Different welding techniques

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CHAPTER 1: INTRODUCTION

1.1 INTRODUCTION OF THE FSW TECHNIQUE

In today’s modern world there are many different welding techniques to join metals. They range from the conventional oxyacetylene torch welding to laser welding. The two general categories in which all the types of welding can be divided is fusion welding and solid state welding.

The fusion welding process involves chemical bonding of the metal in the molten stage and may need a filler material such as a consumable electrode or a spool of wire of the filler material, the process may also need a inert ambience in order to avoid oxidation of the molten metal, this could be achieved by a flux material or a inert gas shield in the weld zone, there could be need for adequate surface preparations, examples of fusion welding are metal inert gas welding (MIG), tungsten inert gas welding (TIG) and laser welding. There are many disadvantages in the welding techniques where the metal is heated to its melting temperatures and let it solidify to form the joint. The melting and solidification causes the mechanical properties of the weld to deteriorate such as low tensile strength, fatigue strength and ductility. The disadvantages also include porosity, oxidation, microsegregation, hot cracking and other microstructural defects in the joint. The process also limits the combination of the metals that can be joined because of the different thermal coefficients of conductivity and expansion of different metals.

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The solid state welding is the process where coalescence is produced at temperatures below the melting temperatures of the base metal with out any need for the filler material or any inert ambience because the metal does not reach its melting temperature for the oxidation to occur, examples of solid state welding are friction welding, explosion welding, forge welding, hot pressure welding and ultrasonic welding. The three important parameters time, temperature and pressure individually or in combinations produce the joint in the base metal. As the metal in solid state welding does not reach its melting temperatures so there are fewer defects caused due to the melting and solidification of the metal. In solid state welding the metals being joined retain their original properties as melting does not occur in the joint and the heat affected zone (HAZ) is also very small compared to fusion welding techniques where most of the deterioration of the strengths and ductility begins. Dissimilar metals can be joined with ease as the thermal expansion coefficients and the thermal conductivity coefficients are less important as compared to fusion welding.

Friction stir welding (FSW) is an upgraded version of friction welding. The conventional friction welding is done by moving the parts to be joined relative to each other along a common interface also applying compressive forces across the joint. The frictional heat generated at the interface due to rubbing softens the metal and the soft metal gets extruded due to the compressive forces and the joint forms in the clear material, the relative motion is stopped and compressive forces are increased to form a sound weld before the weld is allowed to cool.

Friction stir welding is also a solid state welding processes; this remarkable upgradation of friction welding was invented in 1991 in The Welding Institute (TWI) [4]. The process starts with clamping the plates to be welded to a backing plate so that the plates do not fly away during the welding process. A rotating wear resistant tool is plunged on the interface between the plates to a predetermined depth and moves forward in the interface between the plates to form the weld. The advantages of FSW technique is that it is environment friendly, energy efficient, there is no necessity for gas shielding for welding Al, mechanical properties as proven by fatigue, tensile tests are excellent, there is no fume, no porosity, no spatter and low shrinkage of the metal due to welding in the solid state of the metal and an excellent way of joining dissimilar and previously unweldable metals.

1.2 ALUMINUM ALLOYS AND WELDING OF ALUMINUM ALLOYS

Aluminum is the most abundant metal available in the earths crust, steel was the most used metal in 19th century but Aluminium has become a strong competitor for steel in engineering applications. Aluminium has many attractive properties compared to steel it is economical and versatile to use that is the reason it is used a lot in the aerospace, automobile and other industries. The most attractive properties of aluminum and its alloys which make them suitable for a wide variety of applications are their light weight, appearance, frabricability, strength and corrosion resistance. The most important property of aluminum is its ability to change its properties in a very versatile manner; it is amazing how much the properties can change from the pure aluminum metal to its most complicate alloys. There are more then a couple of hundreds alloys of aluminum alloys and many are being modified form them internationally. Aluminium alloys have very low density compared to steel it has almost one thirds the density of steel. Properly treated alloys of aluminum can resist the oxidation process which steel can not resist; it can also resist corrosion by water, salt and other factors.

