Review on Status of Welding for Automotive Application

Pankaj Kumar Chauhan1a, Ayush Kumar1b, Rajesh P Verma2

1Automobile Engineering Department, Graphic Era University, Dehradun

E-mail: apankajchauhan231093@gmail.com, bayushkhokhar.ak@gmail.com

2Asst. Prof., Mechanical Engineering Department, Graphic Era University (GEU), Dehradun.E-mail: rajesh_diva1@yahoo.in

Abstract

In this we are giving a review of welding techniques which are being employed now a days in automotive industry for both environment legislation and customer demands for better results and also concerning about luxurious and safety features, with a light weight products by substituting aluminium in place of steel in the body structure. Aluminuim product substantially overcome the weight of the body as compared to the weight of the steel, whistle maintaining essential strength and stiffness.This paper reviews solid and liquid state welding techniques, including laser beam welding, arc welding containing TIG and MIG, resistance spot welding, friction stir welding.

Keywords: Automotive, Aluminium Alloy, Joining, Welding1. IntroductionThe automotive industry, too, has been making positive efforts to reduce CO2 gas emissions through reduction of the weight of car bodies. The most striking of these efforts is the application of high strength steel sheets (590 to 1,470 MPa) to automotive parts [1]. Considerable volumes of high-strength steel sheet have already been applied to not only reduce the weight of car bodies but also enhance their crashworthiness. It is expected that efforts to further reduce car body weight by making most effective use of high-strength steel sheet will be continued in the future. However, considering the stiffness required of each member, there is a certain limit to the reduction of weight through the use of thinner steel sheets. If a further weight reduction of 30% or more is called for in the future, it will become necessary to develop and deploy a multi-material structure composed partly of lightweight materials [2].The finally realised weight reduction, however, is often not the lowest technically achievable weight. Generally, cost considerations and/or production issues are an overriding issue. In many cases, the opportunities offered by appropriate aluminium solutions are also exploited to increase the vehicle stiffness to obtain a performance enhancement at a modest increase in cost and weight. The Audi space frame structure is its first version is an example of a vehicle structural design where a considerable enhanced stiffness was achieved in addition to a lower weight and - based on the target production volume at acceptable cost [3].2. Review of joining techniquesDue to different characteristics of aluiminum many difficulties has been being facing for welding. The most significant of these is the formation of a surface oxide layer, the result of aluminium reacting with oxygen in the atmosphere. This film protects the metal from corrosion, but also has a melting point considerably higher than that of the aluminium parent metal. Successful welding therefore depends, in part, on the ability of the technique being applied to breakdown this oxide layer, which will remain stable even after the aluminium itself has melted [4].2.1 Laser beam weldingLaser energy is developing as production process for welding and cutting metals. The word laser means "light amplification by stimulated emission of radiation". The light beam is highly collimated and can be focused to result in weld widths about three times the aluminum sheet thickness. Lasers are available in sizes up to 25 kilowatts power output. Lasers in the size range 3 Kw to 10Kw are usually adequate for the aluminum sheet thicknesses automotive applications. The possibility of welding materials of varying thickness quickly, efficiently, and with resultant small heat affected zones continues to attract more and more industrial interest in laser welding. several types of Lasers those used for welding are most commonly the solid state Nd:YAG (Neodymium: Yttrium-Aluminum-Garnet) and the gas CO types. The CO gas lasers are easily capable of the power range suitable for welding aluminum body sheet gauges. Recent developments in the use of fiber optics in the Nd:YAG laser systems have made them suitable for welding the thinner sheet gauges of aluminum [5].Laser welding aluminum alloys offers many advantages: precise heat input, narrow weld bead, narrow heat-affected zone, minimal thermal distortion, as well as elevated welding speeds on thin sections and deep penetration on thick sections. Due to these advantages aluminium alloy is increasing in automobile sector.Now interaction of laser beam with metals. All metals reflect light to some degree, with gold and silver high on the list and carbon steel low on the list. Gold, silver, copper, and aluminum are therefore difficult to weld requiring intense energy which is available form high energy peaking pulses or restoring to light absorbing coatings to reduce their reflectivity. The 1.06 micron wavelength of the Nd:YAG laser is more readily absorbed than the longer 10.6 micron wavelength of the CO lasers more suited for highly reflective materials [6].2.2 Arc weldingArc-welding techniques such as metal inert gas (MIG), tungsten inert gas (TIG) and plasma arc welding are proven and well established technique for joining the 5xxx and 6xxx series aluminium alloys that are generally used for fabricating structures in rail road and marine transport and for bridges, off-shore oil-platform and buildings. Of these, MIG and TIG are the most widely used production methods. Welding aluminium is made difficult by the presence of a high melting point oxide film which remains intact even after the metal has melted [7]. In MIG/MAG welding, the consumable metal electrode is both filler material and arc carrier. The endless filler wire is fed via two or four drive rollers into the welding torch, where the current is transferred at the so-called contact tube. The free wire end is concentrically surrounded by a gas nozzle. The shielding gas that flows out prevents chemical reactions between the hot workpiece surface and the surrounding air. This maintains the strength and durability of the weld metal. Inert and active gases can be used as shielding gases. This is why we refer to metal inert gas (MIG) welding and metal active gas (MAG) welding.In addition to the arc behaviour and deposition rate, the shielding gas is also partly responsible for the material transfer and shape of the weld seam. The inert gases mostly used are argon and helium, plus their compounds. The term inert comes from the Greek, meaning inactive. Inert gases are suited to practically all metals, and especially aluminium and copper, but not steel. Active gases are mainly argon-based inert gas compounds, yet also contain some oxygen or carbon dioxide, and are comparatively reactive. Active gases are suited to stainless, high-alloy steels, as well as to unalloyed and low-alloy steels. With some limitations, even carbon dioxide on its own is suited to unalloyed or low-alloy steels as an active gas.Flux cored wires can also be used as an alternative to the shielding gases, with their casing that evaporates in the arc, thereby creating a shielding gas environment. Flux cored wires also ensure reliable gas shielding where there are draughts.

