to C1 (24%). The green line 2,llustrating partially fused areas or crack initiation, is considerably

ore pronounced in C1 (7%) than in C2 (5%). The red area (region 3),emonstrating a non-fusion zone of the weld, is noticeably larger

n C1 (69%) than in C2 (46%). For the larger weld zone (C2), a more

esirable weld quality was proved.

ig. 11. Ultrasonic echo result of non-destructive weld qualification in C1 and C2.

Fig. 13. Schematically removing flash after FW of drive pinion.

3.5. Light weighting and FEM simulation of optimized design

An optimal weld design was obtained with a gap size limited to0.6 mm. The weight of the new component is approx. 2 kg less thanthe total weight (14.8 kg) of the series production part.

JSS FW was simulated on the one-weld optimized new design.The original dimension of the drive pinion was assessed and theapplied stresses were considered too. Fig. 12 shows the calcu-lation result for this design. As illustrated, a suitable weld zonepreventing the concentration of maximum stress in the weld zonewas obtained. As highlighted, 509 MPa compression stress wouldbe endured by the new weld zone. Referring to Fig. 3, 1305 MPacompression stress was calculated on the previous JSS zone of thecomponent. Therefore, the new design would endure less stress.

3.6. Finish-treatment of weld upset

By considering the low gap height, the softened material is pre-vented from escaping in the form of a flash. Macrographs images(Fig. 9) show the existence of cracks or non-fused zones at the endof the joint. Gage and Mich (1972) introduced different methodsto remove this flash, because of its stress concentration and con-sequently the reduction in component fatigue strength. Machiningoff the spline end of the weld, including removing the weld flashand reducing the area to a smaller length, improved the fatiguestrength (Mohammadzadeh Polami et al., 2013). Clearly, the exist-ence of such an upset facilitates crack propagation due to the notcheffect. Fig. 13 shows schematically removing the weld flash.

4. Conclusions

For light weighting of drive pinion down to 2 kg (the pro-duction part weighs 14.8 kg), different optimization steps usingJSS FW were followed for this component. The new drive pinion

terial

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S. Mohammadzadeh Polami et al. / Journal of Ma

as envisaged to endure the applied component stresses. Theptimization processes are listed as:

Initial situation with two weld zones was tried out. The resultingweld lengths were too small to withstand the applied componentstresses.One weld zone was considered avoiding the non-fusion areabetween the two joints.The weld zone was relocated radially (approx. 6 mm) to preventcreation of the weld zone in the most stressed area. The gap sizewas varied for the relocated weld zone. A micrograph showed thelargest weld zone, for which better weld quality was proved usingthe ultrasonic test. The influence of weld hardness determinedthe phase transformation of the materials. It turned out that aone-weld zone, moved away from the most stressed area andhaving a gap size of 0.6 mm, exhibits the longest weld zone withmore proper quality.FEM simulation of the optimal variant showed less stress distri-bution in the weld zone. Due to the stress concentration, the weldflash had to be removed.

cknowledgments

This work was supported by the Trucks Technology Manage-ent Department of Daimler AG. The authors wish to express their

incere thanks to Mr. Alexander Berndt for useful technical discuss-ons during the course of this project.

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