Relationships between weld parameters, hardness distribution and temperature history in alloy 7050...

Relationships between weld parametershardness distribution and temperature historyin alloy 7050 friction stir welds

A P Reynolds1 W Tang1 Z Khandkar1 J A Khan1 and K Lindner12

Aluminium alloy 7050 was friction stir welded using three different ratios of tool rotation rate to

weld travel speed Welds were made using travel speeds of between 085 and 51 mm sndash1 Weld

power and torque were recorded for each weld An FEM simulation was used to calculate the

timendashtemperature history for a subset of the welds For each weld the hardness distribution with

and without post-weld heat treatment was determined The hardness distributions within the

welds are rationalised based on the friction stir welding parameters and the resulting temperature

histories The analysis provides a basis for manipulation of weld parameters to achieve desired

properties

Keywords Friction stir welding 7xxx series alloys HAZ

IntroductionSince its invention at The Welding Institute1 the frictionstir welding (FSW) process has proven to be a superiortechnique for joining of a variety of aluminium alloys Inparticular for the precipitation hardenable 2xxx and7xxx alloys which may be quite difficult to weld viafusion techniques it is of great utility2 While copiousdata have been generated on the properties of FSWs inmany alloys few systematic studies of weld parametereffects on properties in 7xxx alloys have been producedmost published work has been performed on singlewelds

In order to correlate weld hardness with weldparameters it is necessary to understand the range ofmetallurgical reactions possible for the alloy in questionThere is a substantial body of literature regarding theaging of 7050 (eg Ref 3) however much of it dealswith industrial aging practice and overaging of the alloyat temperatures associated with industrial aging prac-tice The timescales for precipitation aging are generallymuch longer than the transient heat treatments pro-duced by welding processes (eg thousands of secondscompared to tens of seconds) Also the temperaturesused for precipitation aging are substantially lower thanthose encountered in the heat affected zone (HAZ)hardness minima and the weld nuggets of the weldsunder consideration here More data are available foralloy 7075 than for 7050 Fortuitously the kinetics andthermodynamics of precipitation in 7075 and 7050 aresimilar even thought the absolute values of hardnessattained in the two alloys may differ3 In general the

strengthening precipitates precipitationdissolutionsequences are similar in many 7xxx alloys and it is wellestablished that the primary strengthening precipitate in7050-T7451 alloy is the coherent g9 phase4 Examinationof the literature reveals the following regarding pre-cipitate stability in the 7xxx series alloys56

(i) dissolution of the strengthening g9 phase occursat T190uC

(ii) the incoherent g phase precipitates betweenapproximately 215 and 250uC This phasecontributes much less to strengthening thandoes g9 Near 250uC g begins to coarsen rapidly

(iii) g phase begins to dissolve at T320uC(iv) there is a maximum in the formation rate of the

high temperature non-strengthening incoher-ent M-phase at approximately 350uC Hencethe solute will be most rapidly depleted from thematrix at this temperature

Further there is pertinent information regarding thethermal conditions associated with HAZ formation inwelding of 7075 Mahoney et al7 found the minimumHAZ hardness in a 7075 friction stir weld in a regionwhere the maximum temperatures were in the range of300ndash350uC Hwang and Chou8 performed weld simula-tion of alloy 7075 and found that the minimum strengthresulted from a weld thermal cycle with a peak tem-perature of 377uC This was not necessarily the tem-perature which would result in the absolute minimumhardness as a continuum of peak temperatures was notexamined (adjacent temperatures were 288 and 445uC)Hwang and Chou ascribed the low strength at 377uC torapid formation of coarse g Temperatures above 377uCwere considered partial solution treatments with sub-sequent natural aging leading to higher strength whilethose below 377uC resulted in less dissolution ofg9 and hence higher strength Based on the work ofArchambault and Godard6 it seems likely that the

1USC Dept of Mechanical Engineering 300 Main Street Room A224Columbia SC 29208 USA2University of Karlsruhe Karlsruhe Germany

Corresponding author email aprscedu

2005 Institute of Materials Minerals and MiningPublished by Maney on behalf of the InstituteReceived 9 December 2003 accepted 9 April 2004DOI 101179174329305X37024 Science and Technology of Welding and Joining 2005 VOL 10 NO 2 190

minimum hardness at a peak temperature of 377uCmight also be ascribed to rapid formation of the Mphase and concomitant solute depletion Regardlessbased on Mahoney et al and Hwang and Chou forwelding of 7075 peak temperatures near 350uC appearto be most effective in reducing the strength or hardnessin the HAZ when HAZ temperatures gt350uC arepresent It is important to keep in mind that it ispossible to produce friction stir welds in which the peaktemperature never reaches 350uC

The literature also contains several references tomicrostructure property correlation in 7xxx FSWs Oneof the earliest reports of microstructure in a 7xxx FSWwas made by Rhodes et al in 19979 In that paper theauthors investigated the microstructure resulting fromFSW of 635 mm thick 7075 plate The weld was made ata speed of 211 mm sndash1 Tool rotation speed and designwere proprietary Based on TEM determination of themetallurgical condition of the weld nugget region (theportion of the thermo-mechanically affected zone com-prised of fine equiaxed grains) the authors concludedthat the nugget temperature during FSW was between450 and 480uC In a subsequent paper Mahoney et al7

reported additional data on a similar friction stir weld(also made at 211 mm sndash1) in 635 mm thick 7075 plateTimendashtemperature histories for the nugget andHAZwerereported as were hardness and strength both in the aswelded condition and after a post-weld aging heattreatment A positive response to the post-weld heattreatment (PWHT) was observed in the nugget that is itwas strengthened by the PWHT As for most precipita-tion hardening alloys friction stir welded in a T6 or T7condition the minimum hardness was found in the HAZBased on tensile test results it is apparent that the post-weld aging resulted in reduced strength in the HAZ Thepeak temperature in the weld nugget was reported to beapproximately 475uC while that in the HAZ where thehardness was minimum was between 300 and 350uC

