1. INTRODUCTION
Friction stir welding is an emerging solid state, hot shear joining technology used to join materials which are difficult to weld. Initially this method was used for joining Aluminium and Aluminium alloys. Today this technology is finding increasingly widespread industrial acceptance for joining various similar and dissimilar materials. The finite element method is a powerful computerized numerical tool to analyze the engineering problems. One more advantage is this is very much appreciated by the industrial communities worldwide. Friction stir welding utilizes a non-consumable rotating tool with no shielding gas and no need of filler material during the welding process. This method can be used for many geometries and positions because gravity has no effect on the solid state joining method. The concept of the friction stir welding is quite simple. A non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of plates to be joined and moves along the line of joint. Friction stir welding is considered as the most significant development in the metal joining in a decade and is a green technology due to its energy efficiency, environmental friendliness, and versatility. Transition joint made by metallurgical welding of Aluminium to Copper is widely used in electrical and electronics industries. In the field of welding technology, Aluminium and Copper are incompatible metals to welding due to a high affinity to each other, at a temperature more than 125C0 and produce inter-metallic compounds, which are brittle in nature. These inter-metallic components are mechanically and electrically unstable, because they contain a non-metallic covalence bond. Hence, an attempt to welding of Al/Cu using traditional methods with the application of thermal energy, to melt and fuse, the two materials can result in an unreliable weld. Solid state joining methods such as friction welding, explosion welding and friction stir welding (FSW) were accepted as the qualified joining processes of Al /Cu materials due to negligible inter-metallic formation and relatively good joint interface. FSW is more flexible, compared to all other solid state welding processes, in joining components rather large or complex in shape.
1.1 WELDING
Today's modern welding technology started just before the beginning of the 20th century with the development of technologies to producing high heat in localized areas. Welding usually needs a heat source to generate a high temperature zone to melt the material, though it is possible to weld two metal plates without much increase in temperature. There were various methods and standards adopted and there is still a continuous research for improved and new technologies of welding. As the demand for welding of larger thickness components and new materials increases, mere gas flame welding method which was first known to the welding engineer is no longer satisfactory and enhanced methods such as Tungsten Inert Gas welding, Metal Inert Gas welding, electron and laser beam welding, friction welding, friction stir welding have been developed.
1.1.1Welding Process
Welding is a process of permanent joining of two or more than two components (usually metals) by localized coalescence resulting from the suitable combination of pressure, temperature and metallurgical conditions. Based on the combination of pressure and temperature from a high pressure to low temperature and low pressure to high temperature, a wide range of joining methods have been developed.
In order to get coalescence between two plate materials there should be a combination of proximity and activity between the molecules of the plates to be joined, to result in the forming of common metallic crystals.
Proximity and activity can be enhanced by plastic deformation in solid state welding process or by melting the two surfaces to be joined so that fusion will occur in fusion welding method. In solid state welding process the surfaces of the plates being joined are chemically or mechanically cleaned before welding while in fusion welding process the contaminant layer is removed from a molten pool by using the fluxes. In outer space or in vacuum removing of contaminant layers are very easy and welds are produced under low pressure.
Welding is the most efficient and economical process to join components permanently. Welding ranks high among industrial processes and involves more sciences and variables than those involved in any other industrial process. In most of the cases welding is the structurally sound and most cost effective joining method. Welding can be carried out almost anywhere indoors, outdoors, in space and under sea. Some of the methods cause sparks where as others do not even need extra heat. Many of the products we use in our daily life are welded.
1.1.2 Classification of Welding Processes
The great variety of welding processes now in use may be classified according to a number of schemes. One possible classification, developed by the American welding society, is presented in APPENDIX A. Which provide a comprehensive reference for a variety of welding methods, although it is either technique or equipment oriented.
