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steel friction-welded components
Materials Characterization 49 (2003) 421 – 429
Interface properties of aluminum/steel friction-welded components M. Yılmaza, M. C¸o¨l a,*, M. Acetb b
a ¨ niversitesi, I˙zmit, Turkey Metalurji ve Malzeme Mu¨hendislig˘i Bo¨lu¨mu¨, Kocaeli U Tieftemperaturphysik, Gerhard-Mercator Universita¨t Duisburg, D-47048 Duisburg, Germany
Received 23 September 2002; received in revised form 28 March 2003; accepted 28 March 2003
Abstract The study of the metallurgy of the interface of metal/metal friction-welded components is essential for understanding the quality of bonding. We have studied, through optical and electron microscopy, and tensile strength measurements, the bonding properties of Al and interstitial free steel and Al and stainless steel frictionwelded components. The samples were produced by varying the friction time and rotational speed, friction pressure, upsetting pressure, and upsetting time constant at optimized values reported earlier. The bonding occurs over an intermetallic phase, which, when too thick, influences the bonding properties adversely. The thickness of the intermetallic interlayer depends linearly of on the square root of the friction time, indicating that the growth is caused by diffusion. The effect of oxidation on the bonding is also studied on samples prepared under argon atmosphere and normal atmosphere. D 2003 Elsevier Science Inc. All rights reserved. Keywords: Metallurgy; Interface properties; Friction welding; Aluminum/steel friction-welded components; Quality of bonding; Bonding properties
1. Introduction Friction welding is a technique that is used to join bulk components essentially having rotational symmetry. In this welding method, the components are brought into contact, and with one of them remaining stationary, the other is rotated while pressure is applied. When the temperature of the interface has reached an appropriate value, the rotation is halted, while the pressure remains unchanged or increased [1 – 4]. This method, while consuming little time, leads
* Corresponding author. Tel.: +90-262-742-3290; fax: +90-262-742-4091. E-mail address: [email protected] (M. C¸o¨l).
to intensive plastic deformation at the welding temperature. The welding of aluminum to steel is of particular interest, since the resulting products join the very different but favorable properties of each component, namely, the high thermal conductivity and low density of Al, and the low thermal conductivity and the high tensile strength of steels [5]. The demand for aluminum/steel and especially aluminum/stainless steel joints has therefore increased in many areas including cryogenic applications, spacecraft, high vacuum chambers and cooking utensils owing to their superior properties. In these structures aluminum has been partially replaced by stainless steel. In this case, it is necessary to join stainless steel to aluminum alloys [6,7]. The earlier application of
1044-5803/03/$ – see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1044-5803(03)00051-2
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Table 1 Tensile and yield strength of various alloys Alloys
Tensile strength (MPa)
Yield strength (MPa)
Alloy
Tensile strength (MPa)
Yield strength (MPa)
Al99.5 AlMg4.5Mn AlZn4.5Mg1 AlMgSi1 AlMgCu2
65 270 350 310 480
20 140 270 260 340
St37-2 C60 25CrMo4 X5CrNi189 X10CrNiMoTi1810
360 – 470 700 – 850 650 – 1100 500 – 750 550 – 700
220 – 240 360 420 – 700 230 200
aluminum/steel friction welding which has resulted in considerable cost saving, is the production of downhanger assemblies. These consist of a mild steel billet joined to aluminum alloy bar, for use in aluminum smelters [8]. The properties of the interface of Al/ steel components depend on the choice of the material to be welded. The most commonly used are pure Al or Al-Mn-Si as the aluminum component, and ferritic carbon steels or austenitic stainless steels as the steel component [9 – 15]. The problems concerning friction welding of different metals is not only associated with their individual properties such as hardness, melting point, etc., but also with the reactions taking place at the interface. These reactions can lead to the occurrence of brittle intermetallic phases or other undesired components. The presence of intermetallic phases at the interface of Al/steel components influences the bonding unfavorably [9 – 12]. These intermetallic phases are FeAl, FeAl2, Fe2Al5 and FeAl3 and the intermetallic phases are stable up to high temperatures. It can be expected that these phases occur at the component interface, thereby affecting the mechanical properties of the welded component [6,12]. The thickness of the intermetallic phase in friction-welded components is an important parameter that contributes to the mechanical properties and, therefore, must be controlled [7]. In Al/steel friction welding, plastic deformation of the carbon steel or stainless steel component has also been observed [5,12]. The deformation causes a reduction in the grain size at the outer sections of the weld leading to an increase in the microhardness. Common to all Al alloys is that an Al2O3 layer on the Al weld component initially acts as a barrier to producing a bond. However, this layer is broken by the strong deformation occurring as a result of high rotational speeds and pressures, allowing for an oxidefree surface of the aluminum component to be welded [9]. Nevertheless, it is possible to observe the presence of Al2O3 layers in microscopic investigations. Imperfections in Al/steel bonds such as cracks, pores, intermetallic phases, unconnected areas, distortions, etc., unfavorably influence the strength of the weld. The quality of a weld is determined by the properly set welding parameters.
The choice of the welding parameters influences the microstructure. If the friction time is held long, a broad diffusion zone with intermetallic phases can be generated. For short friction times, low friction pressures, and low upsetting pressures, the bond is weak, and voids are commonly found. To achieve a high strength, the friction time should be held as short as possible, while the friction and upsetting pressures should be as high as possible [5]. 1.1. The mechanical properties of al/steel frictionwelded components Table 1 gives an overview of the tensile strengths of friction-welded Al/steel components. Aluminum to ferritic or austenitic steel components, using pure Al (Al99.5), exhibit a higher tensile strength than that of the Al99.5 material itself. In tensile strength measurements, these components fail not at the interface, but in the aluminum material. On the other hand, components with aluminum alloys, other than AlMgSi1, having a tensile strength higher than that of Al99.5, fail at the interface and at lower tensions than that applied in the Al99.5 case [5].
