JOURNAL OF ADVANCES IN MECHANICAL ENGINEERING AND SCIENCES
Joining of AISI 1040 Steel to 6082-T6 Aluminium Alloy by Friction Welding
*CH. Muralimohan1, S. Haribabu2, Y. Hariprasada Reddy2 V. Muthupandi 1, K. Sivaprasad 1
1Department of Metallurgical and Materials Engineering, National Institute of Technology
Tiruchirappalli 620015, Tamilnadu, India
2Department of Mechanical Engineering, Sri VenkatesaPerumal College of Engineering and Technology
Puttur 517583, Andhra Pradesh, India
*
ABSTRACT
AA 6000 series are being used as automotive applications due to the competence to be strengthened by factitious aging subsequently forming. The joining of aluminium to itself as well as other materials becomes increasingly important. Joining of aluminium to steel offered distinctive metallurgical and mechanical properties and recognition of suitable joining methods are challenging. This paper describes a friction welding of 6082-T6 aluminum alloy to 1040 steel via several processes parameters. The influence of friction time and forging pressure on mechanical and metallurgical properties were evaluated. Results of these analyses showed that strength of the welded joints on the order of 220 MPa. The tensile strength made little difference by a rise in friction time under constant forging pressure. However, at the welds made under constant friction time, the ultimate tensile strength of the welds increased through gradual rise in forging pressure. Hence, the forging pressure acting a major role over the influencing of other two major parameters such as friction pressure and friction time on strength of the welds. Microstructural characterization was done by using optical and SEM analysis. The elemental composition of welds was analyzed by EDS technique. The presence of Al-Fe chemical species were characterized via a thin layer of transition zone on bond lines and the significance of percentage of compositions seems related to the Fe2Al5 and FeAl intermetallic compounds. The tensile fracture of the welded joint occurred in 6082-T6 aluminium side near the interface. The characterization of fracture surfaces reveals the pattern of dimples structure.
Keywords:Aluminium,steel,friction welding, dissimilar metals, mechanical properties, microstructural characterization.
1
JOURNAL OF ADVANCES IN MECHANICAL ENGINEERING AND SCIENCES
1. INTRODUCTION
Friction welding process is a solid state welding technique; it has been developed and achieved to more advanced state encompassed by other welding methods. The importance of this friction welding method has been expanded the prominence in the manufacturing industry. The dominance of friction welding influencing the most effective welding method of analogous and divergent metals for the maximum inexpensive and higher production rates and greatly efficient joints, higher in emulate, very less fabrication time and low vitality input [1]. The utmost significant specifications in friction welding are friction pressure, friction time, forging pressure, forging time and rotating speed [2, 3]. The parameters impacting friction welding has been deliberated antecedently [4]. Friction welding processes experienced to heating of welding samples to get the anticipated temperatures at the faying façades with the effect of friction, developed by rubbing weld surfaces of the joining parts.
The joining of contradictory materials passes the probable to exploit the benefits of dissimilar materials repeatedly given that distinctive elucidation to many industrial applications [5, 6]. The main object to join dissimilar metals is owing to the amalgamation of beneficial properties of mechanical and microstructure of one object and either extensive corrosion resistance or low specific weight or virtuous properties of electrical of the other substantial metals [7, 8]. In recent years, the joining processes for dissimilar materials demanding especially in automobile industry outstanding the saving weight of vehicle accessories and body structures. It is essential to magnify the usage of trivial structures in automobile industries has enlarged attention in the practice of both aluminium (Al) and carbon steels as important construction components. Though, the cost of aluminium related to carbon steels limits its applications intended for many industries. For instance a result of this, it is necessary to produce joints between aluminum and steel. However, it is very difficult to produce a reliable joint because of their poor metallurgical compatibility and mismatch in mechanical properties. Conventional fusion welding cannot be feasible to make a reliable joint due to the occurrence of stress concentration, chemical segregation and formation of secondary intermetallic phases.
Literature reports that experiments had overseen on the joining techniques of steel to aluminium ranges from conventional welding methods to sold state welding procedures such as gas metal arc welding [9], explosive welding [10], solid state welding methods such as friction stir welding techniques [11] and vacuum diffusion bonding processes [12]. Many of the studies on friction welding of similar materials available, but the challenging in joining of dissimilar materials studied by Murti et al. on friction welding [13]. They were attentive a practical and theoretical studies over optimization of processes parameters in joining of dissimilar metals by friction welding technique. The investigation has done by Yilbas et al. [4] on physical and microstructural characterization on friction-welded structural steel-aluminium alloys, and aluminium to copper. The behavior of friction welding on aluminium and its alloys were investigated by Yilbas and his co-workers [14]. The examinations of microstructure evaluation on 7075 Al with friction-stir process [15]. The characteristic investigation of Fukumoto et al. [16] on amorphization processes allying Al alloy and stainless steel (SS) by friction welding process. The strength of the aluminium alloys with the carbon steel welds impacts by some of the inadequacies existed being micro-cracks, weld pores, unwanted secondary phases and alterations. The character of a weld is comprehensively depending on the proper optimized parameters, these will establish the metallurgical alterations and tension strength of the welded components. One of the friction welding parameter such as friction time is influencing other parameters, the gradual increment in this, a wide transmission area next to the weld line with presence of secondary particles. For low friction pressure, short friction times with less in forging pressures the weld in fragile at the joint interface and these leads to the formation of inappropriate bonds with the presence of voids. To accomplish maximum efficient joints the friction time necessary to maintain as low as required, although the friction and forging pressure can be taken for higher ranges [17]. Therefore, the aim of the present experimentations is to examine the characterization of metallurgical properties and changes in mechanical properties varying with process parameters of friction welded joints of 6082-T6 aluminium to AISI 1040 steel.
