Materials and Design 49 (2013) 433–441

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Materia ls and Design

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Investigations on the effects of friction stir welding parameters on intermetallic and defect formation in joining aluminum alloy to mild steel

M. Dehghani ⇑, A. Amadeh, S.A.A. Akbari Mousavi School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 17 October 2012 Accepted 5 January 2013 Available online 20 January 2013

Keywords:Dissimilar friction stir welding Tensile strength Intermetallic compound Defect formation Aluminum alloy Carbon steel

0261-3069/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2013.01.013

⇑ Corresponding author. Tel.: +98 21 82084090; faxE-mail addresses: mdehghany@ut.ac.ir, dehghani.m

a b s t r a c t

Joints of Al 5186 to mild steel were performed by using friction stir welding (FSW) technique. The effects of various FSW parameters such as tool traverse speed, plunge depth, tilt angle and tool pin geometry onthe formation of intermetallic compounds (IMCs), tunnel formation and tensile strength of joints were investigated. At low welding speeds due to the formation of thick IMCs (which was characterized as Al6Feand Al5Fe2) in the weld zone the tensile strength of joints was very poor. Even at low welding speeds the tunnel defect was formed. As the welding speed increased, the IMCs decreased and the joint exhibited higher tensile strength. The tunnel defect could not be avoided by using cylindr ical 4 mm and 3 mmpin diameter. By using a standard threaded M3 tool pin the tunnel was avoided and a bell shape nugget formed. Therefore tensile strength of the joint increased to 90% of aluminum base alloy strength. Athigher welding speed and lower tool plunge depth, the joint strength decreased due to lack of bonding between aluminum and steel. Based on the findings, a FSW window has been developed and presented.

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1. Introduction formatio n of IMCs could be decreased noticeably. Several experi-

High corrosion resistance and exceptional mechanical proper- ties of 5XXX series aluminum alloys has progressed their industrial usage, especiall y in car and marine industrie s. However, use of alu- minum in industries may result in some problems such as the pro- cess of joining of aluminum to steel. Joining of aluminum to steel isgenerally difficult due to differences between their physical and chemical properties [1]. Both alloys have incompa rable melting point, thermal conductivity, coefficient of linear expansion and heat capacity. Considering the reference phase diagram of Al–Fesystem [2] the low solubility of iron in aluminum promote s the for- mation of brittle intermet allic compounds (IMCs) such as Fe2Al5,FeAl3 and FeAl, in the weld zone. Therefore, it seems that obtaining strong joint between aluminum and steel sounds impossible orvery difficult by using common fusion welding techniques. Differ- ent techniques such as diffusion bonding [3], friction welding [4],ultrasonic welding [5] and laser welding [6] have been used to join aluminum to steel. Friction and explosive welding are limited to afew weld joint geometries. Ultrasonic and laser welding are almost limited to joining of thin plates; however, laser welding of dissim- ilar thick plates has been reported [7].

Friction stir welding patented by The Welding Institute (TWI), iswidely employed in industry for joining of aluminum alloys [8].Due to the nature of this process, with correct selection of welding parameters, no melting of base alloys is expected. Thus, the

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ments have been published on joining of aluminum to steel, e.g.[9–13]. Uzun et al. [9] investigated the microstructure, hardness and fatigue properties of friction stir butt welded 4 mm thick alu- minum 6013-T4 to X5CrNi18 -10 stainless steel. They successfu lly obtained sound joints and characterized the microstructure ofdissimilar weld such as HAZ and TMAZ in both base metals; how- ever did not investigate the effect of process paramete rs on tensile propertie s of joints. Watanabe et al. [10] investigated the effect oftool rotation speed and pin position on the tensile strength of2 mm thick aluminum 5083 alloy and mild steel sheets. The joint exhibited 86% of ultimate tensile strength of aluminum base alloy.Tanaka et al. [11] analyzed the effect of rotation speed on the tem- perature rise factor and joint strength of aluminum 7075-T65 and mild steel in the condition of constant welding speed and pin position. The effect of heat input on the formation of IMCs and resultant tensile strength was investigated . Chen and Kovacevic [12] reported the effects of pin position on the temperature distri- bution and microstru cture of weld zone at constant tool rotation and traverse speed in joining Al 6061 to AISI 1018 steel. Lee et al.[13] examined the type of IMCs produced in the reaction layer be- tween friction stir weld of Al 6056-T4 and 304 austenitic stainless steel.

In dissimilar welds, due to the high tendency of aluminum toreact with iron, controlling the formation of IMCs is the matter ofconcern. According to the experiments the appropriate selection of welding parameters, especially the tool rotation, traverse speed and tool design controls the amount of heat input and therefore the microstru cture of the joint [14]. In dissimilar FSW of aluminum to

Table 1Summary of the welding parameter s and ultimate tensile strength of joints.