Alternatively, the welding processes may be classified by the type of imposed conditions or based on the metallurgical state of the resulting weld and adjacent regions. A slightly different method is used here: the physical state of the metal at interface to be welded - solid, liquid, or gas- is specified and coupled to the resulting microstructure near the weld. Accordingly, welding processes were classified in four broad categories:
a. Fusion joining b. Liquid-phase joining
c. Atomic joining and d. Solid state joining
Fusion welding process involves methods where significant melting of the constituent metals and alloying with the filler material occur during the welding process. It includes much common welding methods as shielded metal arc welding and more specialized methods such as laser welding and electron beam welding.
Liquid phase welding process involves methods where only the filler material is in the liquid state during the welding process and includes such methods as brazing and soldering.
Atomic welding process involves methods wherein a material is joined to a substrate on an atomic scale by deposition of individual atoms or ions and includes such methods as chemical vapor deposition, electroplating, vacuum evaporation and ion sputtering.
Solid-state welding process involves methods where the constituent material structures of the completed weld results mainly from solid state reaction. It includes such methods as diffusion welding, roll bonding, ultrasonic welding, explosive welding, friction welding, friction stir welding, etc. some of these methods involve "transient" liquid phases, which are largely expelled during the welding process to generate joints similar to those produced exclusively by solid state reactions.
SOLID STATE WELDING PROCESS
Solid state welding process involves methods where the constituent metal structures of the completed joint result essentially from solid state reaction that occurs during the welding processes. Many solid state joining methods are utilized in modern technology, but all rely on various combinations of time, temperature, and deformation. Solid state joining involves atomic bonding and/or inter-diffusion obtained by mechanically bringing appropriately prepared mating surfaces into intimate atomic contact. Removal of surface films and asperities is necessary to obtain the required compressive contact. This can be achieved by mechanical dispersion within the joint area during upsetting of mating parts or by gross lateral expulsion. In either case, joining must occur more rapidly than reintroduction of contaminants from the local environment. All solid state joining processes involve dispersion, solution, or expulsion of surface films when applied in conventional environment.
Solid state processes are frequently used to produce joints that cannot be made by other means, particularly in joints where components are not metallurgically compatible with one and another. More often, solid state joining is used because of the tremendous financial savings afforded. Forge welding is the oldest solid state welding process and the other solid state welding processes are:
a) Explosive welding b) Ultrasonic welding
c) Friction welding d) Friction stir welding
The explosion welding is a solid phase welding process. In this process, the weld joint is made with high relative velocity at a high pressure using high explosives. As the plate moves at high velocity and meets the other plate with massive impact, high stress waves created between the plates, which clear all the oxides and scales present in the interface and make a clean joint.
In ultrasonic welding, the weld joint is obtained by applying pressure and high frequency vibration motions. Pieces to be welded are placed between horn and anvil. The combined clamping pressure and vibratory forces introduce dynamic interfacial stresses between the pieces to be joined, and then the local deformation occurs at the interface. Due to the pressure the work piece gets welded. This method is used to weld thin sections. The deformation is in base metal is minimum in this method.
Friction welding is a solid state welding process in which joining is made by conversion of friction energy into heat and simultaneous application of axial force. The two parts are held axially aligned. One part is rotated at a predetermined speed while the other remains stationary. The non- rotating part is gradually advanced towards the rotating part till contact is made. Axial pressure applied during rotation generates sufficient heat to facilitate plastic state. When sufficient heat is produced, the rotation is stopped and the pressure may be increased until the parts are welded. Surface preparation for welding is not necessary in this process. Friction welding is mostly used for butt welding of rods and tubes.
1.3 FRICTION STIR WELDING
Friction stir welding technology was an emerging and recent thermo-mechanical solid state welding method developed and patented by TWI (welding institute) of United Kingdom in 1991. This technology was developed from the traditional friction welding process. In friction welding process the plasticized material was constrained in only two directions namely horizontal and vertical, while in friction stir welding process the plasticized material layers are constrained in three directions.
