Influence of the coupling between the mechanical characteristics and the welding conditions by the FSSW process : case of the bi-material aluminum-steel

he increasing demand for energy saving as well as environmental issues in different industry sectors has led to the necessity of using lightweight alloys and advanced high strength steels. Such interest has created the need to develop reliable joining technologies which can enable multilateral design and low cost fabrication processes [1, 2]. The assembly between two different materials (Al/Steel, Al/Cu,...) by conventional technologies of welding by fusion is rather complicated and can be due completely difficult to their various physical and chemical properties, in particular with the difference in the cast iron temperatures and with mutual solubility. Furthermore, these processes have the disadvantage of severe heating and cooling cycles which can result in the formation of brittle cast structure, porosity and large amounts of intermetallic phases (FenAlm) that have a detrimental effect on the mechanical properties of the joint, multiaxial fatigue, fatigue properties and fracture mechanism of load [16, 17, 29, 30]. Brazing has also been used as a tentative process to join aluminum and steel. Strong joints have been formed using a eutectic Al–Si brazing alloy. However, holding time must be kept as minimum as possible to avoid the formation of a second, more fragile FenAlm intermetallic layer [18]. Other processes ABSTRACT. The present study investigates the effect of joining parameters on the mechanical characteristics of dissimilar friction stir spot welding (FSSW) between aluminum alloys and galvanized steel. Mechanical performance has been evaluated by shear and microhardness testing. A macrostructural examination has revealed the creation of mechanical interlocking in the Al steel connections. Shear failure load has increased with increasing both tool rotational speed and plunge depth for all FSSW connections. Higher plunge depth has improved the mechanical interlocking between lower and upper sheet due to the formation of hook.

Influence of the coupling between the mechanical characteristics and the welding conditions by the FSSW process: case of the bi-material aluminum-steel INTRODUCTION he increasing demand for energy saving as well as environmental issues in different industry sectors has led to the necessity of using lightweight alloys and advanced high strength steels.Such interest has created the need to develop reliable joining technologies which can enable multilateral design and low cost fabrication processes [1,2].The assembly between two different materials (Al/Steel, Al/Cu,…) by conventional technologies of welding by fusion is rather complicated and can be due completely difficult to their various physical and chemical properties, in particular with the difference in the cast iron temperatures and with mutual solubility.Furthermore, these processes have the disadvantage of severe heating and cooling cycles which can result in the formation of brittle cast structure, porosity and large amounts of intermetallic phases (Fe n Al m ) that have a detrimental effect on the mechanical properties of the joint, multiaxial fatigue, fatigue properties and fracture mechanism of load [16,17,29,30].Brazing has also been used as a tentative process to join aluminum and steel.Strong joints have been formed using a eutectic Al-Si brazing alloy.However, holding time must be kept as minimum as possible to avoid the formation of a second, more fragile Fe n Al m intermetallic layer [18].Other processes ABSTRACT.The present study investigates the effect of joining parameters on the mechanical characteristics of dissimilar friction stir spot welding (FSSW) between aluminum alloys and galvanized steel.Mechanical performance has been evaluated by shear and microhardness testing.A macrostructural examination has revealed the creation of mechanical interlocking in the Al steel connections.Shear failure load has increased with increasing both tool rotational speed and plunge depth for all FSSW connections.Higher plunge depth has improved the mechanical interlocking between lower and upper sheet due to the formation of hook.used for joining Al/steel, particularly in the automotive industry, are self-pierce riveting [24], mechanical clinching as well as adhesive bonding coupled with another joining technique [25].By using solid state welding processes, such as friction welding [19,20,27], diffusion bonding [21] magnetic pressure/ pulse seam welding [22] and ultrasonic welding [23], sound Al/steel joints have been produced.However, these processes have several disadvantages such as longer joining times as well as geometry requirements, among others.Although the formation of the intermetallic phases is kept as minimum as possible due to the minimised energy input, a very thin layer of FenAlm phases is still formed at the Al/steel interface in all aforementioned processes.The Friction Stir Welding (FSW) process was developed by TWI in 1991 [10], as a novel method for joining Al-alloys and, since that time, the welding process has been employed when fabricating non-ferrous alloys (aluminum, titanium, magnesium, zinc and copper alloys), as well as steel and thermoplastic substrates having thicknesses from 1 to 50 mm.The FSSW process is a variant of the FSW technique [13,28], in which the tool is not traversed and instead creates the joint by plunging into and retracting out the tool of the overlapping sheets [14,15].The influence of welding parameters on the failure load properties of Al/Al-alloy friction stir spot welds has been examined at length [5,11,12].A schematic illustration of the FSSW process joining two metal sheets is shown in Fig. 1.As shown in this figure, this process consists of three stages.First a rotating tool R S with a threaded probe is plunged into the upper sheet with a clockwise rotation rate RS of 1000, 1400, 2000 rpm and a plunge rate Vp of 16 mm/min and with penetration depth of length D p (mm).The process control is a displacement (position) control.Second when a shoulder contact the upper sheet, the force control (load) is used, characterised by the tool being driven into the specimen at a rate controlled by the axial force on the tool.The plunging continues as the axial force F increases until a predetermined value F of 1400Kg is reached.The force is held constant for a 2 seconds dwell time to generate a frictional heat.The heated and soften material, is plastically deformed and a solid state bond is made between the sheets.After which, thirdly the tool is retracted with a withdrawal rate Vw of 100 mm/min [4], each parameter in this study is characterised with the software MODDE 5.0 (Modeling and Design) [26] is used for the model elaboration and the statistical analysis of the experimental design.The present work concerns on the study of rupture of a FSSW assembly which consists of single lap of two different thin sheets of aluminum alloy 6061 T5 and galvanized steel having thickness 1.7mm.