Sato et al10 have produced a relatively comprehensiveexamination of the effects of tool rotation speed at aconstant welding speed on the hardness microstructureand response to PWHT in alloy 6063 friction stir weldsIn the present study welds were made at a constantwelding speed of 6 mm sndash1 and tool rotation ratesbetween 800 and 3600 rev minndash1 Thermocouple mea-surements made beneath the welding tool at the weldcentreline showed that the peak temperature increasedwith increasing spindle speed Also the time of thetemperature transient was governed by the weldingspeed ie it was the same for all of the welds examinedThe authors determined that there was an Arrheniusrelationship between the nugget grain size and themaximum temperature The authors proposed that thisrelationship resulted from grain growth in the weldregion after the deformation associated with the FSWprocess was complete In addition it was observed thatthe best response to the PWHT was found for thosewelds which had achieved the highest temperature Theauthors surmised that the observed response was as aresult of differences in precipitate free zone volumeassociated with the different weld microstructures Itseems possible that the response to PWHT may alsohave been influenced by differences in solute andvacancy concentrations stemming from the differentthermal histories of the welds

In another study by Rhodes et al11 lsquoquenchedplungersquo simulations of the early stages of FSW wereperformed The goal of these studies was to determinethe microstructural state of material being friction stirwelded before the end of deformation The authorsfound grain sizes on the order of tens of nanometres andthey determined that increasing tool rotation rate leadsto increased grain size Post-weld heat treatments of thelsquoquenched plungersquo specimens resulted in grain sizestypical of most conventional friction stir weld nuggets2ndash5 mm The authors concluded that the mechanism ofgrain formation in the FSW nugget was as described bySato et al10 The observation connecting grain size torotation speed implies higher temperature is associatedwith greater rotation speed

Jata et al4 studied a 64 mm thick 7050-T7451 FSWproduced at a welding speed of 17 mm sndash1 and a toolrotation rate of 396 rev minndash1 They surmised fromTEM results that the temperature in the weld nugget wassufficient for solution heat treatment (450uC) APWHT of 24 h at 121uC resulted in increased nuggethardness as a result of reprecipitation of strengtheningphases and reduced hardness in the heat affected zone

Su et al12 have performed a substantial TEM study ofthe microstructure in a 7050-T651 friction stir weld(64 mm thick) The reported welding speed was15 mm minndash1 (which seems quite low) The tool rotationrate was 350 rev minndash1 The authors observed a nuggetgrain size varying between 1 and 4 mm and proposedthat the grain formation mechanism in the weld nuggetis continuous dynamic recrystallisation Observation ofvariable dislocation densities in nugget grains was thebasis of their conclusion regarding the operativerecrystallisation mechanism (although all dislocationdensity measurements were qualitative) They alsoobserved that the nugget was solution heat treatedduring the welding process and that the precipitatedistributions in the HAZ were coarsened

Yang et al13 have used the FSW process thermalsimulation described in a subsequent section1415 to aidin explanation of hardness distributions in 2024 and2524 FSWs They found that higher weld temperaturescorresponded to greater nugget hardness presumablybecause of in-process solution heat treatment followedby natural aging of the nugget material

In another recent paper Hassan et al16 havepublished a study of the thermal stability of nuggetgrain structures in friction stir welds on 64 mm thickalloy 7010 They observed that welds made with lowpower resulted in fine nugget grain sizes relative to weldsmade at higher power and that at low power themaximum weld temperatures could be substantiallybelow the solvus temperature for the alloy Post-weldsolution heat treatment showed that the very fine grainstructures were unstable and abnormal grain growth wasobserved during the heat treatments These observationsare in accord with the effects of peak weldingtemperature on grain size that have been discussedpreviously10

Taken together the results of the studies discussedabove particularly Refs 10 13 and 16 indicate that themetallurgical condition of the weld nugget and the heataffected zone may be significantly influenced by thewelding conditions over and above the occurrence ofwelding defects One of the most interesting aspects of

Reynolds et al Alloy 7050 friction stir welds

Science and Technology of Welding and Joining 2005 VOL 10 NO 2 191

FSW relative to fusion welding techniques is that themaximum temperature in the solid metal during fusionwelding is found at the solidliquid boundary and is themelting temperature This is not the case in FSWDepending on the conditions of welding a relativelylarge range of maximum temperatures can be producedCoupling this with the ability to manipulate the heat-upand cool-down times (which are controlled primarily bythe welding speed) it should be possible to accomplish avery wide range of property modification during FSWIt should be noted that the weld power heating andcooling rates are not capable of completely independentmanipulation Given the potential importance of theability to systematically vary weld properties by chan-ging welding parameters it is surprising that there is stillvery little data in the literature that systematically linksFSWrsquos key controlled variables (tool rotation speedwelding speed tool geometry and z-axis force) with theimportant process responses eg peak temperaturemetallurgical structure hardness and weld power

In the present study a broad range of weldingconditions has been used to friction stir weld 64 mmthick 7050-T7451 plate The goal of the work presentedhere is to improve the understanding of the relationshipsbetween FSW parameters and weld hardness bysystematic variation of some of those parameters(welding speed and tool rotation rate) while holdingothers constant