1.3.1: Friction Stir Welding Process:
The concept of the friction stir welding is very simple. The plates to be joined are firmly kept in position using mechanical clamps on machine table. A rotating non-consumable tool composed of a shoulder and specially designed pin is plunged into the plates to be joined until the shoulder touches the upper surface of the sheets and moves along the axis of the joint as shown in figure 1.1. The rotating tool mainly performs two functions such as
a) Generating of heat in the base metal by friction and
b) Stirring of the material by pin to make joint.
Heat produced by friction between base metal and shoulder and plastic deformation of base metal softens the material around the pin and that allows the tool to move in forward direction with little effort. Due to the localized heating of base metal around the pin and rotation of the tool leads the material to move from the front of the tool to the back of the tool where it consolidates and cools down to produce a solid state weld. Due to the various tool pin geometries, the material flow around the tool pin is quite complex. At the time of friction stir welding process the base metal undergoes plastic deformation and results in producing of equiaxed and fine re-crystallized grains.
Figure1.1: Schematic diagram of FSW.
Friction stir welding process has got the acceptance of industrial production communities' worldwide in the last decade. FSW is accepted as green technology due to its environment friendliness, clean, energy efficient and versatility. When Compared to traditional welding process friction stir welding uses less energy. In this process no need of flux and cover gases hence it makes the process environmentally free from pollution. This process carried out without use of filler metal and therefore all Aluminium alloys can be welded without concern for the composition compatibility, which is one of the main issues in the conventional welding. When require dissimilar and composite materials can be welded with equal ease. Unlike the conventional friction welding, which is generally used to weld small axi-symmetric components that can be rotated and pressed against each other to produce a welded joint, FSW can perform various configurations of joints such as lap joints, butt joints, T joints etc. The important merits of friction stir welding are given in table 1.1.
Table1.1: Important Merits of FSW.
Metallurgical Advantages
Environmental Advantages
a. Solid state welding method
b. Base metal distortion is less
c. Excellent repeatability
d. Excellent mechanical and metallurgical properties
e. No cracking
f. Good dimensional stability
a. No need of cover gas
b. No need of filler metal
c. Non-consumable tool
d. No need of solvents
e. Eliminates surface cleaning.
f. Energy efficient
1.3.2. Friction Stir Welding Terminology
Friction stir welding terminology is shown in figure3.1. The terms relating to the friction stir welding are defined as follows.
a. Non-consumable tool: The whole of the rotating device between base plates to be joined and machine spindle is called as non-consumable tool.
b. Tool pin or probe: Tool pin is a part of the non-consumable rotating tool. It is generally truncated cone shape. The pin extends from tool shoulder and plunges into the base metals to be joined. The width of the nugget depends on the size of the tool pin.
c. Tool shoulder: It is also a part of the non-consumable tool, which is pressed on the base plate surface during the welding. Heat is produced due to the rubbing action between shoulder and base metal. The shoulder generates the weld cap.
d. Advancing side: Advancing side is the side of the welding tool where the rotation of the tool and welding speed are in same direction.
e. Retreating side: Retreating side is the side of the welding tool where the rotation of the tool and welding speed are in opposite direction.
f. Tool shoulder footprint: The total area on the base plate is defined as the tool shoulder footprint.
g. Leading and trailing edge: The front face of the tool shoulder during welding is leading edge and the rare face of the tool shoulder during welding is trailing edge. Pin leading face is front face of the tool pin during welding and pin trailing face is rare face of the tool pin during welding.
h. Heel and Heel plunge depth: Some circumferences are tool tilted with a small angle about the spindle axis in order to increase the part of the tool shoulder that penetrates into the base plates to be joined. The part of the shoulder which experience more plunging is called as 'heel' and the maximum depth of the tool shoulder that plunges below the base plates surface is referred as the 'heel plunge depth'. The angle of tilt with respect to spindle axis is known as travel angle or title angle.
i. Rotational speed: It is defined as rate of angular rotations per minute of the welding tool around its axis. This is measured in rpm.
j. Feed or Welding speed: It is defined as the rate of the travel of the tool through the weld axis line.
k: Plunging depth: This is defined as the maximum depth that the pin inserted into the base metal.