2. Experimental Materials in the form of rods with 9.5 mm diameter chosen for the friction welding components in the present experiments were Al of 99.5 purity, IF steel (interstitial free) with a carbon content less than 10 ppm, and AISI 304 stainless steel. In the present
Table 2 Friction welding parameters of Al/IF steel samples (AlC) welded under a normal atmosphere Sample
n (rpm)
pfr (MPa)
pup (MPa)
tup (s)
tfr (s)
Preheat temperature (C)
AlC07 AlC12 AlC17
2300 2300 2300
20 20 20
160 160 160
1 1 1
7 12 17
1000 1000 1000
M. Yılmaz et al. / Materials Characterization 49 (2003) 421–429 Table 3 Friction welding parameters of Al/IF steel (AlC) and Al/ stainless steel (AlP) samples welded under an argon atmosphere Sample
n (rpm)
pfr (MPa)
pup (MPa)
tup (s)
tfr (s)
Preheat temperature (C)
AlC20 AlC30 AlC40 AlP20 AlP30
2300 2300 2300 2300 2300
20 20 20 20 20
160 160 160 160 160
1 1 1 1 1
20 30 40 20 30
1000 1000 1000 1000 1000
experiments, the friction pressure, upsetting pressure, and the upsetting time are held constant at the optimum values reported in earlier studies. Only the friction time is varied to investigate its influence on the properties of the bonding. The first group of Al/IF steel friction-welded samples were prepared under a normal atmosphere. The friction time was between 7 s and 17 s. To compensate for the very different melting temperatures and the plastic deformations of Al and IF steel, preheating up to about 1000 C was applied to the IF steel component over a region of about 40 mm from the interface for 3 – 4 minutes. In this manner some homogeneity in the temperature distribution over the two components was achieved. The welding commenced directly after preheating. The experimental conditions for the friction welding of Al to IF steel are collected in Table 2.
Table 4 Tensile strength of friction-welded Al/IF steel (AlC) and Al/ stainless steel (AlP)
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Table 5 Tensile strength of friction-welded Al/Al2O3 (AlS07) and IF steel/Al2O3 (SC) Sample Test. Test specimen Tensile properties No Diameter Area Load Strength Mean strength (mm) (mm2) (N) (MPa) (MPa) AlC07 SC200
01 01 02 03 SC400 01 02 03 SC1300 01 02 03
9.5 10 10 10 10 10 10 10 10 10
70.84 3825 54 78.54 – – 78.54 – – 78.54 – – 78.54 103 1.3 78.54 114 1.5 78.54 132 1.7 78.54 194 2.5 78.54 – – 78.54 – –
54 –
1.5
2.5
To minimize oxidation, the preheating and the friction welding were preformed under an argon atmosphere on a second set of samples of Al/IF steel and Al/stainless steel. Ar-gas was simply blown through an orifice directed onto the preheated area and the weld interface. In this group of samples, other than Al/IF steel samples, Al/stainless steel samples were also produced. Due to the cooling effect of Ar, the friction times were chosen between 20 s and 40 s, which is considerably longer than for the samples of the first group. The experimental conditions are given in Table 3. The properties of the samples were investigated by various macroscopic and microscopic techniques. The mechanical properties were investigated by tensile and hardness tests. The samples were turned to remove wings before tensile testing. The hardness tests over the whole of the sample were preformed
Sample Test Test specimen Tensile properties No. Diameter Area Load Strength Mean strength (mm) (mm2) (N) (MPa) (MPa) AlC07 AlC12 AlC17 AlC20 AlC30 AlC40 AlP20 AlP30
17 18 11 13 05 09 21 22 31 32 41 42 51 52 61 62
9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 8.0 8.0 8.0 8.0
70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 50.24 50.24 50.24 50.24
4000 4800 3000 3450 4600 3650 3450 1250 3100 3650 3100 2300 4200 4550 4300 1150
57 68 42 49 65 52 49 – 44 52 44 – 84 91 86 –
63 46 59 49 48 44 88 86
Fig. 1. Variation of tensile strength with friction time for steel – Al joints.
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3. Results 3.1. Mechanical tests
Fig. 2. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph as a fracture surface (IF steel side) area 1: fracture at the interface, area 2: Alremaining on the steel after the failure.
The result of the tensile strength measurements for the Al/steel friction-welded pair are given in Tables 4 and 5. Fig. 1 compares the friction time dependence of the tensile strength of the Al/steel and Al/stainless steel components. From this figure it can be seen that the tensile strength of the Al/stainless steel bond is appreciably larger than that of Al/IF steel. The tensile strength of the Al/IF steel bond is seen to decrease slowly with increasing friction time. The tendency appears to be similar for the Al/stainless steel bonds. However, the limited number of samples in this group prevents an accurate conclusion being drawn. In tensile
using the Vickers method under a load of 100 g. However, since the interface region is narrow, hardness tests near this region could be investigated only under a load of 20 g. Scanning electron microscopy was used to investigate the surfaces obtained after tensile failure. The compositions of the material formed at the interface were determined by quantitative energy dispersive X-ray analysis (EDX). Optical microscopy was used in metallographic studies, for which samples were prepared by standard micro- and macroetching techniques. To improve intermetallic contrast, an interference contrast layer was cathodically deposited in air by magnetron sputtering using a Cu cathode.
Fig. 3. Microhardness profile of (a) Al/Fe and (b) Al/stainless steel joints.