There are many different methods available for joining aluminum and its alloys. The selection of the method depends on many factors such as geometry and the material of the parts to be joined, required strength of the joint, permanent or dismountable joint, number of parts to be joined, the aesthetic appeal of the joint and the service conditions such as moisture, temperature, inert atmosphere and corrosion.

Welding is one of the most used methods for aluminum. Most alloys of aluminum are easily weldable. MIG and TIG are the welding processes which are used the most, but there are some problems associated with this welding process like porosity, lack of fusion due to oxide layers, incomplete penetration, cracks, inclusions and undercut, but they can be joined by other methods such as resistance welding, friction welding, stud welding and laser welding. When welding many physical and chemical changes occur such as oxide formation, dissolution of hydrogen in molten aluminum and lack of color change when heated.

The formation of oxides of aluminum is because of its strong affinity to oxygen, aluminum oxidizes very quickly after it has been exposed to oxygen. Aluminum oxide forms if the metal is joined using fusion welding processes, and aluminum oxide has a high melting point temperature than the metal and its alloys it self so it results in incomplete fusion if present when joined by fusion welding processes. Aluminum oxide is a electrical insulator if it is thick enough it is capable of preventing the arc which starts the welding process, so special methods such as inert gas welding, or use of fluxes is necessary if aluminum has to be welded using the fusion welding processes.

Hydrogen has high solubility in liquid aluminum when the weld pool is at high temperature and the metal is still in liquid state the metal absorbs lots of hydrogen which has very low solubility in the solid state of the metal. The trapped hydrogen can not escape and forms porosity in the weld. All the sources of hydrogen has to be eliminated in order to get sound welds such as lubricants on base metal or the filler material, moisture on the surface of base metal or condensations inside the welding equipment if it uses water cooling and moisture in the shielding inert gases. These precautions require considerable pretreatment of the workpiece to be welded and the welding equipment.

Hot cracking is also a problem of major concern when welding aluminum, it occurs due to the high thermal expansion of aluminum, large change in the volume of the metal upon melting and solidification and its wide range of solidification temperatures. The heat treatable alloys have greater amounts of alloying elements so the weld crack sensitivity is of concern. The thermal expansion of aluminum is twice that of steel, in fusion welding process the melting and cooling occurs very fast which is the reason for residual stress concentrations.

Weldability of some aluminum alloys is an issue with the fusion welding processes. The 2000 series, 5000 series, 6000 series and 7000 series of aluminum alloys have different weldabilities. The 2000 series of aluminum alloys have poor weldability generally because of the cooper content which causes hot cracking and poor solidification microstructure and porosity in the fusion zone so the fusion welding processes are not very suitable for these alloys. The 5000 series of aluminum alloys with more than 3% of Mg content is susceptible to cracking due to stress concentration in corrosive environments, so high Mg alloys of 5000 series of aluminum should not be exposed to corrosive environments at high temperatures to avoid stress corrosion cracking. All the 6000 series of aluminum are readily weldable but are some times susceptible to hot cracking under certain conditions. The 7000 series of aluminum are both weldable and non-weldable depending on the chemical composition of the alloy.

Alloys with low Zn-Mg and Cu content are readily weldable and they have the special ability of recovering the strength lost in the HAZ after some weeks of storage after the weld. Alloys with high Zn-Mg and Cu content have a high tendency to hot crack after welding. All the 7000 series of aluminum have the sensitivity to stress concentration cracking.

All these problems associated with the welding of these different alloys of aluminum has lead to the development of solid state welding processes like Friction Stir Welding technique which is an upgraded version of the friction welding processes. This process has many advantages associated with it, and it can weld many aluminum alloys such as 2000 and 7000 series which are difficult to weld by fusion welding processes. The advantages of the Friction Stir Welding processes are low distortion even in long welds, no fuse, no porosity, no spatter, low shrinkage, can operate in all positions, very energy efficient and excellent mechanical properties as proven by the fatigue, tension and bend tests.

1.3 Conventional Welding Processes of Aluminum

A brief description of the most common processes, their applications on aluminum and limitations are given below.