1.3.3 Mechanism of Friction Stir Welding Process
Friction stir welding mechanism is very simple and friction stir welds are formed by following mechanism:
a. The cavity is formed when the welding tool pin penetrates into the base plates. The pin profile decides the shape of cavity. At this time the material under the tool shoulder and around the tool pin is in plasticized condition. The plasticized parent material is surrounded by cooler parent material in the radial direction. The material flow path is decided by direction of rotating tool and welding speed.
b. When tool is moving in longitudinal direction, the material ahead to the tool pin progressively plasticized and moves to the front side of tool pin to the back side of the pin in two different types, namely pin and shoulder driven material flow.
c. The material movement by the pin is in the form of layer by layer and the layers covered in the line of the weld. The material flow due to the shoulder is in bulk and moves from the front side of the tool to the backside of the tool and finishes the weld.
The mechanism of the friction stir welding process is shown in figure 1.2.
Figure 1.2: Mechanism of Friction stir welding a) Formation of cavity during penetration of tool b) Formation of layers due to pin driven material flow c) Tool pin and shoulder driven flow merging area d) Parent metal into nugget zone.
1.3.4 Friction Stir Welding Process Parameters
A large number of process parameters affect the quality of friction stir welded joints. A complex material flow and deformation is involved in friction stir welding process. The various parameters that affect the quality of the friction stir welded joints are given below:
1) Base material
a. Composition of base material
b. Thickness of base materials
c. Properties of base material
2) Clamping of base plates
a. Clamping force b. Clamping design
c. Direction of clamping force
3. Tool geometry
a. Geometry of shoulder b. Design of tool pin.
c. Length of the pin
4. Welding process parameters
a. Tool rotational speed b. welding or traverse speed
c. Plunging speed d. Plunging depth
e. Frictional heating time.
5. Joint configuration
a. Butt joint b. Lap joint c. T- joint etc.
From the above list the tool geometry, welding parameters and joint configuration plays an important role in material flow pattern and distribution of temperature; thereby affecting the microstructural behavior of base material.
1.3.4.1: Tool Geometry
One of the most influential aspects in process development of friction stir welding is tool geometry. Tool geometry plays a major role in vertical flow of material, stirring action and heat generation in base metal at the time of welding. The pin and shoulder geometry are adapted to the base material and plate thickness. A welding tool consists of a pin and shoulder as shown in figure 1.3.
Fugure1.3: Friction stir welded tool
The quality of friction stir welded joints mainly depends on amount of heat generation. Heat is generated due to the rubbing action between base metal and shoulder. Design of tool shoulder is important because, the amount of heat generation depends on diameter of the shoulder. Large shoulder diameter generates excess heat and results in bad metallurgical properties and thereby reduces the mechanical properties of the joint. On the other hand, too smaller shoulder diameters plunges into the base metal instead of rubbing and it leads the breakage of tool. Voids are developed in the joints due to lack of heat generation. Defferent shoulder geometries are shown in figure 1.4.
Figure1.4: Different shoulder geometries
Vertical flow of the material plays an important role in generation of defect free joints. The design of pin is important because the vertical flow and mixing of the material depends on pin geometry. Moreover the width of the weld depends on size of the pin. Voids are generated due to the lack of vertical flow of material. The plastic deformation of the material depends on length of the tool pin. A short pin length generates defects in the root and decreases the tensile and bending strength of the joints. This type of voids acts as an easy path for propagation of cracks. On the other hand, if length of pin is excessive, the pin will touch the backing plate and suffer damage. Different pin geometries are shown in figure 1.5.