The TIG welding process has been facing greater and greater competition from the ever-perfected MIG/MAG process and its related processes. These processes drastically increase productivity without concessions to quality. Despite its slower welding speed and lower deposition rate, the TIG process has been and still is for many applications the best guarantee for the highest quality results. Last but not least, innovations in the power source sector ensure a sustained future for TIG welding.2.3 Resistance spot weldingResistance spot welding (RSW) is a welding process that joint sheet metal pieces together by applying pressure and passing a large current through localized area while the sheets are fixed together. Resistance spot welding power supply type is divided into SCR (silicon controlled rectifier) type and inverter type [8]. The principle of the SCR type power supply is that by phase controlling the single phase with SCR the output welding current is made, which will be converted into large current through a transformer and applied to the base metal [9].

Aluminum alloys have higher thermal and electrical conductivity. In general high thermal conductivity necessitates a high rate of heat input for fusion welding. Aluminums high thermal and electrical conductivity require higher current, shorter weld time, and more precise control of the welding variables than steel welding. With the application of aluminum alloy sheet to the car body, a new problem on the weldability of aluminum has been raised. The study on resistance spot welding which occupies almost part of assembly process of car body in the car manufacturing line has become an important issue [10-12].The total resistance of workpieces and contact surfaces have an impact on the amount of heat created in resistance welding. This total resistance is also termed as dynamic resistance which is the sum of the specific resistance of the material and contact resistances between electrodes. The specific resistance of the workpiece forms the main part of total resistance. The specific resistance of steel increases as the temperature rises or when the alloy content increases.For example, HSLA steels can be welded using lower current levels than when welding non-alloyed steels. Transfer resistance depends greatly on the surface quality of the workpiece. Oxides and impurities increase transfer resistance. Too high a transfer resistance makes welding more difficult and disturbs the heat balance of the weld. The level of transfer resistance does not cause problems when welding cold-rolled steels but in case of aluminium, for example, the rapidly forming oxide film often causes difficulties in resistance welding [13].Because aluminium has such a low electrical resistance & is a good conductor of heat, high welding currents, delivered for very short times, are necessary (see table). The table (for a relatively thin closure-sheet gauge) illustrates the need for more powerful equipment when spot welding aluminium. Thicker gauges (e.g. for structural Aluminium needs higher currents, higher electrode-forces and shorter weld times when compared to steels) [14].0.9 mm GaugeBare

Aluminium*Bare

SteelZn Coated

Steel

Weld Time

(50 Hz cycles)37-109-12

Current Rang

(kA)18.0-23.07.0-10.08.5-11.0

Force (kN)4.1-5.01.9-2.62.2-2.9

* AA6111 mill finish+lubricantTypical spot welding parameters for aluminium and some steels; Source: Ref 14

2.4 Friction stir weldingFriction stir welding, a process invented at TWI, Cambridge, involves the joining of metals without fusion or filler materials; in other terms, it joins materials by using friction heat. This joining technique is energy efficient, environment friendly, and versatile. It is already used in routine, as well as critical applications, for the joining of structural components made of aluminum and its alloys. FSW is a solid-state process, which means that the objects are joined without reaching the melting point. This opens up whole new areas in welding technology [15].The industrial use of FSW by other industries, have been achieved through a Group sponsored project (GSP) conducted at TWI for an international group of TWI member companies. Systematic welding trials have covered various 2xxx (Al-Cu), 5xxx (Al-Mg), 6xxx (Al-Mg-Si), 7xxx (Al-Zn) and 8xxx (Al-Li) series alloys and in each case a high level of weld quality and process repeatability has been observed [16].