2.MATERIALS AND METHODS
In this study, the materials AISI 1040 steel and AA 6082- T6 aluminium rods of diameter and length were machined in the dimensions of 10 mm and 80 mm respectively. The X-ray fluorescence (XRF) spectroscopy analysis has made to resolute the elemental admixture of the parent materials, as shown in (Table 1). The base materials were tested for the assessment of physical properties, as shown in (Table 2).
Table 1.Chemical composition (wt.%) of base materials
AA 6082 / Si / Fe / Cu0.7-1.3 / 0.4 / 0.1
AISI 1040 / C / P / S
0.36-0.45 / 0.04 / 0.05
The continuous drive friction welding method was used to perform the friction welding joints. The welds which were made between AISI 1040 steel and AA 6082- T6 aluminium by friction welding, each deformation resistance differs greatly, in that the aluminium base metal deforms by plastic deformation during joining. The aluminum alloy maintained as a rotating part and 1040 steel sample positioned in a static condition. Resulted welds obtained the circular flash diameter on Al side is larger than the steel side, so that the flash was machined on lathe to required size. In this experiment, the friction welding parameters such as friction time, forging pressure and friction pressure are varied while the speed of the rotating part per minute and forging time held at constant. Parameters employed in performing the various trails are given in (Table 3). After completion of the welding the sample taken out from the machine and tensile sample is prepared as per ASTM-E8 standard. The specimens for V-notch impact of 55x10x10 mm and V-notch is prepared at weld interface of 2 mm depth and 45° angle. The toughness of the resulted welds is recorded at with increase in forging pressure and increase in friction time. The metallographic techniques were applied for all the welds made by different process parameters. The evaluation of joint strengths was conducted after making joints immediately performed a drop test for all the welds at various welding parameters.
Table 2.Mechanical properties of materials used in the experiment
Samples / YS (MPa) / UTS (MPa)AA 6082 / 215 / 240
AISI 1040 / 348 / 510
Microstructural observations of the as-received rods and weld interfaces were accomplished via optical metallography technique and scanning electron microscope (SEM) analysis. Frictions welding of the sectioned transverse welds were prepared for microstructural observation as per standard metallography procedures. Energy dispersive X-ray spectroscopy (EDS) studies were executed to confirm the intermetallic formation and elemental distribution at the weld interface. Microhardness survey was performed at the bond interface of the joints and the zones immediate both the steel and aluminium alloy by with a digital microhardness tester. A load of 300 g was applied for 10 s for 1040 steel, and 100 g for 10 s for aluminium alloy. The tensile failure regions of the resulted joints existed examined using SEM for the fracture location and the mode of failure.
3.RESULTS AND DISCUSSION
3.1.Microstructure Characterization
The resulted friction welded joints evinces the existence of weld flash of mixed material owing to severe plastic deformation with frictional heat and movement of aluminium in the interface of heterogeneous welds. Joint flash formed at the aluminium side while the steel side is not externally changed. (Figure 1) shows the macro structural features of the interfaces with increasing upset pressure. With an increase of forging pressure, the amount of flash increases and the shape of the flash
Figure1.Macrograph showing the transverse section of the weld with interface
Figure 2.Optical microstructure of the 6082 Al alloy base material
tilts toward aluminium from its original radial direction. The center of faying surfaces has a flat shape regardless of forging pressure the periphery of joint is a mechanical mixer of aluminium and steel. This is due to the periphery of joint is faster than the center of the joint, so the frictional heat of the periphery of the joint is larger than the central part. Consequently, at the periphery of the joint plastic flow is easier than in central part. This produces an increase of surface area and is deliberated to contribute to joint strength increases [18].
The microstructure of the base material 6082-T6 aluminium alloy is depicted in (Figure 2), with the insubstantial grains formed in equiaxed structure. Microstructure details existed studied at the aluminium–steel interfaces. In all the joints due to the effect of high rotational speed combined with friction pressure and upset pressure, aluminium grains close to the interface are mechanically deformed and recrystallized as fine equi-axed grains. With an increase in temperature due to friction the yield strength of aluminium decreases and the atomic diffusivity increases which results in more interfacial deformation and facilitates the metallurgical bonding. The presence of deformed grains in steel indicates that deformation is not confined to aluminium because of its low strength and melting point but steel is also subjected to some amount of deformation in the region close to the interface. Itcan be seen from (Figure 3) SEM image of the Al–steel interfaces. The SEM images reveal good bonding along Al–steel interfaces of the friction welded joints, and interfaces are free from discontinuities and micro-cracks. The interface between aluminium and steel is characterized by very thin diffusion transition layer was revealed. The transition layer thickness increases with increasing friction time. The elongate be influenced by of the transition layer wideness owing to the development of weld lines are instigated by diffusion is entails the square root of the friction time [17]. The transition layer performs through homogeneous and seems with a pale overcast divergence.