Figure1.5: Different tool pin geometries a. cylindrical pin b. tapered pin c. cylindrical threaded pin d. tapered threaded pin e. cylindrical straight fluted pin f. tapered straight fluted pin
1.3.4.2 Welding Parameters
In friction welding process two parameters are most important such as rotational speed of the welding tool in clockwise or anticlockwise direction and welding speed along the seam of the weld. The rotation of welding tool provides stirring and mixing of the base metals around the tool pin and the traverse speed of tool results the flow of stirred material from front of the tool to the back of tool and finishes joining process. Very large rotational speed of the tool generates more temperature due to more frictional heating. This leads the grain growth. A very low rotational speed decreases the stirring and mixing of the base metals. Higher welding speeds reduce the vertical flow of material and generate voids. On the other hand, a too low welding speed generates excess heat.
Apart from tool rotational speed and welding speed, another critical welding process parameter is tool tilting angle with respect to the surface of the base metal. A tilting of the welding tool towards trailing side provides efficient flow of stirred material from front side to back side of the tool and enhance the forging of the material at back side of the welding tool shoulder. Large tilting angles results the flash on both the retreating and advancing sides.
Plunging depth of the tool into base metal also plays an important role in producing quality welds. The plunging depth of tool is associated with length of the tool pin. If the plunging depth is too short, it results in the insufficient contact between tool shoulder and surface of the base metal. This leads to the insufficient flow of the stirred material from the front of the tool to the back of the tool, results generation of joints with defects like surface groove or inner channels. On the other hand, too deep plunging depth generates excessive flash due to the plunging of tool shoulder into the base metal. This produces the significantly concave welds and leads the local thinning of the joined sheets.
Plunging speed and frictional heating time (initial heating time) plays a critical role to ensure effective stirring action. These parameters help in conditioning of material prior to start of welding. Very low plunging speeds generate more heat. On contrary, the too high plunging speed generates vibrations, leading to the breakage of the tool.
1.3.4.3 Joint Configuration or Design
The butt and lap joint configurations are easily joined by friction stir welding technology. The simple lap joint configurations are shown in figure 1.6. In this joint, the two overlapped plates to be joined are firmly clamped on backing plate. The rotating tool plunges into the upper plate and bottom plate at the center of overlap and traverse along the axis of weld, joining the two overlap metal plates. Figure 1.7 shows the butt joint configurations. In this joint, the rotation tool plunges into the abutting edges and traverse along the line of seam to weld the two metal pieces.
(a) (b) (c)
Figure1.6: Different lap joint configurations for FSW a) Lap joint b) T-lap joint c) Multiple lap joints.
(a) (b) (c)
Figure 1.7: Different butt joint configurations for FSW a) Square lap joint b) T-butt joint c) Edge butt joint
1.3.5 Classification of Microstructure
Frictional heating and involvement of intense plastic deformation in the nugget zone during the friction stir welding results in the continuous re-crystallization and development of fine grains within the stirred zone and dissolution of precipitation and grain growth within and around stirred zone. Three different zones such as heat affected zone, thermo-mechanical affected zone and nugget zone (stirred zone) have been observed depending on microstructure characterization of grains and precipitates. The three different zones in the friction stir welded joint are shown in figure1.8.
Figure1.8: Various zones in a friction stir welded joint
A. Nugget zone B. Thermo-mechanically affected zone (TMAZ)
C. Heat affected zone (HAZ) D. Unaffected zone or Base material
1.3.5.1 Nugget Zone
The re-crystallized fine grain microstructures are observed in the nugget zone due to the high temperature exposure and great plastic deformation during friction stir welding process. This zone also referred as stirred zone or weld nugget or dynamically re-crystallized zone. The interface between base metal and re-crystallized stirred zone becomes sharp on the advancing side and relatively diffuse on the retreating side. Various nugget zones are developed based on tool geometry, welding parameters, heat generated in the base metal and thermal conductivity of the parent metal. Usually, the weld nugget zone is classified as two types such as elliptical nugget and basin shaped nugget.