Experimental procedures

Friction stir weldingA series of welds was made in 64 mm thick 7050-T7451plate Welds were made at speeds between 086 and51 mm sndash1 using three different ratios of welding speedto tool rotation rate (weld pitch) 056 042 and028 mm revndash1 All of the weld parameters are shownin Table 1 All welds were performed under z-axis forcecontrol the z-axis force was adjusted for the differentwelding speeds and tool rotation rates so as to producegood quality welds A typical FSW tool having athreaded cylindrical pin and a dished shoulder was usedfor all welding The shoulder diameter was 203 mm thepin diameter was 71 mm the pin length was 61 mm

and the thread pitch on the pin was 085 mm threadndash1 Alead angle of 25u was used for all welds During thewelding process the torque supplied to the spindlemotor is monitored continuously After application ofcertain efficiency factors (supplied by the systemmanufacturer) the measured torque is used to calculatethe input weld power

Microhardness testingThe hardness of all welds was measured along the platemidplane on the weld transverse cross-sections as afunction of position relative to the weld centreline AVickers indenter a load of 100 g and a load applicationtime of 15 s were used for all hardness testing Thehardness HV(100 g) was measured in the as weldedcondition and after a post-weld heat treatment of 4 h at120uC In order to minimise the effects of natural agingall specimens were stored at 218uC before hardnesstesting in the as welded condition and the time betweenweld production and hardness measurement was main-tained nearly constant at approximately 2 weeks

Numerical simulation of temperaturehistoryOne barrier to the correlation of FSW timendashtemperaturedata to weld properties and process parameters is thereal difficulty of obtaining high quality weld temperaturedata during the FSW process For example accurateplacement of thermocouples relative to the weld positionis a tedious and difficult task of great importance forobtaining quality temperature data The very hightemperature gradients produced during FSW near theweld tool dictate that small uncertainties in the positionof a thermocouple could lead to large errors in measuredtemperature In addition thermocouples near the toolduring FSW are often moved during the process becauseof the material flow that accompanies the process Thiscan result in either failure of the thermocouple or at theleast uncertainty regarding the location of the tempera-ture measurement In the present study the difficultywith thermocouple based temperature measurement iscircumvented by the use of a numerical analysis whichtakes as input the weld power (measured during

Table 1 Friction stir welding process parameters

Spindle rotation rate rev minndash1 Welding speed mm sndash1 z-axis load kN

90 085 289135 127 30180 17 278270 254 378315 296 378405 381 456120 085 245180 127 245240 17 245360 254 30540 381 41720 51 378180 085 20270 127 22360 17 24540 254 335630 296 36810 381 39900 42 365



welding) the tool geometry and boundary conditionsthat have been determined by fitting to measuredthermocouple data15 This numerical model which willbe described in some detail below can be used to easilyobtain the timendashtemperature histories of friction stirwelds

Details of the input torque based thermal simulationand its validation against experimental work have beenpublished previously1415 but its structure will be brieflydescribed here The energy balance equation of the FSWis described as

+(k+T)zqrc

T~0 (1)

where T is the temperature T is the rate of change oftemperature c is the specific heat r is the density k isthe thermal conductivity and q is the rate of moving heatgeneration per unit volume The heat input is correlatedto the experimentally measured torque data by assuminga constant average interfacial shear strength at theworkpiecetool interface The heat flux is attributed tothe tool interfaces in contact with the workpiece on thebasis of the torques involved

Qinterface~

Qtot|

Torquegeneratedbytheinterfacialarea

Totaltorque(2)

The interfaces of interest are the shoulderworkpieceinterface vertical pin surfaceworkpiece interface andthe pin bottomworkpiece interface Total torque at theshoulder interface can be expressed as

Mshoulder~

ethro

ri

(tr)(2pr)dr (3)

Torque at the pin bottom is given by

MPinBottom~

ethri

0

(tr)(2pr)dr (4)

Torque at the vertical pin surface is given by

MPinSurface~(tri)2prih (5)

where r is the radial distance ri is the pin radius ro is theshoulder radius h is the pin length and t is the assumedaverage interfacial shear stress This shear stress may beconsidered either the weld material shear flow stress

(for sticking conditions) or some shear stress owing tofriction when there is slip between the tool and theworkpiece For purposes of using the model it is notnecessary to know the actual conditions at the toolworkpiece interface as the actual measured torque isused for the calculations The total torque which is thesum of the three torque components is related to theaverage power (Pav) input and hence the total heat input(Qtot) by

Mtotv~Pav~Qtot (6)

For each of the welding conditions simulated the valueof the input torque was continuously measured duringexperimental investigations and was multiplied by therotational velocity to arrive at the experimental powerinput Boundary conditions at the workpiece backsurface and top surface were determined previously byfitting to experimentally determined temperature dataSo in essence the input torque based model uses amoving heat source with the same geometry as the actualwelding tool The heat flux over the surface of the tool isrelated to the actual weld torque and is distributed overthe tool surface based on the moment produced by theshearing stress on a given differential area of the toolassuming constant interface shear strength The outputof the model is the temperature history of the entiremodelled workpiece The primary assumptions asso-ciated with the model include the shape of the heatsource the neglect of tool traversing work and that allof the spindle work is transformed to heat in theworkpiece As will be shown subsequently this timendashtemperature history can be used to rationalise observeddifferences in weld hardness distributions and micro-structure that result from changes in the weld power andthe welding speed

Results

Weld process responseThe weld power for each of the welds made in thepresent study is graphed in Fig 1 as a function ofwelding speed In Fig 2 the specific weld energy (weldpower divided by welding speed) is graphed as afunction of welding speed As can be seen in Fig 2