1.3.1 Gas Tungsten Arc Welding (GTAW):

In gas tungsten arc welding process the heat generated by an arc, which is maintained between the workpiece and a non-consumable tungsten, electrode is used to fuse the joint area. The arc is sustained in an inert gas, which serves to protect the weld pool and the electrode from atmospheric contamination as shown in Figure 2.3.

The process has the following features:

  • It is conducted in a chemically inert atmosphere;
  • The arc energy density is relatively high;
  • The process is very controllable;
  • Joint quality is usually high;
  • Deposition rates and joint completion rates are low.

The process may be applied to the joining of a wide range of engineering materials including stainless steel, aluminum alloys and reactive metals such as titanium. These features of the process lead to its widespread application in aerospace, nuclear reprocessing and power generation industries as well as in the fabrication of chemical process plant, food processing and brewing equipment.

1.3.2 Shielded metal arc welding (SMAW):

Shielded metal arc welding has for many years been one of the most common techniques applied to the fabrication of steels. The process uses an arc as the heat source but shielding is provided by gases generated by the decomposition of the electrode coating material and by the slag produced by the melting of mineral constituents of the coating. In addition to heating and melting the parent material the arc also melts the core of the electrode and thereby provides filler material for the joint. The electrode coating may also be used as source of alloying elements and additional filler material. The flux and electrode chemistry may be formulated to deposit wear- and corrosion-resistant layers for surface protection as shown in Figure 2.4.

Significant features of the process are:

  • Equipment requirement are simple;
  • A large range of consumables are available;
  • The process is extremely portable;
  • The operating efficiency is low;
  • It is labor intensive.

For these reasons the process has been traditionally used in structural steel fabrication, shipbuilding and heavy engineering as well as for small batch production and maintenance.

1.3.3 Plasma welding:

Plasma welding uses the heat generated by a constricted arc to fuse the joint area; the arc is formed between the tip of a non-consumable electrode and either the work piece or the constricting nozzle as shown in Figure 2.5. A wide range of shielding and cutting gases is used depending on the mode of operation and the application.

In the normal transferred arc mode the arc is maintained between the electrode and the work piece; the electrode is usually the cathode and the work piece is connected to the positive side of the power supply. In this mode a high energy density is achieved and the process may be used effectively for welding and cutting.

The features of the process depend on the operating mode and the current, but in summary the plasma process has the following characteristics:

  • Good low-current arc stability
  • Improved directionality compared with GTAW
  • Improved melting efficiency compared with GTAW
  • Possibility of keyhole welding

The keyhole technique is the high heat concentration can penetrate completely through the joint.

These features of the process make it suitable for a range of applications including the joining of very thin materials, the encapsulation of electronic components and sensors, and high- speed longitudinal welds on strip and pipe.

1.3.4 Laser welding

The laser may be used as an alternative heat source for fusion welding. The focused power density of the laser can reach 1010 or 1012 Wm-2 and welding is often carried out using the ‘keyhole’ technique.

Significant features of laser welding are:

  • Very confined heat source at low power
  • Deep penetration at high power
  • Reduced distortion and thermal damage
  • Out-of-vacuum technique
  • High equipment cost

These features have led to the application of leaders for micro joining of electronic components, but the process is also being applied to the fabrication of automotive components and precision machine tool parts in heavy section steel.

1.4 Weld Defects using Conventional Processes

Because of a history of thermal cycling and attendant micro structural changes, a welded joint may develop certain discontinuities. Welding discontinuities can also be caused by inadequate or careless application of established welding technologies or substandard operator training. The major discontinuities that affect weld quality are described below.