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steel, due to significant differences between flow propertie s of base alloys, it is expected that the material movement in the weld zone to be more inhomogene ous in comparison with similar FSW. This inhomogene ous movement of material may be the source of defect formation. Generally, in FSW defects can be eliminated by increas- ing heat input. Increase in heat input may result in brittle IMCs for- mation. To the best knowled ge, authors could not find any report about the effect of process parameters on defect formation and its removal in dissimilar FSW of aluminum to steel concerning IMCs formation. In addition, aluminum 5XXX series contain high amount of magnesium, as the main alloying element, which in- creases the strength of alloys by solid solution mechanism [15].It has been reported that magnesiu m in aluminum promotes the formation of brittle IMCs in aluminum – steel joints [16]. As a re- sult, joining of 5XXX series to steel seems to be more sensitive towelding paramete rs due to the presence of Mg and formatio n ofIMCs in the weld zone.

Due to existence of several effective paramete rs in FSW, it isnecessary to optimize various paramete rs to obtain a defect free joint with an acceptable tensile strength. This might be done through demonstrat ing a welding window, e.g. in similar FSW [17,18]. Gieger et al. [19] presente d a process window for friction stir knead welding of aluminum to steel by considering tilt angle,rotation and traverse speed to obtain a high strength joint. Tanaka et al. [11] reported the relationship between joint strength and temperature rise paramete r which was calculated based on differ- ent rotation speeds at constant traverse speed and shoulder diam- eter. Clearly, finding a FSW window for producing sound joint is amatter of concern.

Therefore, one of the aims of this research is the investiga tion ofprocess parameters to produce sound joint by controlling the amount of IMCs in FSW of Al 5186 to mild steel and also proposin ga welding window. Hence, another aim of this research is the inves- tigation of the effect of plunge depth and tool tilting on the forma- tion of defect free joint and amount of IMCs that, to the best of our knowledge, was not investigated for aforementione d base alloys.

Weld number

Welding speed (mm/min)

Plunge depth (mm)

UTS (MPa)

Tool

S1 14 0.3 0 T1S2 28 0.4 0 T1S3 40 0.4 121 T1S4 56 0.4 208 T1S5 80 0.4 142 T1S6 56 0.8 29 T1S7 40 0.4 0 T1S8 28 0.4 0 T2S9 56 0.4 – T2S10 56 0.4 246 T3S11 56 0.3 187 T3S12 56 0.2 – T3

2. Experimental procedur e

A mild steel (St 52) sheet with thickness of 3 mm was welded to5186 aluminum sheet with the same thickness. Ultimate tensile strength of steel and aluminum were 520 MPa and 275 MPa,respectively . The FSW was carried out using a conventional milling machine. The schemati c arrangement of FSW process for dissimilar joints used in this study is presented in Fig. 1. As the tool rotates,the friction between tool shoulder and work piece generates suffi-cient heat to plasticize material beneath tool shoulder. The rotating tool pin moves the plasticized material from the front to the back of the weld line which results in joining between the butted plates.

Fig. 1. Schematic of the process: (a) friction stir welding pr

The tool rotation speed for all samples was fixed at 355 rpm.The welding speed, pin size, tool plunge depth, tool tilt angle and pin geometry were changed to find the optimum welding condi- tion in which the tunnel defects were prevented and formation of IMCs was restricted. In addition, the effect of different plunge depth on tensile strength of the strongest joint was investigated .The tool shoulder diameter was 18 mm. The pin diameter and pro- file were: (a) 4 mm, non-threaded cylindrical, (b) 3 mm, non- threaded cylindrical and (c) standard M3 threaded pin. The tools are nominated as T1, T2 and T3, respectively. To prevent the over- heating of aluminum alloy and reducing tool wear, the pin was in- serted into aluminum alloy with an offset of 0.2 mm into the steel plate as shown in Fig. 1b. During the welding process, aluminum and mild steel sheets were positioned in retreatin g and advancing side, respectively .

Tensile test specimens were machined from the welds perpen- dicular to the weld line according to the ASTM: E8/E8M- 11 stan- dard (i.e., 25 mm gauge length and 6 mm gauge width). Toremove flash from the top surface and unwelded region and contin- uous tunnel on the underside, the specimens were machined from the both top and underside. The weld areas were located in the cen- ter of the test specimens. Tensile tests were conducted at a constant crosshea d speed of 1 mm/min. The lists of various FSW tests and re- lated tensile strength are summarized in Table 1. The tool tilt angle for weld S7 was 5� while it was 3� for other welds.

Metallogr aphic studies were conducted by utilization of optical microscop y (OM) and electron scanning microscopy (SEM). The samples were etched with Keller and Nital reagents to reveal microstru cture of aluminum and steel base plates, respectively .The etching time was 15 s. For SEM observation, cross sections ofthe welds were ground up to 2500 grit sand paper and polished with 0.3 lm alumina paste. SEM was equipped with an energy dis- persive X-ray spectroscopy (EDS) apparatus. X-ray diffraction

ocess, (b) position of the rotating pin in present study.