1 Weld power plotted against welding speed for 7050

FSWs made using three different weld pitches2 Specific weld energy plotted against welding speed for

7050 FSWs made using three different weld pitches



the specific weld energy declines with increasing weldingspeed and is most strongly dependent on weld pitch atlow welding speed That is at high welding speed allthree weld pitches produce approximately the samespecific weld energy although higher rotation speed at agiven welding speed generally correlates to somewhatincreased weld power and energy At the lowest speedthe weld energy (and power) is substantially higher forthe weld made with high rotation speed (low weld pitch)compared to the welds made with lower rotation speed

These data are viewed in another way in Fig 3 whichis a plot of the weld power as a function of the rotationspeed Like symbols are indicative of welds made at thesame welding speed The data indicate that there isgeneral trend toward higher weld power with higherrotation speed although this trend is stronger at lowerrotation speed than at higher speeds The data alsoindicate that for a given rotation speed a higher weldingspeed requires greater power This point is illustratedmost clearly for the three data points at 180 rev minndash1the two points at 540 rev minndash1 and for those data near400 rev minndash1 These data may also be interpreted asmeaning that the required torque for a given rotationspeed increases with the tool advance per revolution orweld pitch sensible because higher advance per revolu-tion requires deformation and transport of a greatervolume of material per revolution

Weld hardnessMost of the welds created for the present study exhibitthe hardness distribution that is characteristic of manyfriction stir welds in precipitation hardening aluminiumalloys that is the base metal exhibits the highesthardness and the hardness of the nugget is a localmaximum placed between minima in the advancing andretreating side HAZs Only the weld with the lowestweld power made at 082 mm sndash1 at a weld pitch of056 mm revndash1 showed no local maximum in the weldnugget that is the weld nugget exhibited the minimumhardness

The values of the nugget and HAZ hardness wereaffected by both the welding speed and by the weld pitchfor a given welding speed In Fig 4 hardness traversesare shown for welds made at 085 and 38 mm sndash1 at aweld pitch of 042 mm revndash1 The nugget hardness andHAZ hardness are both substantially higher for thefaster weld

In Fig 5 the average nugget hardness for each of thewelds is shown The average hardness is determined bythe arithmetic average of all of the hardness measure-ments inside the fine grain region of the TMAZ (thenugget) Figure 5 reveals some interesting trends Foreach weld pitch the nugget hardness rises with increas-ing welding speed up to some maximum at which pointit levels off The plateau hardness value is achieved at alower welding speed for the lowest weld pitch At everywelding speed the nugget hardness is greatest for theweld made at the lowest pitch the ranking of hardnessmay also be related to the weld power or rotation speedthrough reference to Figs 1 and 3 In Fig 1 it can beseen that in most cases the lowest weld pitch corre-sponds to the highest weld power (and rotation speedsee Fig 3) for a given welding speed Therefore therelationship between hardness and weld pitch might beattributable to weld power or rotation speed It may alsobe seen that the effect of weld pitch on the nuggethardness for a given welding speed is greatest at thelower welding speeds also corresponding to the effect ofrotation speed on weld power and weld specific energy(Figs 1 and 2)

In Fig 6 the effect of the post-weld heat treatment onnugget hardness is shown The data are plotted as thechange in the average Vickers hardness of the weldnugget (DVHN) as a function of welding speed for each

3 Weld power as a function of weld rotation speed data

are organised by welding speed

4 Representative hardness traverses through welds made

at 085 and 38 mm sndash1 both welds were made with an

advance per revolution of 042 mm revndash1

5 Effect of weld pitch and welding speed on average

nugget hardness in as welded condition



of the weld pitches For each weld pitch there is someminimum welding speed above which the weld nuggetexhibits a positive response to the PWHT The minimumrequired welding speed for a positive response to thePWHT is an inverse function of the tool advance perrevolution That is for a given welding speed a weldmade with a higher rotation speed exhibits greaterhardening in the nugget because of the PWHT thisagain corresponds to higher weld power for a givenwelding speed

While nugget hardness is interesting in many casesthe performance of the welded structure will be governedby the minimum hardness of the weld region As for thecase shown in Fig 4 the minimum hardness of a frictionstir weld in a precipitation hardening aluminium alloy istypically found in the HAZ The effect of welding speedand weld pitch on the minimum weld hardness (found inthe HAZ in every case but the one mentioned above) isshown in Fig 7 The minimum hardness data areaverages of advancing and retreating side minimumvalues however the depths of the hardness minima weregenerally symmetric with respect to the weld centrelineThe data show a definite trend for higher minimumhardness with increasing welding speed The trendsrelated to weld pitch are somewhat less clear cut (as

compared to the nugget hardness) At the highest andlowest welding speeds the welds made with the highestpower exhibit the highest minimum hardness Atintermediate speeds there is no particular order

In Fig 8 the minimum hardness after PWHT isplotted against the welding speed for each weld pitchThree effects of the PWHT may be noted (1) the spreadin the data is reduced by the PWHT as evidenced by thereduction in the space between the 99 confidenceinterval lines (2) the effect of welding speed onminimum hardness is reduced by the PWHT (the slopeof the regression line is lower in Fig 8 than in Fig 7)and (3) for every weld the HAZ shows a negativeresponse to the PWHT