The automotive industry, featuring large manufacturing batches, six sigma requirements and challenging material combinations, from wrought and cast aluminium to magnesium alloys, provides a perfect field for FSW applications. The cycle time is less than one minute per seat, using dual welding heads. Welding speed depends on the alloy to be welded and tool geometry. However, speeds up to 6 metres/minute on 5 mm AA6082 are possible [17].2.4.1 Automotive ApplicationsIn principle, all aluminium components in a car can be friction stir welded: bumper beams, rear spoilers, crash boxes, alloy wheels, air suspension systems, rear axles, drive shafts, intake manifolds, stiffening frames, water coolers, engine blocks, cylinder heads, dashboards, roll-over beams, pistons, etc. In larger road transport vehicles, the for applications is even wider and easier to adapt long, straight or curved welds, chassis, frame, fuel and air container, engine parts, traller beams, etc. Welding aluminium wheels was one of the earliest automotive applications for FSW. FSW was first used for longitudinal welding of aluminium tube, which was later cut to the proper length and spin-formed to the right shape. Marine Aluminium is one of the few companies operating commercial-scale Friction Stir Welding units in Northern Europe. The Friction Stir Welding machine, by welding several profiles together, can produce panels in thicknesses from 1.8 to 12 mm, up to a maximum size of 16x20 meters.2.5 Other welding processes

2.5.1 Cold Metal Transfer

CMT is a revolutionary short-circuit welding method, which works with digitally controlled short-circuit metal transfer in the welding arc. For the first time, this method integrates the wire motion into the welding process and into the overall control of the process. In this way, the, thermal input can be limited [18]. The CMT process evolved from the continuous adaptation of the MIG/MAG process to resolve the problems posed by the joining of steel and aluminium. CMT is a controlled process and allows the material transfer to take place with barely any flow of current. The aluminium base material melts together with the aluminium filler material, with the melt wetting the galvanised steel.2.5.1.1 Machine Technology of Cold Metal TransferCMT welding is carried out exclusively using digital inverter power sources. The welding system basically uses the same latest state-of-the-art hardware as a MIG/MAG system, while at the same time taking certain specific requirements into account. Particularly noteworthy is the highly dynamic wirefeeder mounted directly on the welding torch. The moment the power soure detects a short circuit, the welding current drops and the filler wire starts to retract. Exactly one droplet is detached, with no spatter whatsoever. The filler wire then moves forwards again and the cycle is repeated. High frequency and extreme precision are the basic requirements for carefully controlled material transfer. The wire drive on the welding torch is designed for speed, not high tractive forces. The wire is therefore fed by a more powerful but, due to the above, slower main wirefeeder. A wire buffer on the wirefeeding hose is used to convert the superimposed, high-frequency wire movement into a linear wirefeed [19].2.5.2 Diffusion BondingDiffusion bonding, is a subdivision of both solid-state welding and liquid-phase

welding, is a joining process wherein the principal mechanism is interdiffusion of atoms across the interface. The International Institute of Welding (IIW) has adopted a modified definition of solid-state diffusion bonding, proposed by Kazakov (1985).Diffusion bonding of materials in the solid state is a process for making a monolithic

joint through the formation of bonds at atomic level, as a result of closure of the mating surfaces due to the local plastic deformation at elevated temperature which aids interdiffusion at the surface layers of the materials being joined[20].2.5.2.1 Influence of the Process Parameters on the Stages of Diffusion Bonding [21](a) Pressure

Compressive pressure is required during Stage B, to achieve a large area of contact by localised plastic deformation of asperities on nominally flat surfaces. Where appropriate, pressure is also used to bring about the creep mechanisms which contribute to bonding. The applied pressure (P) however, must not be so high as to cause macroscopic deformation of the components, as stated earlier, and hence is

limited to the yield stress (ys). P < ys(b) Temperature

Plastic deformation, creep and the various diffusion mechanisms are all temperature dependent. Temperature determines: (a) the extent of contact area which dictates the size of voids to be eliminated during Stage B, (b) the rate of diffusion which

governs void elimination during Stages C, D, and E. Since solid state diffusion is a thermally activated process, the temperature dependence of the

diffusion coefficient (D) is given by:D = D0 exp(-Q/RT) (2)

where, D0 and Q are pre-exponential factor and activation energy, respectively.

(c) Dwell time

The creep and diffusion mechanisms are time dependent. Hence, sufficient time must be allowed for void closure by material transfer. Bonding time is affected by temperature, the materials and specimen size. Therefore, the time and temperature

for each case should be optimized. The width of the interdiffusion zone (X) formed at the interface between the two materials during diffusion bonding is given by:

X = kt (3)

where, k is the kinetic rate constant at the temperature (T) of bonding and t is duration of bonding.

(d) Surface roughness

The grade of surface roughness determines the extent of initial surface contact and the size of voids. This in turn, influences the bonding rate. Surfaces may be prepared by machining, grinding and polishing. In general, a finish better than approximately 0.4 m is necessary to ensure good initial contact. The removal of surface contaminants and thick oxides prior to bonding is also crucial.

(e) Interlayer materials

Interlayer materials are very useful when bonding dissimilar materials. They serve to reduce temperature and/or pressure required for bonding and also prevent the formation of intermetallic compounds. Soft interlayer materials enhance contact and accommodate the residual stress developed at the interface of dissimilar materials

due to the thermal expansion mismatch. Therefore, it is evident that the process and materials variables are interrelated and will affect the relative contributions to bonding from each of the possible bonding mechanisms.3. Conclusion:In this review paper we discuss about the different welding process which are using for joining of aluminum at different scales and how to weld with other elements in automotive industry References

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