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(XRD) was used to examine the structure and the intermetalli c lay- ers formed in the weld zone and at Al/Fe interface. To distinguish the peaks of formed phases, the scan rate of XRD was reduced and the time of test was prolonged (�5 h.).

3. Results and discussion

3.1. Microstructu re observations

3.1.1. Effects of welding speed In this section the effects of welding speed on formation of IMCs

in microstructure of joints are discussed. Fig. 2 illustrates the OMphotographs of the cross-sectio ns of welds S1, S2 and S3. It is obvi- ous that all weld nuggets are filled with large fragments of steel and small platelets sheared off from the steel plate, which is the re- sult of the abrasion , wear and shearing by tool rotating action. Ataluminum side of welds, two distinct regions with different color contrast can be distinguished in the microstructur e. The light re- gion consists of aluminum 5186 base alloy. The EDS analysis per- formed on dark region showed that it was mainly compose d ofIMCs, the results are presented througho ut this section. Moreover,at aluminum side of the welds the typical microstructur e of FSW isvisible. The weld nugget (WN), thermo-mec hanically affected zone (TMAZ) and heat affected zone (HAZ) are distingui shable in the weld zone. At the welding speed of 14 mm/min (weld S1), a wide IMCs region is formed in the weld nugget (Fig. 2a). By increasing the welding speed from 28 to 40 mm/min, the amount of IMCs de- creases noticeably (Fig. 2b and c). In addition, in welds S1 and S2,the IMCs are formed not only at the upper and middle zone of the weld nugget but also at the weld root (Fig. 2a and b), whereas inweld S3 the IMCs are produced just at the upper region of the weld nugget (Fig. 2c). Therefore, at low welding speed the whole thick- ness of weld nugget undergoes high temperat ure cycle; however since in the FSW the main part of heat input is generated by shoul- der, so at higher welding speed the formatio n of IMCs is limited tothe closest area to the shoulder.

A high magnification OM image of selected areas in Fig. 2a (asoutlined by squares) is illustrated in Fig. 3. Fig. 3a shows the

Fig. 2. OM images of cross-sections of welds (a) S1, (b) S2 and (c) S3 showing the Fefragments, defects and wide IMCs region in the weld zone.

photograp h of aluminum weld zone showing different size and shape of grains at WN, TMAZ and HAZ in weld S1. The weld nugget contains very fine grains produced through dynamic recrystal liza- tion (DRX) mechanism . The maximum width of the fine grain zone,which is called dynamically recrystal lized zone (DRZ), is about 4.2 mm, as shown in Fig. 2a. The width of DRZ is almost similar to the diameter of tool T1 pin. It is well accepted that DRX during friction stir welding results in generation of fine strain-free grains in the weld nuggets [20,21]. As the result of rotation action of the pin the grains at TMAZ deform and elongate (see Fig. 3a). However,the dynamic recrystallization process does not occur in the TMAZ.As the base alloy is a non-heat treatable type of aluminum alloy,there is no evidence of major changes in the HAZ (Fig. 3a). As the pin offset is set to be 0.2 mm into steel plate, the Fe fragments and steel butt face undergo high amounts of plastic strain. The dar- ker contrast of steel at Al/Fe interface and also Fe fragments is the result of their finer grains, as shown in Fig. 3b.

Fig. 4 illustrates the SEM micrographs and correspondi ng points of EDS analysis in the weld S1. It can be seen that the dark region,referred as IMCs in Fig. 2a, has a different contrast in the SEM back- scattered electron image (Fig. 4a). A high magnification SEM image of selected area in Fig. 4a (outlined by square) is presente d inFig. 4b. Fig. 4b shows the existence of a continuous IMC layer (point 1) at the Al/Fe interface and scattered particles of IMC with- in a matrix composed of IMC (point 2). The results of quantitative EDS analysis of the correspond ing points in this figure are pre- sented in Table 2. From the Al to Fe atom ratio and considering the Al–Fe phase diagram [2], the Al5Fe2 is found at Al/Fe interface and the wide IMCs region in Figs. 2a and 4a is characterized as Al6-

Fe, as shown in Fig. 4. The tool rubbing action shears off Fe frag- ments from the steel plate and scatters them into the weld nugget. In addition, the pin rotating action results in re-breaking and redistributi on of Fe fragments and turning them to tiny parti- cles. The tiny particles react with aluminum and form Al5Fe2

(Fig. 4b).The XRD pattern of S1 sample presented in Fig. 5 confirms the

presence of such phases. The same phases are also identified inthe welds S2 and S3. The presence of IMCs was reported in the FSW of aluminum to steel [13,22,23].

Thicknes s of Al6Fe IMCs region in welds S1, S2 and S3 is about 1.4, 0.65 and 0.43 mm, respectively . Additionally , the thickness ofAl5Fe2 IMC layer at the Al/Fe interface is measured as 2.1, 1.4 and 0.9 lm, respectively . Approximately , there is a linear relationshi pbetween decrease in IMCs thickness and increase in welding speed which results in decrease of weld heat input, as discussed below.