Timendashtemperature simulationsSome knowledge of the timendashtemperature history of thewelds is needed in order to rationally connect the weldhardness data to the welding parameters In this sectionselected histories are presented and related to the variouswelding parameters Figure 9a is a plot of fourrepresentative simulated timendashtemperature historiesfor welds made at different speeds and an advance perrevolution of 028 mm sndash1 The temperature at themidplane centreline of each weld is plotted against thetime relative to the time at which the peak temperaturewas attained The length (time) of each temperaturetransient is inversely related to the welding speedTherefore increasing welding speed increases the heat-up and quench rates this is true for welds made at anyweld pitch For the weld pitch shown in Fig 9a thehighest maximum temperature is attained in the fastestweld and the lowest in the slowest weld The two weldsmade at intermediate speeds have nearly the same peaktemperatures but the weld made at 17 mm sndash1 and360 rev minndash1 has a very slightly higher peak T than thatmade at 254 mm sndash1 and 540 rev minndash1 From the datashown in Fig 9a it is apparent that the peaktemperature will be a function of both the weldingspeed and the rotation speed Examination of all of thesimulated timendashtemperature histories indicates that theheat-up and cool-down (or quench) rates are dependentalmost exclusively on the welding speed These twopoints are further illustrated in Fig 9b which is a plot ofthe timendashtemperature histories of three welds made at

6 Effect of welding speed and weld pitch on nugget

response to PWHT

7 Average minimum hardness for all welds in as welded

condition

8 Average minimum hardness for all welds after PWHT



three different welding speeds but the same rotationspeed 180 rev minndash1 In the figure it can be seen thatthe two welds made at 127 and 17 mm sndash1 have nearlythe same peak temperatures whereas the slower weld(085 mm sndash1) has a substantially lower peak T Theheating and quench rates are proportional to thewelding speeds Examination of Fig 3 shows thatthe two faster welds require similar power whereas theslower weld requires substantially less power

Figure 10a shows the peak nugget temperatureplotted as a function of weld power with the datagrouped according to weld pitch Overall the peaktemperature may be approximated as a monotonicallyrising function with a direct relationship to weld powerHowever for each weld pitch there is a plateau (or inthe case of the lowest weld pitch a dip) in thetemperature as a function of weld power This plateauoccurs at intermediate welding speeds (17 and254 mm sndash1) and amplifies the effect observed inFig 9 the peak temperature depends on the rotationspeed and the welding speed Of course as shownpreviously (Figs 1 and 3) the weld power is a functionof the welding speed and also the rotation speedFigure 10b is a plot of the peak T versus the specificweld energy The relationship shown in Fig 10b iscounterintuitive in that a lower peak T is generallyassociated with higher weld energy An analogy for theresults shown in Fig 10a and b would be the differencebetween a metal plate left outdoors in the sun for an

extended period of time (low powerhigh energy)compared to one on which a high power laser wasbriefly focused The plate in the sun might absorb moreenergy yet achieve a much lower peak temperaturecompared to the plate exposed to the laser Anotherfactor that must be borne in mind is that neither powernor weld energy are controlled variables they areprocess responses and are not subject to independentmanipulation This may be compared to the situationduring fusion welding in which weld energy and powermay be controlled by for instance manipulation of arccurrent and voltage

As mentioned in the section lsquoWeld hardnessrsquo abovethe critical hardness is in many cases not the nuggethardness but the minimum hardness Hence the timendashtemperature history at the position of the hardnessminimum may be as or more important than the peakweld temperature observed in the nugget Comparisonbetween the temperature histories for the positions ofthe HAZ hardness minima and the corresponding weldnuggets shows that the length of the temperaturetransient is similar and dependent on the welding speedso the heating and cooling times are similar In Fig 11the centreline peak temperatures and the peak tempera-tures at the positions of the corresponding HAZhardness minima are plotted against the welding speedwith the data grouped according to weld pitch In everycase except the weld made at the lowest welding speedwith the highest pitch (for which the minimum hardnessposition and the nugget coincide) the peak temperature

9 a Timendashtemperature histories for welds made at a

pitch of 028 mm revndash1 b timendashtemperature histories

for three welds made using a tool rotation rate of

180 rev minndash1

10 a Peak weld nugget temperature as a function of

weld power b peak weld temperature as a function

of specific weld energy



in the HAZ at the location of minimum hardness is lessthan that at the centreline The data in Fig 11 alsoindicate that the difference in peak temperature betweenthe weld centreline and the HAZ minimum hardnessposition increases with increasing welding speed

DiscussionIn the preceding section the primary emphasis wasplaced on illumination of the relationships betweencontrolled friction stir welding process parameters andvarious process responses For example the effects ofwelding speed and rotation speed on nugget and HAZhardness were presented and the weld power (actuallya process response variable) was used as an input toa simulation tool to provide information on timendashtemperature histories in the various weld zones In thissection an attempt will be made to cross-correlate theobserved process responses the welding parameters andthe precipitation behaviour of 7050 in order to shed lighton the development of the hardness profiles observed inthese 7050 friction stir welds

In Fig 12 the nugget hardness data are plottedagainst the peak temperatures at the midplane centre-lines of the corresponding welds The data are groupedby welding speed that is data for the same weldingspeed have the same type of symbol Hardness datawithout accompanying simulated temperature data areexcluded Grouping of the data by welding speedenables the influence of time at temperature to be morereadily included in the data analysis the higher thewelding speed the more rapid the rise and fall of thetemperature during the welding process A linear leastsquares fit line has been drawn through all of the datashowing a nominally linear relationship between nuggethardness and peak temperature in the nugget Howeverthe correlation coefficient for this line is only 075 If thethree data points within the circle are omitted from thefit the correlation coefficient for the line improves to092 This will be discussed in detail subsequently

Based on the precipitation behaviour of 7050described in the lsquoIntroductionrsquo it can be stated that allof the nugget temperatures were sufficient to causedissolution of the primary strengthening phase g9 (T