Fig. 4. Macro images of Al/stainless steel joint. (A) Al: aluminum; St: IF steel (Fe); P: region of plastic deformation in Al. (B) Higher magnification of area P.
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test measurements, fractures of these components occur mainly at the interface as seen in Fig. 2—area 1. In all cases, there is always some Al material remaining on the steel after the failure (Fig. 2—area 2). The hardness profile at the interface of Al/IF steel components can be compared to those of Al/stainless steel components, by comparing the results obtained for the AlC30 and the AIP30 samples, which were produced using identical welding parameters. The profiles are shown in Fig. 3a and b. Both samples exhibit the same amount of hardening on the Al side, which is caused by the heavy plastic deformation. Hardening is also observed on the steel side of the interface. The increase in the hardness is found to be more in the stainless steel component as compared to
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that of the IF steel component. High plastic deformation in both the IF steel and stainless steel side of the interfaces is also observed. The deformation causes a strong decrease in the grain size, which leads to hardening in the region of the interface. The stronger increase in the hardness of the stainless steel near the interface can be related directly to the high deformation hardening in austenitic steels in general. 3.2. Metallographical and microanalytical studies of the interface Samples of the friction-welded Al/IF steel rods were cut along their axes, perpendicular to the interface, for metallographical and microanalytical studies.
Fig. 5. Micrograph of Al/IF steel joint for (a) tfr = 20 s; interface layer contrast, (b) tfr = 30 s; interface layer contrast, (c) tfr = 40 s; interface layer contrast, and (d) micrograph of Al/stainless steel for tfr = 20 s: interface layer contrast (IM: intermetallic, O: iron oxide).
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Table 6 The thickness of the interlayers formed at the interface for various friction times Al/IF steel (in air)
Al/IF steel (in Ar)
Al/stainless steel
Friction 7 12 17 20 30 40 20 30 time (s) Thickness 1.75 2.08 2.53 5.33 5.9 6.55 2.8 3.18 (mm) S.D. 0.4 0.42 0.54 0.59 0.89 0.57 0.26 0.49
Since the deformation is stronger on the Al side, this end of the component was etched in Beraha II for about 10 min. The macro images are shown in Fig. 4A and B. Fig. 4A shows the full cross section and Fig. 4B shows a detail. The strong plastic deformation of Al at the interface, marked with a ‘‘P’’ in Fig. 4A, is clearly seen. Under the influence of the upsetting pressure, Al flows to cover the steel completely. The interference contrast images of Al/IF steel and Al/stainless steel are shown in Fig. 5a – d. The bright spots in the Al component and the bright streak along the Al side of the interface are artefacts caused by topological disturbances due to metallographical sample preparation conditions. The different grey shades of the phase contrast among the figures is related to the different thickness of the interference layer on each sample. Fig. 5a – c are the images of Al/IF steel samples for the respective friction times ranging from 20, 30, to 40 s. The bonding is found to occur on the whole of the interface for all samples. In the sample with tfr=20 s, shown in Fig. 5a, the aluminum matrix is dark, whereas the steel component appears with a light grey contrast. The interface appears as a new phase with a halftone lighter contrast. The boundary of this phase with IF steel is not sharp and contains imperfections. The contrast at the interface region is uniform, indicating that the composition of the interlayer phase is homogeneous. The inclusion designated by ‘‘O’’ in the figure is an iron oxide particle, which is swept away from the interface. In the sample with tfr=30 s (Fig. 5b), the Al matrix is light grey and the steel matrix is dark. The interface, which is dark grey, is somewhat thicker than that in Fig. 5a. An iron oxide particle in the Al matrix is also found here. In the tfr=40 s sample shown in Fig. 5c, Al appears dark with grey inclusions, whereas steel appears as light grey. The interface has become even thicker here. The Al side now contains components of both the interface material, denoted by IM, and oxides. Oxides, swept away from the interface, are found on the Al side in the region of strong plastic deformation in all samples. At longer friction times interface particles are also encountered in the Al matrix.
The appearance of Al/stainless steel components is different when compared to the Al/IF steel components. Fig. 5d shows the interface region of an Al/ stainless steel component for a sample with tfr=20 s. The Al matrix appears with a light grey contrast, and stainless steel is dark. A new phase between the steel and Al components is very narrow and can be seen as a dark grey strip along the boundary on the steel side. The thickness of the interlayers, obtained by averaging over 10 points for Al/IF steel and 5 points for Al/stainless steel and the standard deviations, are listed in Table 6. The interlayer thickness increases with increasing friction time. For the samples produced under an Ar-atmosphere and same friction times, the interlayer thickness in Al/IF steel is found to be about twice as large as that in Al/stainless steel. The linear dependence of the interlayer thickness on the square root of the friction time implies that the growth of the interface is caused by diffusion. As seen in Fig. 6, there are two types of behavior of the friction time dependence of the interlayer thickness. The interlayer thickness grows rapidly in the Al/IF steel component when welded under an Ar-atmosphere. On the other hand, the Al/IF steel component, welded under a normal atmosphere, and the Al/stainless steel component, welded under Ar gas, exhibit a similar growth. In the Al/IF steel samples produced under normal atmosphere, the presence of a thick and porous oxide layer is observed. The Al stainless steel sample exhibits, on the other hand, a thin but dense layer of chromium oxide. Both oxides inhibit the growth of the interface phase. The main difference in the effect of the two oxides is that the iron oxide is swept away from the interface giving way to the occurrence of an
Fig. 6. Variation of thickness of the intermetallic layer with friction time for steel – Al joints.
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Fig. 7. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph (center of sample).
Fig. 8. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph (side of sample).