3.1.2. Determination of heat input factor (HIF)To define the optimum welding conditions for an acceptab le

joint and to study the formation of IMCs, it is necessary to investi- gate the relationshi p between the joint strength and the heat input for each welding condition. In the literature, several equation swere develope d to describe the effect of friction stir welding paramete rs on the extent of heat flow into the welding zone [24,25]. Frigaad et al. [24] suggested the following equation for heat generation per unit area and time during FSW process:

Q0 ¼ 4=3p2lPxR3 ð1Þ

where Q0 is the net power (W), l is the friction coefficient, P is the applied pressure (Pa), x is the tool rotation speed (rev/s), and R isthe tool shoulder radius (m). The rotation of tool in the friction stir welding process causes to flow the heat into the weld zone. The heat input per unit length of the weld can be evalua ted by Q0/v, where v isthe tool traverse speed (m/s). In this study we could not measure the real pressure in the vertical direction quant itatively and also the fric- tion coefficient at each welding temperatur e. It is reported that at

Fig. 3. OM images of weld S1 showing (a) different grain size of WN, TMAZ and HAZ and (b) Al/Fe interface and heavily deformed Fe fragment in Al.

Fig. 4. SEM micrographs of weld S1 showing (a) wide IMCs region in the weld nugget and (b) IMC layer at Al/Fe interface and scattered particles of IMC in the weld nugget.

Table 2EDS analysis of corresponding points in Fig. 4b.

Point Al (at.%) Fe (at.%) Mg (at.%) Al/Fe Atom ratio Phase

1 71.3 25.2 3.5 2.8 Al5Fe2

2 82.3 11.7 5.9 7.0 Al6Fe

Fig. 5. XRD of the weld S1 showing the corresponding peaks of Al6Fe and Al5Fe2.

436 M. Dehghani et al. / Materials and Design 49 (2013) 433–441

constant plunge depth, the applied pressure remains almost con- stant [26]. However, the friction coefficient during friction stir weld- ing remains almost constan t [21]. Furthermor e, Tanaka et al. [11]and Hirata et al. [21] reported that the relations hip betwee n the heat

input and the properties of similar FSW can be expressed without using l and P paramete rs. Therefore, by eliminating l and P fromEq. (1) and dividing both sides of the result by welding speed (v),the ‘‘heat input factor’’ (HIF) can be expressed as:

Q0=v ¼ ð4=3p2xR3Þ=v ð2Þ

Relationshi p between joint strength (UTS) and HIF obtained from data of present study is illustrate d in Fig. 6. It is necessary to note that the HIF of welds with almost similar plunge depth isillustrate d in Fig. 6 and effect of different pin profiles is neglected.The open and close circles show tunnel-contain ing and tunnel-free joints, respectively. According to Fig. 6, the HIF of welds S1–S3 de- creases with increasing the welding speed. At the highest HIF (weld S1), a wide IMCs region and continuous IMC layer are pro- duced in the weld nugget and at the Al/Fe interface (Fig. 4a and b). Clearly, by reducing the heat input the amount of IMCs in the weld zone is decreased and limited to the upper side of the weld cross-sec tion, where is closer to the tool shoulder, as shown inFig. 2c. Comparison between microstructur e of the welds and HIF shows the good conformity between the results and Eq. (2).

As Eq. (2) and Fig. 6 reveal, by increasing the welding speed the heat input decreases, so it is expected that the formation of IMCs isreduced noticeab ly. Fig. 7 demonstrates the optical micrograph ofcross section of the welds S4 and S5 performed at welding speeds of 56 and 80 mm/min, respectivel y. From Fig. 7, it is obvious that the wide IMCs region (i.e. Al6Fe) disappears and is not visible inOM images of microstructure. As the heat input decrease s, the max- imum temperat ure of weld decreases and results in the decrease ofreaction between aluminum and steel. From microstru cture

Fig. 6. Relationship between joint strength and HIF obtained from present study and data of literature.

Fig. 7. OM image of welds (a) S4 and (b) S5, showing no IMCs region formation athigher welding speeds.

Fig. 8. IMCs formation and the removal of tunnel in welds (a) S6 and (b) S7, due tohigher plunge depth and tool tilting, respectively.

M. Dehghani et al. / Materials and Design 49 (2013) 433–441 437

observation and the ultimate tensile strength of welds (Table 1), the welding speed of 56 mm/min is found to be minimum welding speed in which the formation of IMCs region (i.e. Al6Fe) can beavoided. However , a thin layer of IMC (<0.5 lm) exists at the Al/ Fe interface. By increasing the welding speed from 14 to 56 mm/ min, the thickness of IMCs decreases due to decrease in HIF or heat input of the welds.