1.4.1 Porosity:

Trapped gases released during melting of the weld area and trapped during solidification, chemical reactions during welding, or contaminants, cause porosity in welds. Most welded joints contain some porosity, which is generally spherical in shape or in the form of elongated pockets. The distribution of porosity in the weld zone may be random, or it may be concentrated in a certain region. Porosity in welds can be reduced by the following methods:

  • Proper selection of electrodes and filler metals.
  • Improving welding techniques, such as preheating the weld area or increasing the rate of heat input.
  • Proper cleaning and preventing contaminants from entering the weld zone.
  • Slowing the welding speed to allow time for gas to escape.8

1.4.2 Slag inclusions:

Slag inclusions are compounds such as oxides, fluxes, and electrode-coating materials that are trapped in the weld zone. If shielding gases are not effective during welding, contamination from the environment may also contribute to such inclusions. Welding conditions are important, and with proper techniques the molten slag will float to the surface of the molten weld metal and not be entrapped. Slag inclusions may be prevented by:

  • Cleaning the weld-bead surface before the next layer is deposited by using a hand or power wire brush.
  • Providing adequate shielding gas.
  • Redesigning the joint to permit sufficient space for proper manipulation of the puddle of molten weld metal.

1.4.3. Incomplete fusion and penetration:

A better weld can be obtained by:

  • Raising the temperature of the base metal.
  • Cleaning the weld area prior to welding.
  • Changing the joint design and type of electrode.
  • Providing adequate shielding gas.

Incomplete occurs when the depth of the welded joint is insufficient. Penetration can be improved by:

  • Increasing the heat input.
  • Lowering travel speed during welding.
  • Changing the joint design.
  • Ensuring that surfaces to be joined fit properly.8

1.4.4 Weld profile:

Weld profile is important not only because of its effects on the strength and appearance of the weld, but also because it can indicate incomplete fusion or the presence of slag inclusions in multiple-layer welds. Under filling results when the joint is not filled with the proper amount of weld metal Figure 2.7. Undercutting results from melting away the base metal and subsequently generating a groove in the shape of recess or notch. Unless it is not deep or sharp, an undercut can act as a stress raiser and reduce the fatigue strength of the joint and may lead to premature failure. Overlap is a surface discontinuity generally caused by poor welding practice and selection of the wrong materials. A proper weld is shown in Figure 2.7c.5

1.4.5 Cracks:

Cracks may occur in various locations and direction in the weld area. The types of cracks are typically longitudinal, transverse, crater, and toe cracks Figure 2.8. These cracks generally result from a combination of the following factors:

  • Temperature gradients that cause thermal stresses in the weld zone.
  • Variations in the composition of the weld zone that cause different contractions.
  • Embitterment of grain boundaries by segregation of elements, such as sulfur, to the grain boundaries as the solid-liquid boundary moves when the weld metal begins to solidify.
  • Hydrogen embitterment.
  • Inability of the weld metal to contract during cooling is a situation similar to hot tears that develops in castings and related to excessive restraint of the work piece.

(a) crater cracks. (b)Various types of cracks in butt and T joints.8

Cracks are classified as hot or cold cracks. Hot cracks occur while the joint is still at elevated temperatures. Cold cracks develop after the weld metal has solidified. Some crack prevention measures are:

  1. Change the joint design to minimize stresses from shrinkage during cooling.
  2. Change welding-process parameters, procedures, and sequence.
  3. Preheat components being welded.
  4. Avoid rapid cooling of the components after welding.8

1.4.6 Lameller tears:

In describing the anisotropy of plastically deformed metals, we stated that because of the alignment of nonmetallic impurities and inclusions (stringers), the work piece is weaker when tested in its thickness direction. This condition is particularly evident in rolled plates and structural shapes. In welding such components, lamellar tears may develop because of shrinkage of the members in the members or by changing the joint design to make the weld bead penetrate the wearer member more deeply.8

1.4.7 Surface damage:

During welding, some of the metal may spatter and be deposited as small droplets on adjacent surfaces. In arc welding possess, the electrode may inadvertently contact the parts being welded at places not in the weld zone (arc strikes). Such surface discontinuities may be objectionable for reasons of appearance or subsequent use of the welded part. If severe, these discontinuities may adversely affect the properties of the welded structure, particularly for notch-sensitive metals. Using proper welding techniques and procedures is important in avoiding surface damage.8

1.5 Skill and Training requirements:

Many of the traditional welding processes required high levels of operator skill and dexterity, this can involve costly training programs, particularly when the procedural requirement described above need to be met. The newer processes can offer some reduction in the overall skill requirement but this unfortunately been replaced in some cases by more complex equipment and the time involved in establishing the process parameters has brought about a reduction in operating factor. Developments, which seek to simplify the operation of the equipment, will be described below but effective use of even the most advanced processes and equipment requires appropriate levels of operator and support staff training. The cost of this training will usually be recovered very quickly in improved productivity and quality.