190uC) Excluding from discussion for the moment the

data points enclosed by the circle in Fig 12 thefollowing statements can be made

1 The nugget experiencing the lowest peak tempera-ture should be expected to exhibit substantial precipita-tion of the g phase The peak T for this weld is in therange of g-phase formation and because of the lowwelding speed the time available for precipitation wouldbe relatively large

2 The nugget with the next highest peak T (also atthe lowest welding speed) achieves a temperature atwhich some re-dissolution of the precipitated g-phasemight be expected During cooling and subsequent(short) time at room temperature there could besufficient solute available for solution strengthening orformation of GP zones hence higher hardness thanobserved in the weld with the lowest peak temperature

3 With increasing peak temperature (again exclud-ing the data points inside the circle) greater levels ofg-phase dissolution should be expected and hencegreater solute will be retained in the nugget at the endof welding resulting in greater hardness

Now consider the data points within the circle Thepotential metallurgical reactions and resulting hardnesslevels in these welds must be considered in the light ofthe high temperature aging study of Archambault andGodard6 Based on that study the peak temperatures inthese welds correspond to the temperature at which therate of formation of the M phase in 7XXX alloys ismaximum The formation of M phase will not provideany strengthening increment it will only serve to depletesolute from the matrix hence reducing the post-weldhardness Based solely on peak temperature similarlevels of solute should be expected for the encircled dataas for the other points with similar peak temperatures(those data points directly above the encircled points)hence similar hardness levels However the encircleddata points are for welds made at relatively low weldingspeeds therefore the weld nuggets will have spent thegreatest amount of time at temperatures near 350uCallowing the greatest amount of M phase formationhence reduced solute and hardness levels

Figure 13 shows the hardness in the HAZ minimaplotted against the peak temperature in the minimumhardness location As in Fig 12 the data are organisedby welding speed Given the forgoing discussion of

12 Nugget hardness as a function of peak temperature

in weld nugget data are grouped according to weld-

ing speed

11 Peak weld temperature on midplane centreline and at

HAZ minimum hardness position plotted against

welding speed data are grouped according to weld

pitch



Fig 12 Fig 13 may be readily explained For everyweld except one for which an HAZ temperature near350uC is possible the HAZ hardness minimum occurs ina region where the peak temperature was in the range of330ndash370uC (HAZ temperatures near 350uC are possibleonly when the nugget peak temperature exceeds thatvalue) For the welds with HAZ hardness minimacoinciding with a peak temperature ranging between330 and 370uC the HAZ hardness correlates closelywith the welding speed Higher speeds lead to higherhardness This correlation is presumably because of theamount of time available for formation of the M phaseand concomitant solute depletion This observation isalso consistent with the findings of Mahoney et al andHwang and Chou (hardness minima in regions withpeak temperatures near 350uC) The maximum in therate of M phase formation near 350uC explains whyHAZ hardness minima are not found in regions withsubstantially higher or lower peak temperatures (whentemperatures of 350uC or greater occur)

Still referring to Fig 13 the welds made with thelowest welding speed (085 mm sndash1) have peak tempera-tures in the HAZ hardness minimum that are very closeto their corresponding nugget peak temperatures TheHAZ minimum hardness of the lowest temperature weldis the same as its nugget hardness The other 085 mm sndash1

weld has a substantially lower HAZ hardness thannugget hardness (107 versus 132) The reason for thislarge difference is not obvious based on the tempera-tures (differing by less than 10uC) however it can bespeculated that the kinetics of precipitate dissolutionand precipitation in the nugget may be stronglyinfluenced by the simultaneous straining and thermaltransient in the nugget

Figure 14 shows the change in nugget and HAZhardness as a result of PWHT plotted against the peaktemperatures in the corresponding regions As inFigs 12 and 13 the data are organised by weldingspeed The HAZ response to PWHT is in all casesnegative That is the hardness of the HAZ is decreasedby the PWHT regardless of the welding conditions Thisis consistent with the presence of limited solute availablefor precipitation in the HAZ and the coarsening ofexisting precipitate distributions In the weld nuggets apositive response to the PWHT is generally observed for

peak temperatures greater than 355uC For the weldswith peak nugget temperatures close to 350uC it isapparent that increased welding speed leads to a morepositive response to the PWHT This is also consistentwith the argument regarding time available for solutedepletion by M phase precipitation For peak nuggettemperatures significantly greater than 350uC no cleartrend with respect to the effect of welding speed can bediscerned

Summary and concluding remarksIn the present study weld thermal simulation has beenused to provide timendashtemperature histories for a series ofwelds made in aluminium alloy 7050-T7 The peaktemperature attained in the friction stir welds was madeto vary over a rather large range by manipulation of thewelding parameters Several relationships between weld-ing parameters peak temperatures and hardness dis-tributions have been elucidated for this series of welds

1 The heat-up and cool-down rates associated with agiven weld are determined almost solely by the weldingspeed (assuming the same thermal boundary conditionsfor all of the welds) This is probably a generalconclusion

2 The peak temperature at the centreline of the weldis a complex function of the rotation speed and thewelding speed Ranking of a series of welds by peaktemperature cannot be reliably done by use of only therotation speed welding speed advance per revolutionspecific weld energy or even the weld power For weldtemperature data presented here the best correlationappears to be with the weld power

3 For this set of welds nugget hardness may bereasonably correlated with the peak temperature in theweld An exception to this correlation occurs forrelatively slow welds with peak temperatures near350uC which exhibit anomalously low hardness It isspeculated that this exception is because of solutedepletion by M phase precipitation