3.1.3. Tunnel formation According to Figs. 2 and 7, all welds contain tunnel defects and

pores in the retreatin g side of the weld microstructure. At higher welding speed the number of tunnels and pores increases and de- fects move upright into aluminum weld nugget (compare Figs. 7and 2). Because the tool has shorter time to plasticize and move the materials around the pin; and as the maximum temperature of weld decreases, the flow stress of materials increases and causes insufficient plastic deformation of material at welding condition.Under such a condition, the tool cannot accomplish weld line and consolidate materials in the weld zone and therefore the tun- nel forms. It is necessary to note that in similar FSW, the tunnel forms at the advancing side [20,27]. However in dissimilar FSW of aluminum to steel, the tunnel forms at the retreatin g side (Figs. 2and 7). The reason might be attributed to the higher strength ofsteel plate and its resistivity to deformation and also to the appli- cation of pin offset.

Tunnel or wormhol e is a common defect in FSW. It is believed that defects in FSW are the result of improper selection of process parameters (e.g. welding rotation and traverse speed, applied

pressure , etc.) and/or improper design of tool (i.e. geometri cal paramete rs) [28]. Improper selection of process parameters results in insufficient plasticizatio n of materials and imbalanc e in material movement around tool pin, therefore, tunnel defect forms. It has been reported that the tunnel defects form in the welds with low heat input (i.e. cold welds where low rotation speed and/or high welding speed and/or low applied pressure were applied)[17,28,29 ]. Therefore, to fabricate a defect free Al 5186-steel joint,it is necessary to increase the heat input and/or modify tool geom- etry. In present study, the rotation speed and diameter of the tool shoulder are kept constant ; to increase the heat input it is neces- sary to decrease the welding speed. However, at low welding speed the high amount of IMCs form in the microstructur e (Fig. 2); soconsideri ng the selected welding parameters in this study, the de- crease of welding speed is not a proper method to overcome the problem of tunnel defect formation. Therefore, to remove the tun- nel defect other welding parameters are investiga ted as discussed below.

3.1.3.1. Effects of tool plunge depth on IMCs formation and tunnel removal. Zhang et al. [26] investigated the mechanism of defect formatio n and proposed criteria for defect free joints of FSW as:

P �x=v � V=kðrsh � rpÞ ð3Þ

where V is the volume of the defect, K is the proportio n factor, rsh

and rp is the radius of shoulder and pin, respective ly. P, x and vhas the same meaning as in Eq. (2). According to Eq. (3), to avoid the formation of tunnel defect, the welding pressure and/or pin ra- dius should be larger and/or less than a specific value, respec tively.Therefor e, to enhance the welding pressure, the plunge depth of0.4 mm (welds S4) increased to 0.8 mm (weld S6), as shown in Ta-ble 1. Fig. 8a shows the OM image of the weld S6 and it is obvious that the tunnel defect disappears at higher plunge depth (i.e. higher applied pressure). However , the wide IMCs region forms again (compare Figs. 8a and 7). By increasing the plunge depth, the inter- action betwee n shoulde r and work piece increases, the more fric- tional heat is generated which results in sufficient plasticiza tion of materials and facilitates materials moveme nt around tool pin.Therefor e, the tunnel defect disappears, however due to higher heat generation the wide IMCs region forms in the weld zone (Fig. 8a).Similar to welds S1 and S2, Al5Fe2 is produce d at the Al/Fe interface (thickness about 1.6 lm). The wide IMCs region (shown by arrow sin Fig. 8a) is composed of Al6Fe (thickness about 0.8 mm).

We could not measure and control the applied pressure quanti- tatively but by controlling the plunge depth it was possible to con- trol applied pressure qualitatively. By increasing the plunge depth the axial load increases [29] and as a consequence the applied

Fig. 9. OM images of (a) weld S8 showing IMCs formation and (b) weld S9 revealing tunnel defect, using tool T2.

Fig. 10. Removal of tunnel defect using threaded pin in weld S10.

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pressure of welding increases. Kumar and Kailas [29] reported that the increase in the plunge depth of the tool by 0.8 mm increased the axial load from 4 to 104 kN. As shown in Figs. 7 and 8a, the in- crease in plunge depth (i.e. axial load or applied pressure) affected the plasticize d material flow within the whole thickness of the weld, therefore the IMCs formed on both upper and bottom side of the welds. FSW modeling shows the direct relationship between applied pressure and heat input [24,25,27]. It is expected that the maximum temperature of the weld increases with increasing the heat input, however in similar welding of Al 2024, Schmidt et al.[25] have reported that increase in pressure did not show consid- erable increase in maximum temperature . The formation of IMCs in the microstructur e of the weld S6 shows the increase in the weld temperature due to increase in the applied pressure (compareFigs. 8a and 7a), as reported by Tang et al. [30]. Therefore, byincreasing the plunge depth from 0.4 to 0.8 mm, the thickness ofIMC at Al/Fe interface increases from 0.5 (weld S4) to 1.6 lm (weldS6). In addition as Fig. 8a shows, not only the IMC (i.e. Al6Fe) forms at upper side of the weld but also the IMCs move towards the weld root, as the result of increasing downward forging force of the tool.It can be concluded that increasing the plunge depth increases heat generation and downwa rd forging action of the tool. Therefore, inthe present study, increasing the applied pressure in order toeliminate the tunnel is found to be not a suitable method due toformation of IMCs; the authors tried to optimize other process parameters as describe below.