MATERIALS AND COMPOSITION
ommercial grade AA 6061 alloy and galvanized steel sheets of 1.7mm thick were used as the starting materials.AA 6061 is in the T5 condition in the as received condition and a galvanized steel sheet.Their nominal chemical compositions are presented in Tab. 1.

C
Table 1: Chemical compositions of AA 6061-T5 and Galvanized steel.

EXPERIMENTAL METHODS
The welding machine he basic principle of process FSSW is carried out on a modified conventional milling machine.This machine is largely used for the operations of machining in industry.It is one of the first machines used to carry out FSW.It can apply up to 360 KN of axial load [5], which makes it possible to weld plates of aluminium alloys until 15mm thickness.The tool of welding and the system of fastening is shown in Fig. 2.

Joint configuration, tool and its geometry
The main dimensions of the tool made of height strength steel are given in Fig. 3.The fixed probe has a diameter of 5 mm and a length of 1.8 mm.The flat shoulder has a diameter of 10 mm.These geometrical characteristics were selected on the basis of preliminary tests developed by several researchers [5][6][7][8].Dimensions of the assembly in Fig. 4 are fixed according to relations W/H= 2 where W is the width of the specimen and H the distance between the center of the exit hole and the free edge of the substrate in the longitudinal direction [9].The thickness of substrates is 1.7 millimetres and shims of same thickness bulk material were used for the specimens to maintain alignment in shear tensile testing.The monotonic tensile tests were carried out on an electromechanical tensile testing machine.The crosshead displacement speed is set at a rate 0.6 mm min -1 .Spacer was used during testing to compensate for the offset created by the lap joint.

The welding parameters
The welding parameters used for all FSSW welds are listed in Tab. 2. The downward and upward welding speed ranged at 16 mm/min.The rotational speed of the tool ranged from 1000 to 2000 rpm and the penetration depth 1.9 to 2.7 mm.

Temperature profile
he layout of thermocouples is shown in Fig. 5.The transient temperatures from the thermocouples were recorded.The thermocouple placed approximately under the center of stir zone.In Fig. 6 a maximum temperature of the experimental test that it was detected for a penetration of 1.9 mm was approximately 260 °C for the sample with tool rotating speed of 2000 rpm and the minimal is détected with 1000 rpm at 110 °C.The maximum temperature of experimental result that detected for penetration of 2.3 mm was approximately 322 °C for the sample with tool rotating speed of 2000 rpm, and the minimal is detected with 1000 rpm at 175 °C.The maximum temperature of experimental test that was detected for penetration of 2.7 mm was approximately 455 °C for the sample with tool rotating speed of 2000 rpm, and the minimal is detected with 1000 rpm at 287 °C.The temperature profiles have a uniform plot during the welding process which is trending symmetrically toward the peak of thermal cycles, and dropping axisymmetrically after passing through the maximum temperature.