1.3.5.2 Thermo-mechanically Affected Zone
The narrow thermo-mechanically affected zone has been observed between nugget zone and heat affected zone. This zone experiences both plastic deformation and temperature during friction stir welding. This region subjected to the plastic deformation but re-crystallization does not occur due to the lack of deformation strain. The base metal elongated grains are deformed around the nugget zone. The grains in this zone generally contain a great density of sub boundaries.
1.3.5.3 Heat Affected Zone
In between parent metal and TMAZ there is a heat affected zone. HAZ experiences the thermal cycle which changes the mechanical and micro-structural properties of the material. But this zone does not undergo any plastic deformation. The microstructure in this zone was characterized by coarse grains due to the grain growth in this region.
1.3.5.4 Unaffected zone or Base material
This region is remote from the weld nugget. This zone does not undergo a plastic deformation as well as thermal cycling. The temperature in this region does not change the micro structural and mechanical properties of the material.
1.3.5 Defects in FSW Joints
Surface defects and porosity are the common defects observed in the friction stir welding process. The surface defects are shown in figure1.9. The surface defects may be produced due to the improper selection of welding speed.
Figure1.9: Surface defects
At constant rotational speed, an increase in welding speed leads to the initiation of wormhole near the root of the weld. Size of the wormhole increases as welding speed increases due to the insufficient material flow towards the root of the weld. The ratio of the rotational speed and welding speed plays an important role in formation of wormholes. For same tool geometry and material the size of the wormhole increases as this ratio reduces.
Most of the frictional heat is produced at the interface of the base plate and tool shoulder. Significant heterogeneity at this region may lead to formation of defect in the form of excess flash due to the overheating of the surface as shown in figure1.10. The propensity of voids or cracks increases as increase in travel speed even though is an alloying dependence.
Figure1.10: Defect in the form of excess flash
1.5 FINITE ELEMENT METHOD
The finite element method is defined as a computerized numerical method to solve the engineering problems. The finite element method is more powerful and general tool in its application to real world engineering problems that involves more complicated physics, geometry and boundary conditions.
In finite element method, the given domain is divided into subdomains and each subdomain is called as an element. The elements are connected at points are called nodes. The collection of elements is called finite element mesh. In finite element method, a given continuum is viewed as a collection of elements, and over each element the governing equation is approximated by any one of the conventional variational methods. The main reason behind seeking approximate solution on a collection of subdomain is the fact that it is easier to represent a complicated function as a collection of simple polynomials.
The finite element method is computationally intensive, due to the required operations on very large matrices. In the early years, calculations have been performed using mainframe computers, which at that time, were considered to be very high speed powerful tools for use in analysis of engineering problems. In 1960s, the first FEM software code called NASTRAN was developed. In the years since, many software packages were introduced for finite element analysis such as ANSYS, COSMOS, ALGOR etc. In today's computational world, most of these software's are used in computers to get solutions to large problems in static and dynamic structural analysis, fluid flow, heat transfer and electromagnetic analysis.
1.6 ANSYS
ANSYS is integrated design analysis software based on the finite element method developed by ANSYS, Inc. It has its own tightly integrated pre-and post-processor. ANSYS offers a comprehensive software suite that spans the entire range of physics, providing access to virtually any field of engineering simulation that a design process requires. .ANSYS is accepted by all industrial community because of its versatile performance. Engineering capabilities of ANSYS are:
a. Structural Analysis: It includes linear and nonlinear stress, dynamic analysis and buckling.
b. Thermal Analysis: It includes steady state, transient, conduction, convection, radiation and phase change.
c. CFD Analysis: It includes steady state, transient, incompressible, compressible, laminar and turbulent flow analysis.
d. Field and Couple Field Analysis: It includes thermal-structural, thermal-electric, fluid-thermal, fluid-structural, magnetic-structural etc.
e. Electromagnetic Fields Analysis: It includes magneto-static and electro-static analysis.
f. Sub-modeling, optimization and parametric language.