4 The kinetics of M phase precipitation provides areasonable explanation for the observation (in this andother work) of HAZ minimum hardness correspondence

13 HAZ minimum hardness as a function of peak tem-

perature at minimum hardness location data are

grouped according to welding speed14 Nugget and HAZ response to PWHT nugget data are

closed symbols HAZ data are open symbols data

are grouped according to welding speed



to peak temperature near 350uC when HAZ tempera-tures of 350uC or greater are attained

5 As a corollary to 4 faster welding speed corre-sponding to shorter times near 350uC result in higherHAZ minimum hardness also consistent with the solutedepletion by M phase precipitation explanation

6 Maximum nugget hardness correlates with thehighest weld temperatures

Based on the foregoing an optimum weld schedulewould consist of a peak temperature above the solutiontreatment temperature (but less than any incipientmelting temperature) and the highest possible weldingspeed The necessity of creating a defect free weld willplace additional constraints on the welding parameters

The conclusions presented are based largely on theeffects of temperature and do not address the additionalcomplicating effects of differences in strain historybetween the nuggets and HAZs When considering onlythe HAZ or only the nugget data it appears thatreasonable explanations for observed trends may be putforward based solely on considerations of welding speedand peak temperature Additional detailed work will berequired to sort out any conflicting results between HAZand nugget behaviour that could be attributed to thestrain history

Acknowledgements

This work was partially supported by NSF grant DMI-9978611 Dr J Chen program manager and the AirForce Research Laboratory through Contract F33615-96-D-5835 Dr K V Jata TPOC

References1 W M Thomas E D Nicholas J C Needham M G Church

P Templesmith and C J Dawes International Patent Application

No PCTGB9202203 and GB Patent Application No 91259789

1991

2 A P Reynolds W D Lockwood and T U Seidel Mater Sci

Forum 2000 331ndash337 1719ndash1724

3 J T Staley Met Trans 1974 5 929ndash932

4 K V Jata K K Sankaran and J J Ruschau Metall Mater

Trans A 2000 31A 2181ndash2192

5 F Viana A M P Pinto H M C Santos and A B Lopes

J Mater Process Technol 1999 92ndash93 54ndash59

6 P Archambault and D Godard Scripta Mater 2000 42 675ndash

680

7 M W Mahoney C G Rhodes J G Flintoff R A Spurling and

W H Bingel Metall Mater Trans A 1998 29A 1955ndash1964

8 R Y Hwang and C P Chou Scripta Mater 1998 38 215ndash

221

9 C G Rhodes M W Mahoney W H Bingel R A Spurling and

C C Bampton Scripta Mater 1997 36 69ndash75

10 Y S Sato M Urata and H Kokawa Metall Trans A 2002 33A

625ndash635

11 C G Rhodes M W Mahoney W H Bingel and M Calabrese

Scripta Mater 2003 48 1451ndash1455

12 J-Q Su T W Nelson R Mishra and M Mahoney Acta Mater

2003 51 713ndash729

13 B C Yang J H Yan M A Sutton and A P Reynolds Mater

Sci Eng A 2004 A364 55ndash65

14 M H Kandakar J A Khan and A P Reynolds Proc 6th Int

Conf on lsquoTrends in welding researchrsquo Callaway Gardens GA

USA April 2002 218ndash223 Materials Park OH ASM

International

15 M Z H Khandkar J A Khan and A P Reynolds Sci Technol

Weld Joining 2003 8 165ndash174

16 K A A Hassan A F Norman D A Price and P B Prangnell

Acta Mater 2003 51 1923ndash1936



minimum hardness at a peak temperature of 377uCmight also be ascribed to rapid formation of the Mphase and concomitant solute depletion Regardlessbased on Mahoney et al and Hwang and Chou forwelding of 7075 peak temperatures near 350uC appearto be most effective in reducing the strength or hardnessin the HAZ when HAZ temperatures gt350uC arepresent It is important to keep in mind that it ispossible to produce friction stir welds in which the peaktemperature never reaches 350uC

The literature also contains several references tomicrostructure property correlation in 7xxx FSWs Oneof the earliest reports of microstructure in a 7xxx FSWwas made by Rhodes et al in 19979 In that paper theauthors investigated the microstructure resulting fromFSW of 635 mm thick 7075 plate The weld was made ata speed of 211 mm sndash1 Tool rotation speed and designwere proprietary Based on TEM determination of themetallurgical condition of the weld nugget region (theportion of the thermo-mechanically affected zone com-prised of fine equiaxed grains) the authors concludedthat the nugget temperature during FSW was between450 and 480uC In a subsequent paper Mahoney et al7

reported additional data on a similar friction stir weld(also made at 211 mm sndash1) in 635 mm thick 7075 plateTimendashtemperature histories for the nugget andHAZwerereported as were hardness and strength both in the aswelded condition and after a post-weld aging heattreatment A positive response to the post-weld heattreatment (PWHT) was observed in the nugget that is itwas strengthened by the PWHT As for most precipita-tion hardening alloys friction stir welded in a T6 or T7condition the minimum hardness was found in the HAZBased on tensile test results it is apparent that the post-weld aging resulted in reduced strength in the HAZ Thepeak temperature in the weld nugget was reported to beapproximately 475uC while that in the HAZ where thehardness was minimum was between 300 and 350uC