3.1.3.2. Effects of tool tilt angle. In addition to the tool rotation and traverse speed, another important parameter of FSW is tool tilt an- gle. Proper tool tilting ensures that the shoulder holds the plasti- cized material and moves it from the front to the back of the pin and fills the cavity. The weld S7 was performed with a tilt angle of 5�. As shown in Fig. 8b, the increase in tool tilt angle results ina tunnel-free joint due to increase in weld heat input [31,32] anddownward forging force [32], however the wide IMCs region forms in the weld nugget. Unlike the other welds, at higher tool tilt angle,the IMCs are formed at the bottom part of the weld and broadened to the width of the weld nugget. In weld S7, thickness of Al6Fe(shown by arrows in Fig. 8b) and Al5Fe2 (at the Al/Fe interface) isabout 1.2 mm and 3.9 lm, respectively. Comparison between IMCs thickness in the welds S7 and S3 indicates that the increase in tool tilt angle increases the heat input of the weld severely and results in the formation of thick IMCs in the weld zone, and however the downward forging force increases which limits the IMCs at bottom part and spreads them to the width of the weld nugget. To the best knowledge of authors, there is not a direct impleme ntation of the tool tilting in FSW heat input modeling in the literature [e.g.20,27]. In the literature, there is an agreement on increase in heat input due to tool tilting but there is contradic tion between effects of tool tilting angle on tunnel removal. For example, Zhang et al. re- ported that by increasing tool tilt angle from 0� to 3�, the down- ward forging force and weld maximum temperature increased 3times and 100 �C, respectivel y, as the result tunnel disappeared [32]. By increasing the tool tilting from 1� to 3�, the formation oftunnel defects due to the increase in gap between tool shoulder and work piece was reported [33]. In lap FSW of Al 5083 and steel,increasing the tool tilt angle (>1�) resulted in the formation of alu- minum rich IMCs as well as pore defects [34]. This study shows that the increase in tool tilting from 3� to 5� increases the weld heat input and downward forging force and removes the tunnel defect, also, increases the thickness of Al5Fe2 IMC from 0.9 (weldS3) to 3.9 (weld S7) micron and produces a wide zone of Al6Fe. This result indicates that the tool tilt angle has very effective role onheat generation during dissimilar FSW. By proper selection ofparameters, the IMCs and tunnel free joint between Al 5186 and steel was performed at 3� tool tilt angle (see Section 3.1.3.4.).

3.1.3.3. Effects of pin size. As Eq. (3) indicates, it is possible to elim- inate the tunnel defect with the smaller diameter of the pin. There- fore, the welds S8 and S9 were performed by tool T2 and the microstru cture of the welds are shown in Fig. 9. Comparis on ofwelds S8 and S9 with the weld S2 shows that at low welding speed of 28 mm/min and using smaller pin (weld S8) the tunnel defect can be eliminated however the IMCs phases form in the weld microstru cture, as shown in Fig. 9a. Smaller pin has smaller vol- ume and hence the shoulder shall fill a smaller cavity. Similar tothe weld S4, by increasing the welding speed to 56 mm/min (weldS9) the wide IMCs region (i.e. Al6Fe) is eliminated , and in spite ofusing smaller pin, the tunnel defect forms again, (Fig. 9b).

3.1.3.4. Effects of pin profile. Considering the minimum welding speed of 56 mm/min which prevents the formation of wide IMCs region (i.e. Al6Fe), a standard treaded M3 pin (tool T3) was used.Fig. 10 shows that the tunnel defect is eliminated successfu lly and a bell shape nugget forms. Meanwhi le, in the weld nugget,there are two different zones; the darker bands consisted of finergrains surrounded by the lighter zone with slightly coarse grains.The SEM observation of weld microstructure shows no IMCs region in the weld zone (Fig. 11a). However, by using high magnificationimage (Fig. 11b) a very thin layer is found at the Al/Fe interface (thickness 6 0.5 lm). Adjacent to the interface, a few scattered particles are found, as shown by arrows in Fig. 11b. Due to the dif- ferent back scattered SEM image contrast between these phases and base alloys, it can be proposed that the IMC forms at the Al/ Fe interface. The EDS analysis is not applicable on the IMC layer as the result of thickness limitatio n. As diameter of pins are similar in tools T2 (Fig. 9b) and T3 (Fig. 10), the threaded pin increases the downwa rd forging force and facilitates the downwa rd movement of plasticized material, as the result, cavity can be filled success- fully and the tunnel and pores are avoided.