ANSYS Element library contains 189 finite elements. They are broadly grouped into: mass, link, beam, plane, solid, combine, contact, pipe, shell, source, fluid, matrix, visco, hyper, infin, surf etc. Under each type, different shapes and orders complete the list.
1.7 WELDING OF DISSIMILAR METALS
Dissimilar metal joining offers the potential to utilize the advantages of different materials often providing unique solutions to engineering requirements. The main reason for dissimilar joining is due to the combination of good mechanical properties of one material and either low specific weight or good corrosion resistance or good electrical properties of second material. Consequently, joining processes for dissimilar material have received considerable attention in the recent years. Many emerging applications in power generation and the chemical, petrochemical, nuclear, aerospace, transportation, and electronics industries lead to the joining of dissimilar materials by different joining methods especially by solid state welding. Due to the different chemical, mechanical, and thermal properties of materials, dissimilar materials joining present more challenging problems than similar materials joining. A description of a few applications of dissimilar joints is given below.
The joining of Titanium alloy with Aluminium alloy could have a major application in the field of aerospace and automobile industry where high strength and low weight are desirable.
The joining of Aluminium alloys and Zinc-coated steel could have used in the manufacturing of vehicles.
Aluminium and austenitic stainless steel joints find increasing applications in cryogenic systems, chemical plants, nuclear components and the electrical industry.
Pump shafts are required for special pumps to use in a corrosive atmosphere. The shaft of electrical motor is made from carbon steel and the stainless steel is an extension for corrosive medium.
Valve disc assembly involves welding of satellite with carbon steel or martensitic stainless steel.
1.8 WELDING OF ALUMINIUM TO COPPER
Copper and Aluminium are widely used in engineering structures, electrical and electronics industries, power generation, heat sinks etc. due to unique performances such as high electric conductivity, high heat conductivity, corrosion resistance and mechanical properties. In the field of welding technology, Aluminium and Copper are incompatible metals to welding due to a high affinity to each other, at a temperature more than 125C0 and produce inter-metallic compounds, which are brittle in nature. These inter-metallic components are mechanically and electrically unstable, because they contain a non-metallic covalence bond. Hence, an attempt to welding of Al/Cu using traditional methods with the application of thermal energy, to melt and fuse, the two materials can result in an unreliable weld. The general method to obtain this metallic bond was to plate Al with another material that facilitates soldering. This method contains number of steps, is environmentally unfriendly and is used for only small joints. Solid state joining methods such as friction welding, explosion welding and friction stir welding [FSW] were accepted as the qualified joining processes of Al /Cu materials due to negligible inter-metallic formation and relatively good joint interface.
A number of interesting Cu to Al welding applications has been very successfully put into production over the few years. A brief description of a few is given below.
Copper pads welded to Aluminium heat sinks. This application involves the welding of Copper pads to the Aluminium heat sinks. The pads are necessary for the eventual soldering of rectifying diodes to the heat sink.
Copper pads welded to Aluminium ignition module base plates. In this application an ignition module uses an Aluminium back plate for most of the circuit mounting and for heat dissipation. A copper pad was required to locate and solder mount a power circuit.
Copper/Aluminium transition joints used in automotive starter motor field coils. A transition to a Copper conductor was necessary for a conventional electrical connection within the motor. The Copper conductor eliminates problems associated with terminating Aluminium due to the tough, non-conducting oxide layer that forms on Aluminium and the tendency for Aluminium terminations to fail as a result of thermal cycling.
Copper/ Aluminium transition joint in distribution transformers. This is a similar application to the motor field coil. This application is found in some distribution transformers that have been wound with Aluminium wire. A transition weld is required to attach a copper conductor that will allow conventional terminating techniques to be employed.