1.6 Areas for development:

Advances in welding processes may be justified in:

  • Increased deposition rate;
  • Reduced cycle time;
  • Improved process control;
  • Reduced repair rate;
  • Reduced weld size;
  • Reduced joint preparation time;
  • Improved operating factor;
  • Reduction in post-weld operations;
  • Reduction in potential safety hazards;
  • Removal of the operator from hazardous area;
  • Simplified equipment setting.

Some or all these requirement have been met in many of the process developments which have occurred in the ten years; these will be described in detail in the following chapters but the current trends in the of this technology are examined below.

1.7 New processes:

The Primary incentive for welding process development is the need to improve the total cost effectiveness of joining operations in requirement for new processes. Recently, concern over the safety of the welding environment and the potential shortage of skilled technicians and operator in many countries have become important considerations.

Many of the traditional welding techniques described in this Chapter are regarded as costly and hazardous and it is possible to improve both of these aspects significantly by employing some of the advanced process developments described in the following chapters.

The use of new joining techniques such as Friction Stir Welding appears to be increasing since it does not involve melting. The application of these processes has in the past been restricted, but with the increased recognition of the benefits of automation and the requirement for high-integrity joints in newer materials it is envisaged that the use of these techniques will grow.

This is a new process originally intended for welding of aerospace alloys, especially aluminum extrusions. Whereas in conventional friction welding, heating of interfaces is achieved through friction by rubbing two surfaces, in the FSW process, a third body is rubbed against the two surfaces to be joined in the form of a small rotating non-consumable tool that is plunged into the joint. The contact pressure causes frictional heating. The probe at the tip of the rotating tool forces heating and mixing or stirring of the material in the joint.

1.8 Research objectives:

The objectives of our project are to:

  • Adopt FSW to a milling machine
  • Design the FSW tools, select its material and have it manufactured
  • Design the required clamping system
  • Apply FSW to plates of an alloy that is not readily weldable by conventional methods
  • Investigate FSW parameters (RPM, Feed Rate and Axial force)
  • Analyze conventionally welded and Friction Stir welded sections then compare their properties.

The objective of this research is to characterize the mechanical properties of friction stir welded joints and study the micro structure of the base metal and the weld nugget evolved during the friction stir welding of similar and dissimilar alloys of Aluminum.

Aluminum 2024 and 7075 are considered for this investigation. The mechanical properties such as ultimate tensile strength, yield strength, formability, ductility and vicker’s hardness are measured and an effort is made to find out a relation between the process variables and properties of the weld. The optimal process parameters for the Friction-Stir welding of AA2024 and AA7075 will be defined based on the experimental results.

Having understood the significance of FSP, the main objective of this thesis is to investigate the effect of process parameters like rotational and translational speeds on the forces generated during FSP of aluminum alloys and relate these forces with the microstructure evolved in order to optimize the process.

The specific objectives of the work presented are:

  • Design and conduct FS processing experiments on aluminum alloy for different combinations of rotational and translation speeds.
  • Measuring the generated processing forces during FSP of aluminum alloys
  • Examine the microstructural of the processed sheets using transmission electron microscope (TEM).
  • Attempt to establish a correlation between these measured forces and the resulting microstructure.

Chapter 2 Review of Literature

2.1 General Idea of the Friction Stir Technology

This section gives an insight into the innovative technology called friction stir technology.

The action of rubbing two objects together causing friction to provide heat is one dating back many centuries as stated by Thomas et.al [1]. The principles of this method now form the basis of many traditional and novel friction welding, surfacing and processing techniques. The friction process is an efficient and controllable method of plasticizing a specific area on a material, and thus removing contaminants in preparation for welding, surfacing/cladding or extrusion. The process is environmentally friendly as it does not require consumables (filler wire, flux or gas) and produces no fumes. In friction welding, heat is produced by rubbing components together under load. Once the required temperature and material deformation is reached, the action is terminated and the load is maintained or increased to create a solid phase bond. Friction is ideal for welding dissimilar metals with very different melting temperatures and physical properties. Some of the friction stir technologies are shown in the Fig.2-1.