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interlayer phase, whereas the chromium oxide is only broken at some places allowing for the development of an interlayer phase. Scanning electron microscopy on polished surfaces was performed to obtain information complementary to the results obtained by interface contrast. In Fig. 7, the features of the interface of the tfr = 20 s Al/IF steel sample show that the interface is jagged as it grows into the steel component. Similar growth on a smaller scale is also observed in the Al component direction. Additionally, there are three different phase contrasts within the interface. The IF steel side is bright, and the Al side is dark gray. The interface layer is homogenous and appears with a light gray contrast. EDX analysis of this region gives 63.3% Al, 35.2% Fe, and 1.5% O. If it is assumed that the very small amount of oxygen is contributed from a particle lying within the information depth, the composition of the interlayer corresponds typically to that of FeAl3. A different region of the sample is shown in Fig. 8, where high porosity and other contrast features are found. EDX analysis shows that the area with light gray contrast is composed of 4.5% Al, 57.3% Fe, and 38.2% O. The high oxygen content is an indication of the presence of a dense iron oxide region. The iron oxide is surrounded on both sides by a thin layer of FeAl3, as seen by the darker contrast.
4. Discussion In the Al/metal friction-welded samples prepared within this work, no porous parts or other defects have been observed. On applying the upsetting pressure, Al deforms plastically and regulates the cold hardening at the interface. Although there is no direct observable outward flow of the steel component, microhardness testing indicates the presence of deformation in this region as well. The formation of a porous oxide at the interlayer, caused during preheating and friction under normal atmosphere, decreases the strength of the bonding. Welding under an Ar atmosphere enables a strong interface to develop, however the presence of oxides could not be fully eliminated in the present study. While iron oxide is swept away from the interface area into the plastically deformed Al region almost completely in Al/IF steel components, chromium oxide bonds strongly to the steel matrix in Al/ stainless steel samples. Chromium oxide cannot be readily swept away from the interface region. Therefore, it is only possible for an interlayer phase to form in regions where the chromium oxide does break away from the interface. As often discussed in the literature, an aluminum oxide layer in Al/steel
components is essentially not present, since it is driven away from the interface by the high plastic deformation. In all samples, it is observed that the bonding is mediated by an intermetallic phase. Microanalysis studies show that in Al/IF steel components the intermetallic is FeAl3. This phase is also found to occur in Al/stainless steel samples [12], but occurs only regionally and in limited quantities, and is not uniform as in Al/IF steel components. The intermetallic phase occurring at the interface shows a growth front with microporosity indicating that the front is faceted. This crystallographically oriented growth is observed on the IF steel side. The thickness of the intermetallic layer occurring in Al steel samples is about half the size of that in Al/ IF steel under identical welding conditions and increases with increasing friction time. The interlayer compound FeAl3 grows proportionally to the square root of friction time, Which is an indication of diffusion-controlled growth. This is presented in Fig. 6, where the errors associated with temperature gradients, generation of oxides, etc., are also taken into account. In Al/IF steel friction-welded samples, the tensile strength decreases slightly with increasing friction time. Al/stainless steel components also exhibit similar behavior, but with clearly higher tensile strength. The tensile strength is highly influenced by the thickness of the intermetallic phase at the interlayer. Fractures would be expected to occur as the thickness increases leading to a weakening of the tensile strength. Indeed, Fig. 9 shows as a similar linear dependence of the interlayer of the interlayer thickness on the tensile strength, in which the tensile
Fig. 9. Variation of tensile strength with thickness of the intermetallic layer for Al/IF steel (Fe) joints.
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strength decreases with increasing interlayer thickness. It is seen that at small thickness, the value of the tensile strength approaches that of pure Al.
5. Conclusion In this work we have demonstrated that bonding in Al/steel friction-welded components takes place over an intermetallic phase. The bonding can have sufficient strength only under optimized conditions. When the size of the interlayer is either too thick or too thin, the bonding properties are unfavorable. The ideal case would be to eliminate the intermetallic phase completely, which is possible by the correct choice of an insertion layer to be placed between the welding components. Further study is required to match such insertion material, i.e., composition, thickness, etc., for a particular application.
Acknowledgements This work was supported by Forschungszentrum Ju¨lich and the Turkish Scientific and Technical Re¨ PSAN search Council. We gratefully acknowledge SU A.a. for the use of the friction welding unit for the experiments.
References [1] Yılmaz M. Investigation of welding area in the friction welding of tool steels. Ph.D. Thesis. Yıldız Technical University, I˙stanbul, Turkey; 1993. [2] Weller EF. Metals handbook, vol. 6. Metals Park, OH: American Society for Metals; 1983. p. 719 – 28.