Fig. 11. SEM micrographs of weld S10 showing (a) no IMCs region in the weld nugget and (b) thin IMC layer at the Al/Fe interface.

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It is interesting to note that using M3 threaded pin tool, even atwelding speed of 80 mm/min, tunnel does not form. It is worth noting that some welds are performed with standard M4 threaded pin with similar condition s to the weld S10, however the tunnel defect forms again, the results are not shown here. This indicates that in dissimilar FSW of aluminum to steel, due to different flowproperties of base alloys, there is a narrow window for selection of pin diameter, using slightly higher pin diameters even with threaded profile causes the formation of tunnel defect. Therefore,in dissimilar FSW of aluminum 5186 to medium strength steel itis found that for prevention of tunnel formation the pin diameter is more important than its threaded profile.

3.1.3.5. Effects of decreasing plunge depth. To investiga te the effect of plunge depth (i.e. applied pressure ) welds S11 and S12 were per- formed with similar condition to weld S10 but with different plunge depth of 0.3 and 0.2 mm, respectively (see Fig. 12). It isobvious that at the plunge depth of 0.3 mm the tunnel defect is still absent (Fig. 12a) whereas at the plunge depth of 0.2 mm it forms again (Fig. 12b). As stated previousl y, the tunnel formation iscaused by the reduction in downwa rd forging force and heat input of the weld. It can hence be concluded that by proper selection ofprocess paramete rs namely the welding speed, plunge depth and pin geometry, the IMCs formation can be restricted and a bell shape defect-free nugget can be formed.

Fig. 12. Effect of plunge depth on tunnel formation (a) 0.3 mm in weld S11 and (b)0.2 mm in weld S12.

3.2. Ultimate tensile strength

The ultimate tensile strength of the welds and the relationship between UTS and heat input factor (HIF) are presente d in Table 1and Fig. 6, respectively. The welds S1, S2, S7 and S8 fractured dur- ing set-up and before implementation of the tensile test. The zero strength of the welds can be attributed to the existence of high amount of IMCs in the weld zone and at Al/Fe interface (Figs. 2, 8and 9a). By increasing the welding speed from 14 to 56 mm/min,the UTS increases and reaches 208 MPa, i.e. about 75% joint effi-ciency in the weld S4 is achieved. By increasing the welding speed,the HIF (Fig. 6) and accordingly the maximum temperature of the welds decrease s, therefore the reaction between aluminum and steel decrease s noticeab ly. Conseque ntly, the formation of brittle IMCs in the weld zone is restricted and the thickness of IMCs de- creases from 1.4 mm to less than 1 lm, as the result joint strength increases. However, due to the presence of scattered pores, the ten- sile strength is lower than that of aluminum base alloy. Further in- crease in the welding speed to 80 mm/min reduces the UTS of the weld S5 to 142 MPa, due to formatio n of scattered pores. All welds fractured at Al/Fe interface, except weld S4 which fractured atTMAZ/we ld nugget border where a pore existed, as shown by ar- row in Fig. 7.

In spite of similar HIF of welds S10, S9 and S4, weld S10 has the highest UTS (246 MPa), nearly 90% joint efficiency. Good consolida- tion of material and obtaining defect-free joint using threaded pin can produce successfu l joint between Al 5186 and steel with a thin layer of IMC at the Al/Fe interface (thickness 6 0.5 lm). The de- crease in the plunge depth decreases the joint strength of weld S11 to 187 MPa, as shown in Table 1. Due to reduction in plunge depth (i.e. applied pressure ), it can be suggested that the reduction of heat input and maximum temperat ure of the weld prevents the formatio n of strong bond between aluminum and steel and there- fore the strength of the weld decreases. In similar FSW, if the plunge depth is less than a certain value then the tunnel defect forms, whereas the nugget collapses and flash formation occurs at high plunge depth. In dissimilar FSW of Al 5186 to steel, the welds are very sensitive to plunge depth, in a way that decrease in plunge depth by 0.1 mm decreases the UTS of the weld notice- ably. Increase in plunge depth produces high amount of IMCs.Therefore, in welding of Al 5186 and steel, pin profile and plunge depth shall be considered as very effective parameters.

The direct relationship between the UTS, HIF and microstru c-ture of the welds can be elaborated through Figs. 2, 6 and 7. Asthe HIF increases, the thick IMCs form in the weld zone and UTS decrease s. On the other hand by decreasing the HIF the UTS of weld

440 M. Dehghani et al. / Materials and Design 49 (2013) 433–441

increases and reaches a maximum, afterwards by further reduction in the HIF the strength decrease s due to lack of proper bonding be- tween the base alloys and formation of scattered pores. Therefore,it can be concluded that there is an optimum range for the heat in- put which provides proper strength of the welds.