Work carried out at TWI by Thomas et.al [2,3] has demonstrated that several alternative techniques exist or are being developed to meet the requirement for consistent and reliable joining of mass production aluminum alloy vehicle bodies. Three of these techniques (mechanical fasteners, lasers and friction stir welding) are likely to make an impact in industrial processing over the next 5 years. FSW could be applied in the manufacture of straight-line welds in sheet and extrusions as a low cost alternative to arc welding (e.g. in the fabrication of truck floors or walls). The development of robotized friction stir welding heads could extend the range of applications into three dimensional components.

Mishra et.al [4] extended the FSW innovation to process Al 7075 and Al 5083 in order to render them superplastic. They observed that the grains obtained were recrystallized, equiaxed and homogeneous with average grain sizes 300 rpm no abnormal grain size was observed.

Friction stir processing of nanophase aluminum alloys led to high strength ~ 650 MPa with good ductility above 10% [Figure 2-4]. Improvements in ductility were due to a significantly improved homogenization of the microstructure during FSP. The FSP technique is very effective in producing ductile, very high specific strength aluminum alloys, such as the Al-Ti-Cu and Al-Ti-Ni as investigated by Beron et al. [26]. The authors investigated two processes: hot isostatic pressing (HIP) and friction stir process (FSP) and compared the microstructures and corresponding properties resulted from the respective processes on 7075 Al alloy. HIP results in a very high strength alloy with low ductility and inhomogeneous structure. But FSP results in comparatively low strength below 740Mpa but very high ductility at temperatures above 300°C at ~500°C. However the FS processing parameters can be optimized to lower both the operating temperature and time at the temperature in order to improve the strength further. Thus this paper concludes that FSP produces high strength Al alloys with significant ductility.

Sato et al. [27] investigated the effect of rotational speed on the microstructure and hardness during friction stir welding of Al 6063-T5. They concluded that the maximum temperature of the welding thermal cycle increased with increase in rotational speed. And also it is observed that the recrystallized grain size increased exponentially with the increasing maximum temperature. Thus they clearly indicated that there is an increase in grain size as the rotational speed increased.

Sato et al. investigated the precipitation sequence in friction stir weld of 6063 Al alloy during aging [28] and concluded that post weld annealing at 440K for 12hrs gives greater hardness in overall weld than in the as- received base material and also shifted the minimum hardness from as-welded minimum hardness region to the precipitated-coarsened region. They have also studied the micro-texture of the friction stir welded 6063-T5 Al alloy using orientation imaging microscopy [29].

Sato et al. [30] examined the dominant microstructural factors governing the global tensile properties of a FS welded joint of 6063 Al alloy by estimating the distribution of local tensile properties corresponding to local microstructure and hardness. They concluded that the minimum hardness determined global yield and ultimate tensile strengths of the weld joint. They stated that in a homogeneously hard joint, such as a solution heat treated and aged weld, a fracture was observed to be located in a region with a minimum average Taylor factor (M) which is equivalent to s/tc where s is the applied uniaxial stress and tc the shear stress working on active plane systems.

Lockwood et al. [31] studied the global and local mechanical response of FS welded AA2024 both experimentally and numerically. Transverse loaded tensile specimens via the digital image correlation technique obtained full field strain measurements. Assuming an iso-stress configuration, local constitutive data were determined for the various weld regions and were used as input for a 2D finite element model. The numerical results compared well with the experimental results in predicting the global mechanical response especially the strain levels. It was also observed that the global strain level was approximately 4% for both the model and experiment.