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[3] Wallace FJ, et al. Welding handbook, vol. 3. Miami, FL: American Welding Society; 1980. p. 239 – 62. [4] Vill VI. Friction welding of metals. NY: American Welding Society; 1962. [5] Reiners G, Kreye H. Mikrostruktur und mechanische Eigenschaften von Reibschweissverbindungen aus Aluminium und Stahl. Schweiss Schneid 1988;40:123 – 9. [6] Fukumoto S, Tsubakino H, Okita K, Aritashi M, Tomita T. Microstructure of friction weld interface of 1050 aluminium to austenitic stainless steel. Mater Sci Technol 1998;14:333 – 8. [7] Fukumoto S, Tsubakino H, Okita K, Aritashi M, Tomita T. Friction welding process of 5052 aluminium alloy to 304 stainless steel. Mater Sci Technol 1999;15:1080 – 6. [8] Nicholas ED. Friction welding. Engineering 1970;11:275 – 8. [9] Yılbas¸ BS, S¸ahin AZ, Kahraman N, Al-Garni AZ. Friction welding of St-Al and Al-Cu materials. J Mater Process Technol 1995;49:431 – 43. [10] Ochi H, Ogawa K, Yamamoto Y, Suga Y. Friction welding of aluminum alloy and steel. Proceedings of the 7th International Conference on Offshore and Polar Engineering, Honolulu, USA; 1977. p. 541 – 5. [11] Hartwig H, Kouptsidis J. Schweissverbindungen zwischen AlMgSi0.5 und X5CrNi189 fu¨r das Vakuumsystem von Hochenergiebeschleunnigern. Schweiss Schneid 1978;30:45 – 7. [12] Achar DRG, Ruge J, Sundaresan S. Verbinden von Aluminium mit Stahl, besonders durch Schweissen. Aluminium 1980;56:147 – 9. [13] Ochi H, Ogawa K, Yamamoto Y, Hashinaga S, Suga Y, Ohnishi T. Static strength of friction welded joint of 6061 aluminum alloy to SUS304 stainless steel. Light Met 1996;46:500 – 4. [14] Hartwig H. Schweisseu¨bergangsstu¨cke aus Aluminium und nichtrostendem Stahl. Aluminium 1981;57:615 – 8. [15] Mechsner K, Klock H. Grenzfla¨chengefu¨ge von Reibschweissverbindungen aus Aluminium und Stahl. Aluminium 1983;59:850 – 4.
Interface properties of aluminum/steel friction-welded components M. Yılmaza, M. C¸o¨l a,*, M. Acetb b
a ¨ niversitesi, I˙zmit, Turkey Metalurji ve Malzeme Mu¨hendislig˘i Bo¨lu¨mu¨, Kocaeli U Tieftemperaturphysik, Gerhard-Mercator Universita¨t Duisburg, D-47048 Duisburg, Germany
Received 23 September 2002; received in revised form 28 March 2003; accepted 28 March 2003
Abstract The study of the metallurgy of the interface of metal/metal friction-welded components is essential for understanding the quality of bonding. We have studied, through optical and electron microscopy, and tensile strength measurements, the bonding properties of Al and interstitial free steel and Al and stainless steel frictionwelded components. The samples were produced by varying the friction time and rotational speed, friction pressure, upsetting pressure, and upsetting time constant at optimized values reported earlier. The bonding occurs over an intermetallic phase, which, when too thick, influences the bonding properties adversely. The thickness of the intermetallic interlayer depends linearly of on the square root of the friction time, indicating that the growth is caused by diffusion. The effect of oxidation on the bonding is also studied on samples prepared under argon atmosphere and normal atmosphere. D 2003 Elsevier Science Inc. All rights reserved. Keywords: Metallurgy; Interface properties; Friction welding; Aluminum/steel friction-welded components; Quality of bonding; Bonding properties
1. Introduction Friction welding is a technique that is used to join bulk components essentially having rotational symmetry. In this welding method, the components are brought into contact, and with one of them remaining stationary, the other is rotated while pressure is applied. When the temperature of the interface has reached an appropriate value, the rotation is halted, while the pressure remains unchanged or increased [1 – 4]. This method, while consuming little time, leads
* Corresponding author. Tel.: +90-262-742-3290; fax: +90-262-742-4091. E-mail address: [email protected] (M. C¸o¨l).
to intensive plastic deformation at the welding temperature. The welding of aluminum to steel is of particular interest, since the resulting products join the very different but favorable properties of each component, namely, the high thermal conductivity and low density of Al, and the low thermal conductivity and the high tensile strength of steels [5]. The demand for aluminum/steel and especially aluminum/stainless steel joints has therefore increased in many areas including cryogenic applications, spacecraft, high vacuum chambers and cooking utensils owing to their superior properties. In these structures aluminum has been partially replaced by stainless steel. In this case, it is necessary to join stainless steel to aluminum alloys [6,7]. The earlier application of
1044-5803/03/$ – see front matter D 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1044-5803(03)00051-2
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Table 1 Tensile and yield strength of various alloys Alloys
Tensile strength (MPa)
Yield strength (MPa)
Alloy
Tensile strength (MPa)
Yield strength (MPa)
Al99.5 AlMg4.5Mn AlZn4.5Mg1 AlMgSi1 AlMgCu2
65 270 350 310 480
20 140 270 260 340
St37-2 C60 25CrMo4 X5CrNi189 X10CrNiMoTi1810
360 – 470 700 – 850 650 – 1100 500 – 750 550 – 700
220 – 240 360 420 – 700 230 200
aluminum/steel friction welding which has resulted in considerable cost saving, is the production of downhanger assemblies. These consist of a mild steel billet joined to aluminum alloy bar, for use in aluminum smelters [8]. The properties of the interface of Al/ steel components depend on the choice of the material to be welded. The most commonly used are pure Al or Al-Mn-Si as the aluminum component, and ferritic carbon steels or austenitic stainless steels as the steel component [9 – 15]. The problems concerning friction welding of different metals is not only associated with their individual properties such as hardness, melting point, etc., but also with the reactions taking place at the interface. These reactions can lead to the occurrence of brittle intermetallic phases or other undesired components. The presence of intermetallic phases at the interface of Al/steel components influences the bonding unfavorably [9 – 12]. These intermetallic phases are FeAl, FeAl2, Fe2Al5 and FeAl3 and the intermetallic phases are stable up to high temperatures. It can be expected that these phases occur at the component interface, thereby affecting the mechanical properties of the welded component [6,12]. The thickness of the intermetallic phase in friction-welded components is an important parameter that contributes to the mechanical properties and, therefore, must be controlled [7]. In Al/steel friction welding, plastic deformation of the carbon steel or stainless steel component has also been observed [5,12]. The deformation causes a reduction in the grain size at the outer sections of the weld leading to an increase in the microhardness. Common to all Al alloys is that an Al2O3 layer on the Al weld component initially acts as a barrier to producing a bond. However, this layer is broken by the strong deformation occurring as a result of high rotational speeds and pressures, allowing for an oxidefree surface of the aluminum component to be welded [9]. Nevertheless, it is possible to observe the presence of Al2O3 layers in microscopic investigations. Imperfections in Al/steel bonds such as cracks, pores, intermetallic phases, unconnected areas, distortions, etc., unfavorably influence the strength of the weld. The quality of a weld is determined by the properly set welding parameters.