1.9 Al-Cu PHASE DIAGRAM
Figure1.11 shows the Al-Cu phase diagram. The Al-Cu phase diagram shown only goes up to 60% Cu by weight and is split at around 52 wt% Cu by a particular phase. This "split" means that the two parts of the diagram must be considered separately. The diagram up to the 52% point is very similar to the standard phase diagram. Here the phase on the right is named θ, but other than its name it is dealt with in exactly the same way as a beta phase.
A 33% Copper and 67% Aluminium is known as the eutectic composition and temperature is around 5480C. A 36% Cu and 64% Al composition is known as hypereutectic ally. A 25% of Cu and 75% of Al is correspondingly known as the hypoeutectic alloy.
Figure 1.11: Al-Cu phase diagram
1.6.1 Eutectic Alloy
As a liquid is cooled at the eutectic composition, the two phases grow simultaneously as an interconnected structure which forms the solid eutectic phase. The phase has a lamellar structure, which consists of many thin alternating layers of the two components. The lamellar structure ensures that there are very small diffusion field's head of the solid-liquid interface, meaning that atoms do not travel over very significant distances for the two phases to simultaneously form.
Micrographs produced by techniques such as reflect light microscopy (RLM) and scanning electron microscopy (SEM) are shown in figure1.12. The RLM picture indicates the co-operative formation of θ and Al phases which form the eutectic lamellae. The SEM picture indicates an inter-lamellar spacing of about 1μm, as well as some imperfections which form from irregularities and disturbances during growth.
(a) (b)
Figure1.12 Micrographs of eutectic alloy produced by a. RLM b. SEM
1.6.2 Hypoeutectic Alloy
The RLM micrograph of the 25%Cu/75%Al sample is shown in figure1.13. The figure shows primary Al dendrite arms (white). The dendrite trunk has been intersected at an angle by the plane of polishing to give the observed morphology.
Figure 1.13: Micrographs of hypoeutectic alloy produced by RLM
1.6.3 Hypereutectic Alloy
The RLM micrograph of the 36% of Cu and 64% of Al sample, just to the right of the eutectic (hypereutectic) shown in the figure1.14. This figure indicates the primary dendrite formation from the CuAl2 (θ) phase. The remaining liquid transforms to the eutectic at the eutectic temperature.
Figure 1.14: Micrographs of hypereutectic alloy produced by RLM
1.8 DESIGN OF EXPERIMENTS
The design of experiments (DOE) is the simultaneous evaluation of two or more factors (parameters) for their ability to affect the resultant average or variability of particular product or process characteristics. To accomplish this in an effective and statistically proper fashion, the levels of the factors are varied in a strategic manner, the results of the particular test combinations are observed, and the complete set of results is analysed to determine the influential factors and preferred levels, and whether increase or decrease of those levels will potentially lead to further improvement. It is important to note that this is an interactive process; the first round through the DOE process will many times lead to subsequent rounds of experimentation. The beginning round, often reffered to as a screening experiment, is used to find the few important, influential factors out of the many possible factors involved with a product or process design. This experiment is typically a small experiment with many factors at two levels. Later rounds of experiments typically involves few factors at more than two levels to determine conditions for further improvement.
The DOE process is divided into three main phases which encompass all experimentation approches. The three phases are as follows.
a) The planning phase b) The conducting phase and
c) The analysis phase.
The planning phase is by far the most important phase for the experiment to provide the expected information. An experimenter will learn something from any experiment; sometimes the information is in a positive sense and some times negative sense. Positive information is an indication of which factors and which levels lead to improve the product or process performance. Negative information is an indication of which factor do not lead to improvement. If the experiment includes the real, yet unknown, influential factors and appropriate levels, the experiment will tend to yield positive information. If the experiment does not include the real influential factors, the experiments will yield negative information.
The second most important phase is the conducting phase, when test results are actually collected. If experiments are well planned and conducted, the analysis is much easier and more likely to yield positive information about factor and levels.
The analysis phase is when the positive or negative information concerning the selected factors and levels is genarated based on the previous two phases. This phase, however, is the most statistical in the nature of the three phases of the DOE by a wide margin.