Mahoney et al. [32] conducted longitudinal and transverse (to the friction stir welded) tensile testing on AA 7075 alloy, which demonstrated that the weakest region associated with FSW was the low temperature location within the heat-affected zone about 7 to 8 mm from the edge of the weld nugget. The yield strength at this location was 45pct less than that of the base metal while; the ultimate tensile strength was 25pct less. Thus concluded that in weldable Al alloys typically, the weld zone would exhibits a 30 to 60 pct reduction in yield and ultimate strengths, hence the losses due to friction stir process were at the lower end of the range for Al alloys.

Mitchell et al. [33] performed FSW of ¼” thick AA6061 sheets for eight combinations of rotational and translational speeds. In their work they presented the forces generated especially the transverse and translation forces and also the temperatures. The temperature is measured using thermocouples. They observed that the transverse force was greater than translation force for all the combinations of speeds and feeds. Their work clearly showed that there exists a unique combination of shear and normal forces that produces a friction stir weld and have stated that the understanding of the contribution of two forces and the relationship to each other was important in modeling the FSW process.

Jata et al. [34] FS welded Al 7050-T7451 alloy to investigate the effects on the microstructure and mechanical properties. Results were discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121°C for 24 hours (as-FSW + T6) did not result in an improvement either in the strength or the ductility of the welded material. It was evident from TEM analysis that the FS welding process transformed the initial 1mm sized pancake-shaped grains in the parent material to fine 1to5µm dynamically recrystallized grains. Tensile specimens tested transverse to the weld showed that there was a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path was observed in the HAZ. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, showed that the FCG resistance of the weld-nugget region decreased, while that of the HAZ increased.

2.3 Studies on Tool and Tool Wear during FSW

The tool design plays a very crucial role in friction stir technology. Hence it becomes an important area of study to make the process more efficient. There have been few contributions in this area which can be jotted as follows.

The design of the tool is the key to the successful application of the process to a greater range of materials and over a wider range of thickness. A number of different high performance tool designs have been investigated. The investigations by Thomas et al. [35] describe the recent developments using these enhanced tools from the perspective of existing and potential applications. Aluminum alloy plates of thickness 1mm to 50mm have been successfully friction stir welded in one pass and a 75mm thick FSW weld in 6082 T6 aluminum alloy plate. Encouraging results and good performance have been achieved by using the MX TrifluteTM type tools to make single pass welds in a number of materials, from 6mm to 50mm in thickness. Typically, the WhorlTM reduced the displaced volume by about 60%, while the MX TrifluteTM reduced the displaced volume by about 70%.

Tool wear in a right-hand-threaded, carbon steel nib reached a maximum at 1000 rpm counter-clockwise rotation speed in the FSW of an aluminum 6061+20 vol. % Al2O3 MMC where the corresponding, effective wear rate was approximately 0.64%/cm as studied by Prado et al. [36]. Above 1000 rpm the wear rate declined. It was approximately 0.42% /cm at 1500 rpm and 0.56%/cm at 2000 rpm. There was no measurable wear and essentially zero wear rate for the same nib rotating at 1000rpm for the FSW of a commercially Al6061 alloy.

2.4 Microstructural studies on friction stirred alloys

A basic understanding of the evolution of microstructure in the dynamically recrystallized region of FS material and relation of this with the deformation process variables of strain, strain rate, temperature and process parameters is very essential. This section would give an insight into such studies.

Peel et.al. [7] reported the results of microstructural, mechanical property and residual stress investigations of four AA5083 FS welds produced under varying conditions. It was found that the weld properties were dominated by the thermal input (thermal excursion) rather than the mechanical deformation by the tool, resulting in a 30 mm wide zone of equiaxed grains around the weld line. Increasing the traverse speed and hence reducing the heat input narrowed the weld zone. It is observed that the recrystallization resulting in the weld zone had considerably lower hardness and yield strength than the parent AA5083. During tensile testing, almost all the plastic flow occurred within the recrystallized weld zone and the synchrotron residual stress analysis indicated that the weld zone is in tension in both the longitudinal and transverse directions. The peak longitudinal stresses increased as the traverse speed increases. This increase is probably due to steeper thermal gradients during welding and the reduced time for stress relaxation to occur. The tensile stresses appear to be limited to the softened weld zone resulting in a narrowing of the tensile region (and the peak stresses) as the traverse speed increased. Measurements of the unstrained lattice parameter (d0) indicated variations with distance from the weld line that would result in significant errors in the inferred residual stresses if a single value for d0 were used for diffraction based experiment.