Tensile strength test
The aim of this phase is to study the tensile stress-shear on plates welded by friction stir spot welding FSSW and the determination of the optimum parameters on specimens welded by a welding point.Fig. 7 presents the tensile curves (load-displacement) of structure welded by friction stir spot welding (FSSW) assembly with 1 point.Fig. 7(A) present the tensile curves of structure welded by penetration depth fixed in 2.5 mm and three different rotational speed (R S =1000, 1400 and 2000 rpm).It can be seen the best performing sample was that with R S =1400 rpm at 2925 N. The worst performing sample was that which has welding parameters of R S = 1000 rpm with 900 N. We observe that the resistance of a welded point by FSSW depends on the tool rotation speed and this is proportional.Fig. 7(B) show the tensile curves of load-displacement of welds made by friction stir spot welding (FSSW) assembles with a rotation speed 1400 rpm and different penetration depth (D P = 1.9; 2.3 and 2.7 mm).It is observed that the parameters D P = 2.7 mm gives the best tensile strength with 3322 N, and the weakest tensile strength was obtained for DP = 1.9 mm.From these results we can observe the influence of the penetration depth D P on the mechanical behaviour of the joints welded by (FSSW).
According the analysis done with MODDE 5.0 for the tensile curves (load-displacement) of the structures welded by FSSW, the widening while taking into account the interaction between two parameters ( penetration depth and rotational speed).We can be seen in Fig. 8    The graphic exploitation of the results of measurements of the microhardness is carried out on the Tecplot software Fig. 11.From Fig. 11 is possible to observe that under conditions with higher D P (A and B) higher SZ hardness have been obtained.The RS also plays an important role in the hardness behavior, higher RS (B and C) has resulted in higher SZ hardness values.High results of microhardness tests is on the rise of galvanized steel in the sheet of aluminum AA 6061.However, care must be taken while analysing the influence of higher D P since excessive penetration of the tool could cause excessive thinning of the upper sheet leading to a decrease in the weld strength.

Figure 1 :
Figure 1: Schematic illustration of stages of FSSW process.

Figure 2 :
Figure 2: Clamping of the joint configuration on the FSSW machine.

Figure 8 :
Figure 8 : Analysis with MODDE 5.0 of the tensile curves (load-displacement) of the structures welded by FSSW (a).The tensile curves with different parameters (b) Effects of the different parameters on the load-displacement.Microhardness Fig.9 present the method used for the microhardness measurements on three cross sections (A,B and C) of FSSW welded with different parameters, The results of the microhardness tests are presented in fig.10,When the results on BM of the Aluminum alloy 6061 T5 and Galvanized steel is approximately 56 HV and 190 HV respectively.Minimum hardness values in the Aluminum alloy have been found at the TAZ/ TMAZ regions with values varying from 35 HV to 65 HV depending on the FSSW parameters used.In the AA 6061 Al alloy, Hardness was always higher at the BM; SZ hardness values have varied between 65 HV and 90 HV.In the lower sheet Galvanized steel, SZ hardness has varied between 250 HV and 740 HV, whereas slightly higher hardness values than those observed in the BM have been found in the transition region TAZ/ TMAZ (between 150 HV and 220 HV).These values are consistent with the cartography found at the SZ and transition zone TAZ/TMAZ under all conditions.

Figure 9 :Figure 10 :
Figure 9:Schematic illustration of the Method of the microhardness measurements.

Figure 11 :
Figure 11 :Cartography of Microhardness profiles of cross-sections of dissimilar FSSW between AA 6061 and Galvanised steel; (a) (R S 1000 rpm and D P 1.9 mm); (b) (R S 1000 rpm and D P 2.7 mm); (c) (R S 2000 rpm and D P 2.7 mm).