Sato et al10 have produced a relatively comprehensiveexamination of the effects of tool rotation speed at aconstant welding speed on the hardness microstructureand response to PWHT in alloy 6063 friction stir weldsIn the present study welds were made at a constantwelding speed of 6 mm sndash1 and tool rotation ratesbetween 800 and 3600 rev minndash1 Thermocouple mea-surements made beneath the welding tool at the weldcentreline showed that the peak temperature increasedwith increasing spindle speed Also the time of thetemperature transient was governed by the weldingspeed ie it was the same for all of the welds examinedThe authors determined that there was an Arrheniusrelationship between the nugget grain size and themaximum temperature The authors proposed that thisrelationship resulted from grain growth in the weldregion after the deformation associated with the FSWprocess was complete In addition it was observed thatthe best response to the PWHT was found for thosewelds which had achieved the highest temperature Theauthors surmised that the observed response was as aresult of differences in precipitate free zone volumeassociated with the different weld microstructures Itseems possible that the response to PWHT may alsohave been influenced by differences in solute andvacancy concentrations stemming from the differentthermal histories of the welds

In another study by Rhodes et al11 lsquoquenchedplungersquo simulations of the early stages of FSW wereperformed The goal of these studies was to determinethe microstructural state of material being friction stirwelded before the end of deformation The authorsfound grain sizes on the order of tens of nanometres andthey determined that increasing tool rotation rate leadsto increased grain size Post-weld heat treatments of thelsquoquenched plungersquo specimens resulted in grain sizestypical of most conventional friction stir weld nuggets2ndash5 mm The authors concluded that the mechanism ofgrain formation in the FSW nugget was as described bySato et al10 The observation connecting grain size torotation speed implies higher temperature is associatedwith greater rotation speed

Jata et al4 studied a 64 mm thick 7050-T7451 FSWproduced at a welding speed of 17 mm sndash1 and a toolrotation rate of 396 rev minndash1 They surmised fromTEM results that the temperature in the weld nugget wassufficient for solution heat treatment (450uC) APWHT of 24 h at 121uC resulted in increased nuggethardness as a result of reprecipitation of strengtheningphases and reduced hardness in the heat affected zone

Su et al12 have performed a substantial TEM study ofthe microstructure in a 7050-T651 friction stir weld(64 mm thick) The reported welding speed was15 mm minndash1 (which seems quite low) The tool rotationrate was 350 rev minndash1 The authors observed a nuggetgrain size varying between 1 and 4 mm and proposedthat the grain formation mechanism in the weld nuggetis continuous dynamic recrystallisation Observation ofvariable dislocation densities in nugget grains was thebasis of their conclusion regarding the operativerecrystallisation mechanism (although all dislocationdensity measurements were qualitative) They alsoobserved that the nugget was solution heat treatedduring the welding process and that the precipitatedistributions in the HAZ were coarsened

Yang et al13 have used the FSW process thermalsimulation described in a subsequent section1415 to aidin explanation of hardness distributions in 2024 and2524 FSWs They found that higher weld temperaturescorresponded to greater nugget hardness presumablybecause of in-process solution heat treatment followedby natural aging of the nugget material

In another recent paper Hassan et al16 havepublished a study of the thermal stability of nuggetgrain structures in friction stir welds on 64 mm thickalloy 7010 They observed that welds made with lowpower resulted in fine nugget grain sizes relative to weldsmade at higher power and that at low power themaximum weld temperatures could be substantiallybelow the solvus temperature for the alloy Post-weldsolution heat treatment showed that the very fine grainstructures were unstable and abnormal grain growth wasobserved during the heat treatments These observationsare in accord with the effects of peak weldingtemperature on grain size that have been discussedpreviously10

Taken together the results of the studies discussedabove particularly Refs 10 13 and 16 indicate that themetallurgical condition of the weld nugget and the heataffected zone may be significantly influenced by thewelding conditions over and above the occurrence ofwelding defects One of the most interesting aspects of












90 085 289135 127 30180 17 278270 254 378315 296 378405 381 456120 085 245180 127 245240 17 245360 254 30540 381 41720 51 378180 085 20270 127 22360 17 24540 254 335630 296 36810 381 39900 42 365





+(k+T)zqrc

T~0 (1)


Qinterface~

Qtot|


Totaltorque(2)


Mshoulder~

ethro

ri

(tr)(2pr)dr (3)


MPinBottom~

ethri

0

(tr)(2pr)dr (4)





Mtotv~Pav~Qtot (6)


Results




























response to PWHT


condition











180 rev minndash1



















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International
















90 085 289135 127 30180 17 278270 254 378315 296 378405 381 456120 085 245180 127 245240 17 245360 254 30540 381 41720 51 378180 085 20270 127 22360 17 24540 254 335630 296 36810 381 39900 42 365





+(k+T)zqrc

T~0 (1)


Qinterface~

Qtot|


Totaltorque(2)


Mshoulder~

ethro

ri

(tr)(2pr)dr (3)


MPinBottom~

ethri

0

(tr)(2pr)dr (4)





Mtotv~Pav~Qtot (6)


Results




























response to PWHT


condition











180 rev minndash1



















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International









+(k+T)zqrc

T~0 (1)


Qinterface~

Qtot|


Totaltorque(2)


Mshoulder~

ethro

ri

(tr)(2pr)dr (3)


MPinBottom~

ethri

0

(tr)(2pr)dr (4)





Mtotv~Pav~Qtot (6)


Results




























response to PWHT


condition











180 rev minndash1



















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International




























response to PWHT


condition











180 rev minndash1



















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International














180 rev minndash1



















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International




















ing speed




pitch

























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International





























Acknowledgements





1991









680




221




625ndash635




2003 51 713ndash729






International







Relationships between weld parameters, hardness distribution and temperature history in alloy 7050...

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Transcript of Relationships between weld parameters, hardness distribution and temperature history in alloy 7050...