The evolution of the fine-grained structure in friction-stir processed aluminum has been studied by Rhodes et.al. [8] using a rotating-tool plunge and extract technique. In these experiments, the rotating tool introduced severe deformation in the starting grain structure, including severe deformation of the pre-existing sub-grains. Extreme surface cooling was used to freeze in the starting structure. Heat generated by the rotating tool was indicated as a function of the rotation speed and the external cooling rate. At slower cooling rates and/or faster tool rotation speeds, recrystallization of the deformed aluminum was observed to occur. The initial sizes of the newly recrystallized grains were in the order of 25-100 nm, considerably smaller than the pre-existing sub-grains in the starting condition. Subsequent experiments revealed that the newly recrystallized grains grow to a size (2-5µm) equivalent to that found in friction-stir processed aluminum, after heating 1-4 min at 350-450 °C. It is postulated that the 2-5 µm grains found in friction-stir welded and friction-stir processed aluminum alloys arose as the result of nucleation and growth within a heavily deformed structure and not from the rotation of pre-existing sub-grains.

Sato et.al [9] applied FSW to an accumulative roll-bonded (ARBed) Al alloy 1100. FSW resulted in reproduction of fine grains in the stir zone and small growth of the ultrafine grains of the ARBed material just outside the stir zone. FSW was reported to suppress large reductions of hardness in the ARBed material, although the stir zone and the TMAZ experienced small reductions of hardness due to dynamic recrystallization and recovery. Consequently, FSW effectively prevented the softening in the ARBed alloy which had an equivalent strain of 4.8.

The microstructure evolution of a joint of Al-Si-Mg alloys A6056-T4 and A6056-T6 was characterized using transmission electron microscopy (TEM) by Cabibbo et.al. [10]. Metallurgical investigations, hardness and mechanical tests were also performed to correlate the TEM investigations to the mechanical properties of the produced FSW butt joint. After FSW thermal treatment was carried out at 530 °C followed by ageing at 160 °C (T6). The base material (T4) and the heat-treated one (T6) were put in comparison showing a remarkable ductility reduction of the joint after T6 treatment i.e., it was 80-90% of that of the parent material.

The microstructure of a FSW Al-6.0Cu-0.75Mg-0.65Ag (wt.%) alloy in the peak-aged T6 temper was characterized by TEM by Lityska et.al. [11]. Strengthening precipitates found in the base alloys dissolved in the weld nugget, while it was observed that in the heat-affected zone Cu) and s (Althey were coarsened considerably, causing softening inside the weld region. Precipitates of the O (Al2Cu) phase, was considered as the main strengthening phase in base material, grew up to 200-300 nm in the heat-affected zone, but their density decreased. It was observed that they co-existed with F'(Al2Cu), S'(Al2CuMg), F(Al25Cu6Mg2) phases. The density of the F’ and S’ phases as well as their sizes increased in comparison to the base material. The high-resolution observation allowed them to compare the morphology of the O phase plates in the heat-affected zone and in the base material.

The grain structure, dislocation density and second phase particles in various regions including the dynamically recrystallized zone (DXZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) of a FSW aluminum alloy 7050-T651 were investigated and compared with the unaffected base metal by Su et.al. [12]. The various regions were studied in detail to better understand the microstructural evolution during FSW. They concluded that the microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Using the combination of structural characteristics observed in each weld region, a new dynamic recrystallization model was proposed. The precipitation phenomena in different weld regions were also discussed.

The laser beam and friction stir processes were applied to the ECA pressed Al alloy 1050 with the thickness of 1 mm by Sato et.al. [13]. The ECA pressed alloy after two passes through the die consisted of cell structure with cell size of about 0.58 µm, and the hardness value was approximately 46 Hv. The LBW produced as-cast coarse microstructure and coarse equiaxed grain structure at the fusion zone and the HAZ respectively, which led to the hardness reduction to

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