3.3. Al 5XXX to steel FSW window

This study indicates that the heat input model which has been developed for aluminum alloys (Eq. (1)), can be successfully ap- plied for dissimilar FSW where the properties of base alloys are incompatible. Meanwhile, the effect of welding speed is not inher- ently incorporate d in heat input models of FSW [20,27,35,36], the comparison between microstructur e and strength of welds shows the good conformity between results and calculated heat input fac- tor (HIF) based on Eq. (2). From Fig. 6, it is obvious that there is anoptimum HIF range in which the tensile strength of the joint reaches acceptable value. This range is about 0.04–0.08 m2/N,shown by dashed line in Fig. 6. If the process parameters (rotationand traverse speed, tool shoulder diameter) are chosen in a way that HIF exceeds the aforementione d range, the strength of joint will be reduced due to the formation of IMCs (at high HIF) or lack of proper bonding between plates (at low HIF). The data from Ref.[10] are inputted into the Eq. (2) and the result is presented inFig. 6. This reference is selected due to similarity of its base alloys to this study and use of completely different process paramete rs incomparison to our work. The welding parameters of Ref. [10] wereas: rotation speed: 100–1250 rpm, welding speed: 25 mm/min,tool shoulder diameter: 15 mm, plate thickness: 2 mm. Fig. 6shows that the HIF region of the experime ntal results of Ref. [10]is fitted completely to the HIF range of our study justifying the pro- posed welding window. It should be noted that the HIF range determined is suitable for FSW Al 5XXX series to mild steel and dif- ferent HIF range would be obtained for different combinations ofjoints. Incorporatio n of butt FSW data of other aluminum alloys series joint to steel (such as Ref. [11], the data are not shown here)shows that the optimum HIF range still exists but the value of the range differs for different Al alloys.

It can be summarized that in FSW of Al 5XXX series to carbon steel, the Eq. (2) can be used as a fast workshop model to findthe optimum range of main process parameters which should befallen within the proposed welding window in Fig. 6. Afterwards,to eliminate tunnel defects and improve tensile strength the pin profile and plunge depth might be modified.

4. Conclusion

The friction stir butt welding used to fabricate joints between aluminum 5186 and mild steel plates with rotation speed of355 rpm. Effect of welding speed, tool tilting, geometry of pin and plunge depth on the microstructur e and tensile strength were investigated and results obtained are as follows:

1. At the welding speeds of 14 and 28 mm/min the strength ofjoints were 0 and 29 MPa, respectively . The formatio n of thick intermetalli c compound s on the whole thickness of the weld nugget reduced the joint strength. As the welding speed increased to 40 mm/min the amount of IMCs reduced notice- ably and IMCs only formed at the upper side of the weld. There- fore, the tensile strength increased to 121 MPa. The IMCs asAl5Fe2 and Al6Fe were indentified at the Al/Fe interface and inthe weld nugget, respectively .

2. Using a 4 mm diameter pin resulted in the formation of tunnel defect even at low welding speed (14 mm/min). Bydecreasing the pin diameter to 3 mm, the tunnel was removed

successfu lly at the welding speed of 28 mm/min, however atthe welding speed of 56 mm/min the tunnel defect formed again.

3. By increasing the tool tilt angle to 5� the tunnel was removed asthe result of increased heat input and downward forging force.However , due to the formation of thick IMCs, the joint exhibited no tensile strength as 0 MPa. The IMC thickness measureme nts showed that tool tilt angle has a very effective role in heat gen- eration. Similarly, by increasing the tool plunge depth from 0.4 to 0.8 mm, while the tunnel was eliminated, IMCs formed in the weld nugget and the joint strength decreased from 121 to29 MPa.

4. Using a standard M3 pin tool and at welding speed of 56 mm/ min, the joint showed maximum tensile strength as 246 MPa.The bell shape nugget with a thin layer of IMC at the Al/Fe inter- face (60.5 lm) formed. The threaded pin increased the down- ward forging force and facilitated the downward movement ofthe plasticized material. Therefore, the cavity was filled suc- cessfully and the tunnel and pores disappeared. The results showed that decreasing the plunge depth from 0.4 mm to 0.3 decrease d the joint strength from 246 to 187 MPa. At plunge depth of 0.2 mm the tunnel defect formed again. This result indicated that dissimilar FSW of aluminum 5186 and steel was very sensitive to plunge depth, in a way that a decrease of 0.1 mm in plunge depth decreased the UTS of the weld noticeab ly.

5. In dissimilar FSW of aluminum to steel for preventing tunnel formatio n the pin diameter was more important than its threaded profile.

6. The simplified model for obtaining HIF was used to predict the range of process parameters in which the proper joint strength could be obtained. Based on this model, a welding window was demonst rated for FSW of Al 5XXX series to mild steel.

Acknowled gments

The authors gratefully acknowledge Dr. M.G. Maraghe h and Mr.M. Panahi for their staunch support and providing the welding ma- chine. The authors would like to thanks Mr. A.R. Taghizadeh , Mr. M.Zahmatkes h, Mr. M. Farahani, Mr. F. Hasanab adi and Mr. O. Dastani for assistance in performi ng the tests and laborator y evaluations.

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