The choice of the welding parameters influences the microstructure. If the friction time is held long, a broad diffusion zone with intermetallic phases can be generated. For short friction times, low friction pressures, and low upsetting pressures, the bond is weak, and voids are commonly found. To achieve a high strength, the friction time should be held as short as possible, while the friction and upsetting pressures should be as high as possible [5]. 1.1. The mechanical properties of al/steel frictionwelded components Table 1 gives an overview of the tensile strengths of friction-welded Al/steel components. Aluminum to ferritic or austenitic steel components, using pure Al (Al99.5), exhibit a higher tensile strength than that of the Al99.5 material itself. In tensile strength measurements, these components fail not at the interface, but in the aluminum material. On the other hand, components with aluminum alloys, other than AlMgSi1, having a tensile strength higher than that of Al99.5, fail at the interface and at lower tensions than that applied in the Al99.5 case [5].
2. Experimental Materials in the form of rods with 9.5 mm diameter chosen for the friction welding components in the present experiments were Al of 99.5 purity, IF steel (interstitial free) with a carbon content less than 10 ppm, and AISI 304 stainless steel. In the present
Table 2 Friction welding parameters of Al/IF steel samples (AlC) welded under a normal atmosphere Sample
n (rpm)
pfr (MPa)
pup (MPa)
tup (s)
tfr (s)
Preheat temperature (C)
AlC07 AlC12 AlC17
2300 2300 2300
20 20 20
160 160 160
1 1 1
7 12 17
1000 1000 1000
M. Yılmaz et al. / Materials Characterization 49 (2003) 421–429 Table 3 Friction welding parameters of Al/IF steel (AlC) and Al/ stainless steel (AlP) samples welded under an argon atmosphere Sample
n (rpm)
pfr (MPa)
pup (MPa)
tup (s)
tfr (s)
Preheat temperature (C)
AlC20 AlC30 AlC40 AlP20 AlP30
2300 2300 2300 2300 2300
20 20 20 20 20
160 160 160 160 160
1 1 1 1 1
20 30 40 20 30
1000 1000 1000 1000 1000
experiments, the friction pressure, upsetting pressure, and the upsetting time are held constant at the optimum values reported in earlier studies. Only the friction time is varied to investigate its influence on the properties of the bonding. The first group of Al/IF steel friction-welded samples were prepared under a normal atmosphere. The friction time was between 7 s and 17 s. To compensate for the very different melting temperatures and the plastic deformations of Al and IF steel, preheating up to about 1000 C was applied to the IF steel component over a region of about 40 mm from the interface for 3 – 4 minutes. In this manner some homogeneity in the temperature distribution over the two components was achieved. The welding commenced directly after preheating. The experimental conditions for the friction welding of Al to IF steel are collected in Table 2.
Table 4 Tensile strength of friction-welded Al/IF steel (AlC) and Al/ stainless steel (AlP)
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Table 5 Tensile strength of friction-welded Al/Al2O3 (AlS07) and IF steel/Al2O3 (SC) Sample Test. Test specimen Tensile properties No Diameter Area Load Strength Mean strength (mm) (mm2) (N) (MPa) (MPa) AlC07 SC200
01 01 02 03 SC400 01 02 03 SC1300 01 02 03
9.5 10 10 10 10 10 10 10 10 10
70.84 3825 54 78.54 – – 78.54 – – 78.54 – – 78.54 103 1.3 78.54 114 1.5 78.54 132 1.7 78.54 194 2.5 78.54 – – 78.54 – –
54 –
1.5
2.5
To minimize oxidation, the preheating and the friction welding were preformed under an argon atmosphere on a second set of samples of Al/IF steel and Al/stainless steel. Ar-gas was simply blown through an orifice directed onto the preheated area and the weld interface. In this group of samples, other than Al/IF steel samples, Al/stainless steel samples were also produced. Due to the cooling effect of Ar, the friction times were chosen between 20 s and 40 s, which is considerably longer than for the samples of the first group. The experimental conditions are given in Table 3. The properties of the samples were investigated by various macroscopic and microscopic techniques. The mechanical properties were investigated by tensile and hardness tests. The samples were turned to remove wings before tensile testing. The hardness tests over the whole of the sample were preformed
Sample Test Test specimen Tensile properties No. Diameter Area Load Strength Mean strength (mm) (mm2) (N) (MPa) (MPa) AlC07 AlC12 AlC17 AlC20 AlC30 AlC40 AlP20 AlP30
17 18 11 13 05 09 21 22 31 32 41 42 51 52 61 62
9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 8.0 8.0 8.0 8.0
70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 70.84 50.24 50.24 50.24 50.24
4000 4800 3000 3450 4600 3650 3450 1250 3100 3650 3100 2300 4200 4550 4300 1150
57 68 42 49 65 52 49 – 44 52 44 – 84 91 86 –
63 46 59 49 48 44 88 86
Fig. 1. Variation of tensile strength with friction time for steel – Al joints.