Figure3.Scanning electron microstructure of the aluminium–steel weld interface
Figure4.EDS spot scan spectrum of the Al–steel weld interface
(Figure 4) shows the EDS spot analyses were made across the sliced transverse sections of the optimal welds produced with friction pressure of 90 MPa, forging pressure p2–180 MPa and friction time t1–4 s. The EDS energy spectrum (Figure 4) of weld interface as results of chemical analysis made at different locations are recorded. The average chemical composition of elements are Al is ~52.8–60.5 at.% and Fe is ~46.2–50.8 at.%. It is worth noting that establishment of the chemical anatomy at the Al–steel interface; the Al–Fe compound diagram indicates the possibility of presence of FeAl and Fe2Al5intermetallic phases [19]. The formation of these secondary species leads to degradation in the tensile properties.
3.2.Mechanical Properties
The micro-hardness survey was conducted across the Al–steel joint interface covering base material and abutting weld material affected by frictional heat is demonstrates in (Figure 5). It can be envisage from results showing eventually the analogous tendency in hardness distribution away from interface. Whereas, in interface area it deliberates hardness profile slightly strengthens with escalate in forging pressure. The microstructural tendency counsel that inadequate friction pressure and steepforging pressures excel to utmost hardness. These changes caused due to the slighter frictional heat input accessible at the interior resulting outrageous strain hardening effect. The highest hardness value on aluminium side near the weld lines is directly associated to the microstructure crystallized in the weld as a result of high density dislocations during extensive plastic deformation. However, there is no appreciable increase in hardness in 1040 steel side equated to original hardness of the parent material representing that strain hardening is less and the extent of deformation is limited on steel compared to aluminium. The peak hardness recorded at Al/steel interface can be attributed to the formation of intermetallic compounds of Al–Fe and microstructural formation.
Figure5.Hardness profile showing the
variation in hardness at center and periphery regions recorded at the joint interface
Figure6.Variations of tensile strength with forging pressure (p2)
(Figure 6) shows the changes tensile strength and amount of upset with increasing in forging pressure. The resulted tensile strength of the welds enhanced slightly through an augment in forging pressure overt the 160 MPa. Whereas the amount of upset increased constantly with increasing forging pressure, in this regard forging pressure of 160 MPa does not contribute to tensile strength elevation regardless of friction time. Therefore, the optimal weld strength of 220 MPa attained at
friction time t1–4 sec, and forging pressure p2–180 MPa. Though, in all the joints tensile fracture occurred at aluminum side nearer to weld interface with ductile mode.
(Figure 7) shows the variation in tensile strength with friction time for each forging pressure. When the forging pressure is 140 MPa, which is the lowest pressure in the present experimentation, thought the additions of friction time, the strength of friction welded joint is dispirited. That is, however the thermal degradation region increase with friction time, the lowest pressure cannot discharge thermal degradation region from the interface completely [18]. Hence, it is considered that the effect of welding parameters such as forging pressure affected distantly on tensile strength plays a major role beyond the other parameters are friction pressure and friction time, which is mainly under the control of forging pressure.
Impact strength results inveterate the influence of temperature on welds. The toughness of the welds varying through different welding processes and with varies temperatures. Hence, the toughness is depleting with inappropriate assortment of process parameters. The inflated forging pressure ameliorates in associating the bonding of the joints. The experimental impact test results are increased with forging pressure and the maximum strength of 36 J attained at forging pressure-160 MPa, friction pressure–100 MPa and friction time–2 sec. Impact strength is decreasing with enhancement of friction pressure due to the heat development at the bonding area is affected on grain coarsening. Toughness and tension strengths intensifying with amplifying the forging pressure, and this resulting due to equi-axed granulated grain pattern with exorbitant strain hardening effect on heat affected zone and bond lines
3.3.Fracture Surface Analysis
Failure occurred at aluminium base metal is adjacent to the weld interface for all trails. To investigate the etiquette of the fracture was characterized by scanning electron microscopy and the exhibited fracture graphs are materialized in (Figure 8). The SEM fractured graphs propounded that the fracture is assorted manner. Some of the fracture morphology depicted that the tensile shearing surface due to the effect of pulling test. The mode of failure is purely ductile in nature with formation of dimple structure.
Figure 7.Effect of friction time on tensile strength at constant forging pressures
Figure8.SEM fracture showing the
morphology of joint with ductile failure
4.CONCLUSIONS
AA 6082–T6 aluminium to AISI 1040 steel was effectively accomplished by friction welding processes. The alterations in microstructure through welding of bonded specimens were studied in circumstance. The effects of welding process parameters on microstructural interchanges, microhardness and tension tests of friction welded joints were explored and the reciprocity at metallurgical structures and fracture durability weredisputed. The below are the some of the existed conclusions in the study.