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3. Results 3.1. Mechanical tests
Fig. 2. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph as a fracture surface (IF steel side) area 1: fracture at the interface, area 2: Alremaining on the steel after the failure.
The result of the tensile strength measurements for the Al/steel friction-welded pair are given in Tables 4 and 5. Fig. 1 compares the friction time dependence of the tensile strength of the Al/steel and Al/stainless steel components. From this figure it can be seen that the tensile strength of the Al/stainless steel bond is appreciably larger than that of Al/IF steel. The tensile strength of the Al/IF steel bond is seen to decrease slowly with increasing friction time. The tendency appears to be similar for the Al/stainless steel bonds. However, the limited number of samples in this group prevents an accurate conclusion being drawn. In tensile
using the Vickers method under a load of 100 g. However, since the interface region is narrow, hardness tests near this region could be investigated only under a load of 20 g. Scanning electron microscopy was used to investigate the surfaces obtained after tensile failure. The compositions of the material formed at the interface were determined by quantitative energy dispersive X-ray analysis (EDX). Optical microscopy was used in metallographic studies, for which samples were prepared by standard micro- and macroetching techniques. To improve intermetallic contrast, an interference contrast layer was cathodically deposited in air by magnetron sputtering using a Cu cathode.
Fig. 3. Microhardness profile of (a) Al/Fe and (b) Al/stainless steel joints.
Fig. 4. Macro images of Al/stainless steel joint. (A) Al: aluminum; St: IF steel (Fe); P: region of plastic deformation in Al. (B) Higher magnification of area P.
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test measurements, fractures of these components occur mainly at the interface as seen in Fig. 2—area 1. In all cases, there is always some Al material remaining on the steel after the failure (Fig. 2—area 2). The hardness profile at the interface of Al/IF steel components can be compared to those of Al/stainless steel components, by comparing the results obtained for the AlC30 and the AIP30 samples, which were produced using identical welding parameters. The profiles are shown in Fig. 3a and b. Both samples exhibit the same amount of hardening on the Al side, which is caused by the heavy plastic deformation. Hardening is also observed on the steel side of the interface. The increase in the hardness is found to be more in the stainless steel component as compared to
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that of the IF steel component. High plastic deformation in both the IF steel and stainless steel side of the interfaces is also observed. The deformation causes a strong decrease in the grain size, which leads to hardening in the region of the interface. The stronger increase in the hardness of the stainless steel near the interface can be related directly to the high deformation hardening in austenitic steels in general. 3.2. Metallographical and microanalytical studies of the interface Samples of the friction-welded Al/IF steel rods were cut along their axes, perpendicular to the interface, for metallographical and microanalytical studies.
Fig. 5. Micrograph of Al/IF steel joint for (a) tfr = 20 s; interface layer contrast, (b) tfr = 30 s; interface layer contrast, (c) tfr = 40 s; interface layer contrast, and (d) micrograph of Al/stainless steel for tfr = 20 s: interface layer contrast (IM: intermetallic, O: iron oxide).
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Table 6 The thickness of the interlayers formed at the interface for various friction times Al/IF steel (in air)
Al/IF steel (in Ar)
Al/stainless steel
Friction 7 12 17 20 30 40 20 30 time (s) Thickness 1.75 2.08 2.53 5.33 5.9 6.55 2.8 3.18 (mm) S.D. 0.4 0.42 0.54 0.59 0.89 0.57 0.26 0.49
Since the deformation is stronger on the Al side, this end of the component was etched in Beraha II for about 10 min. The macro images are shown in Fig. 4A and B. Fig. 4A shows the full cross section and Fig. 4B shows a detail. The strong plastic deformation of Al at the interface, marked with a ‘‘P’’ in Fig. 4A, is clearly seen. Under the influence of the upsetting pressure, Al flows to cover the steel completely. The interference contrast images of Al/IF steel and Al/stainless steel are shown in Fig. 5a – d. The bright spots in the Al component and the bright streak along the Al side of the interface are artefacts caused by topological disturbances due to metallographical sample preparation conditions. The different grey shades of the phase contrast among the figures is related to the different thickness of the interference layer on each sample. Fig. 5a – c are the images of Al/IF steel samples for the respective friction times ranging from 20, 30, to 40 s. The bonding is found to occur on the whole of the interface for all samples. In the sample with tfr=20 s, shown in Fig. 5a, the aluminum matrix is dark, whereas the steel component appears with a light grey contrast. The interface appears as a new phase with a halftone lighter contrast. The boundary of this phase with IF steel is not sharp and contains imperfections. The contrast at the interface region is uniform, indicating that the composition of the interlayer phase is homogeneous. The inclusion designated by ‘‘O’’ in the figure is an iron oxide particle, which is swept away from the interface. In the sample with tfr=30 s (Fig. 5b), the Al matrix is light grey and the steel matrix is dark. The interface, which is dark grey, is somewhat thicker than that in Fig. 5a. An iron oxide particle in the Al matrix is also found here. In the tfr=40 s sample shown in Fig. 5c, Al appears dark with grey inclusions, whereas steel appears as light grey. The interface has become even thicker here. The Al side now contains components of both the interface material, denoted by IM, and oxides. Oxides, swept away from the interface, are found on the Al side in the region of strong plastic deformation in all samples. At longer friction times interface particles are also encountered in the Al matrix.
The appearance of Al/stainless steel components is different when compared to the Al/IF steel components. Fig. 5d shows the interface region of an Al/ stainless steel component for a sample with tfr=20 s. The Al matrix appears with a light grey contrast, and stainless steel is dark. A new phase between the steel and Al components is very narrow and can be seen as a dark grey strip along the boundary on the steel side. The thickness of the interlayers, obtained by averaging over 10 points for Al/IF steel and 5 points for Al/stainless steel and the standard deviations, are listed in Table 6. The interlayer thickness increases with increasing friction time. For the samples produced under an Ar-atmosphere and same friction times, the interlayer thickness in Al/IF steel is found to be about twice as large as that in Al/stainless steel. The linear dependence of the interlayer thickness on the square root of the friction time implies that the growth of the interface is caused by diffusion. As seen in Fig. 6, there are two types of behavior of the friction time dependence of the interlayer thickness. The interlayer thickness grows rapidly in the Al/IF steel component when welded under an Ar-atmosphere. On the other hand, the Al/IF steel component, welded under a normal atmosphere, and the Al/stainless steel component, welded under Ar gas, exhibit a similar growth. In the Al/IF steel samples produced under normal atmosphere, the presence of a thick and porous oxide layer is observed. The Al stainless steel sample exhibits, on the other hand, a thin but dense layer of chromium oxide. Both oxides inhibit the growth of the interface phase. The main difference in the effect of the two oxides is that the iron oxide is swept away from the interface giving way to the occurrence of an
Fig. 6. Variation of thickness of the intermetallic layer with friction time for steel – Al joints.
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Fig. 7. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph (center of sample).
Fig. 8. SEM micrograph of IF steel – Al joint at a friction time of 20 s; secondary electron micrograph (side of sample).
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interlayer phase, whereas the chromium oxide is only broken at some places allowing for the development of an interlayer phase. Scanning electron microscopy on polished surfaces was performed to obtain information complementary to the results obtained by interface contrast. In Fig. 7, the features of the interface of the tfr = 20 s Al/IF steel sample show that the interface is jagged as it grows into the steel component. Similar growth on a smaller scale is also observed in the Al component direction. Additionally, there are three different phase contrasts within the interface. The IF steel side is bright, and the Al side is dark gray. The interface layer is homogenous and appears with a light gray contrast. EDX analysis of this region gives 63.3% Al, 35.2% Fe, and 1.5% O. If it is assumed that the very small amount of oxygen is contributed from a particle lying within the information depth, the composition of the interlayer corresponds typically to that of FeAl3. A different region of the sample is shown in Fig. 8, where high porosity and other contrast features are found. EDX analysis shows that the area with light gray contrast is composed of 4.5% Al, 57.3% Fe, and 38.2% O. The high oxygen content is an indication of the presence of a dense iron oxide region. The iron oxide is surrounded on both sides by a thin layer of FeAl3, as seen by the darker contrast.
4. Discussion In the Al/metal friction-welded samples prepared within this work, no porous parts or other defects have been observed. On applying the upsetting pressure, Al deforms plastically and regulates the cold hardening at the interface. Although there is no direct observable outward flow of the steel component, microhardness testing indicates the presence of deformation in this region as well. The formation of a porous oxide at the interlayer, caused during preheating and friction under normal atmosphere, decreases the strength of the bonding. Welding under an Ar atmosphere enables a strong interface to develop, however the presence of oxides could not be fully eliminated in the present study. While iron oxide is swept away from the interface area into the plastically deformed Al region almost completely in Al/IF steel components, chromium oxide bonds strongly to the steel matrix in Al/ stainless steel samples. Chromium oxide cannot be readily swept away from the interface region. Therefore, it is only possible for an interlayer phase to form in regions where the chromium oxide does break away from the interface. As often discussed in the literature, an aluminum oxide layer in Al/steel
components is essentially not present, since it is driven away from the interface by the high plastic deformation. In all samples, it is observed that the bonding is mediated by an intermetallic phase. Microanalysis studies show that in Al/IF steel components the intermetallic is FeAl3. This phase is also found to occur in Al/stainless steel samples [12], but occurs only regionally and in limited quantities, and is not uniform as in Al/IF steel components. The intermetallic phase occurring at the interface shows a growth front with microporosity indicating that the front is faceted. This crystallographically oriented growth is observed on the IF steel side. The thickness of the intermetallic layer occurring in Al steel samples is about half the size of that in Al/ IF steel under identical welding conditions and increases with increasing friction time. The interlayer compound FeAl3 grows proportionally to the square root of friction time, Which is an indication of diffusion-controlled growth. This is presented in Fig. 6, where the errors associated with temperature gradients, generation of oxides, etc., are also taken into account. In Al/IF steel friction-welded samples, the tensile strength decreases slightly with increasing friction time. Al/stainless steel components also exhibit similar behavior, but with clearly higher tensile strength. The tensile strength is highly influenced by the thickness of the intermetallic phase at the interlayer. Fractures would be expected to occur as the thickness increases leading to a weakening of the tensile strength. Indeed, Fig. 9 shows as a similar linear dependence of the interlayer of the interlayer thickness on the tensile strength, in which the tensile
Fig. 9. Variation of tensile strength with thickness of the intermetallic layer for Al/IF steel (Fe) joints.
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strength decreases with increasing interlayer thickness. It is seen that at small thickness, the value of the tensile strength approaches that of pure Al.
5. Conclusion In this work we have demonstrated that bonding in Al/steel friction-welded components takes place over an intermetallic phase. The bonding can have sufficient strength only under optimized conditions. When the size of the interlayer is either too thick or too thin, the bonding properties are unfavorable. The ideal case would be to eliminate the intermetallic phase completely, which is possible by the correct choice of an insertion layer to be placed between the welding components. Further study is required to match such insertion material, i.e., composition, thickness, etc., for a particular application.
Acknowledgements This work was supported by Forschungszentrum Ju¨lich and the Turkish Scientific and Technical Re¨ PSAN search Council. We gratefully acknowledge SU A.a. for the use of the friction welding unit for the experiments.
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