Introduction
1.1 FSW process
FSW process fabricates joint by using a constantly rotating cylindrical-shouldered tool with plane or profiled pin that is having traversal feed at a constant rate into a butt joint between two clamped pieces of material. The pin is slightly shorter than the weld depth required with the tool shoulder riding on the top of the work surface. This non-consumable welding tool softens the work piece locally through heat produced by friction and plastic deformation, thereby allowing the tool to stir the joint surfaces. The shoulder makes firm contact with the top surface of the workpiece. Heat generated by friction at the shoulder and to a lesser extent at the pin surface, softens the material being welded. Severe plastic deformation and flow of this plasticized metal occurs as the tool moves along the length of the plate. Material is transported from the front of the tool to the trailing edge where it is forged into a joint. The dependence on friction and plastic deformation for the heat source precludes significant melting in the workpiece, avoiding many of the difficulties arising from a change in state, such as change in gas solubility and volumetric expansion [7]. In Fig. 1.1, the advancing side corresponds to the plate over which the direction of tool rotation is same as the tool translation direction and the other side is called the retreating side. In the advancing side the relative velocity between the tool and the workpiece goes through a maximum while it goes through a minimum in the retreating side. This difference can lead to asymmetry in heat generation, material flow and the properties of the two sides of the weld [8].
Chapter 1
Figure 1.1. Basic principal of FSW.
The operations during FSW consist of several phases of action and each phase can be described as a time period where the welding tool and the work piece are moved relative to each other. In the first operation, the tool is plunged vertically into the joint line between the work pieces while the tool rotates. This action takes place in the plunge period. The plunge period is followed by the dwell period, where the tool is held steady relative to the work piece but still rotates. The mechanical interaction, due to the velocity difference between the rotating tool and the stationary work piece produces heat by frictional work and material deformation.
This heat dissipates into the surrounding material that result in temperature rise and softening of materials. The actual welding process can be initiated by moving either the tool or the work piece relative to each other along the joint line [9]. The traverse velocity is in the range of 1–
10 mms−1 depending on welding parameters i.e. the rotation speed, plunge force or plunge depth and tilt angle, tool design and weld piece materials. When the weld distance is covered, the tool is pulled out of the work piece leaving behind an exit hole as a footprint of the tool [10]. A schematic representation of the sequence of FSW is described in Fig. 1.2.
Introduction
Figure 1.2. Schematic representation of FSW process [10].
In FSW, the tool is obviously a critical component to the success of the process. The tool typically consists of a rotating round shoulder and different profile pins that deforms the work piece, mostly by friction, and moves the softened alloy around it to form the joint as shown in Fig. 1.3. The probe height is limited by the work piece thickness; the probe tip must not penetrate the work piece, or damage the backing plate or support frame is unavoidable [11]. The combination of the shoulder and the pin stretches and folds material and creates a solid joint between originally free surfaces that are intersected by the pin. In general, two tool surfaces are needed to perform the heating and joining processes in the friction stir welding.
The shoulder surface is the area where the majority of the heat is generated whereas the probe surface is where the work pieces are joined together and only a fraction of the total heat is generated. Secondly, the shoulder confines the underlying material so that the void formation and porosity behind the probe are prevented. The conical tool shoulder helps establish a pressure under the shoulder, but also acts as an escape volume for the material displaced by the probe during the plunge action. Newer tool designs employ special features, e.g. multi- facets, threads and flutes [12, 13] which are thought to produce advantageous conditions to assist the joining process. But, the simple cylindrical probe shape has proven to work satisfactorily. However, the FSW tool is subjected to severe stress and high temperatures particularly for the welding of hard alloys such as steels and titanium alloys and the commercial application of FSW to these alloys is now limited by the high cost and short life
Chapter 1 of FSW tools. Although significant efforts have been made in the recent past to develop cost effective and reusable tools, most of the efforts have been empirical in nature and further work is needed for improvement in tool design to advance the practice of FSW to hard alloys [14].
Figure 1.3. Cylindrical threaded pin type probe FSW tool.
1.1.1 Limitation of FSW process
The FSW process offers significant advantages as compared to fusion welding processes of aluminum and aluminum alloy, several of which are particularly important to the automotive industry. Despite of these advantages FSW encountered a number of challenges in the welding of materials with intermediate to high hardness and higher melting points.
There are several complex phases in FSW process which are difficult to visualize during the process. These phases of the FSW process and the metallurgy of the resultant weld depend on the material of the workpiece, tool material, tool geometry and process parameters such as tool rotation speed, tool tilt angle, welding speed and plunge depth which affect the heat generation and material flow during the process. In FSW, the amount of heat input is the most important factor. A high tool traverse speed or harder material would cause the lowering of the coefficient of friction between the tool and the workpiece. This confine the frictional heat generation that leads to insufficient material softening and the material flow presenting the risk of weld defects and issues with weld quality. The necessary heat generation during process is also controlled by optimized FSW process parameters such as rotation speed,
Introduction
traverse speed, etc. The process optimization is time consuming and narrows the process window, thus minimizes the application potential of the FSW process.
It is mostly observed that FSW process has been successful for softer alloys because the load requirements are relatively lower than that for the harder alloys. This indicates that partial or additional softening of the harder alloys is required before or during the FSW. Also additional softening reduces the load requirement on the tool and thus improves the tool performance, tool life, FSW process window, welding efficiency and weld quality. The additional softening of the harder workpiece in FSW can be achieved by applying additional preheating or pre-softening mechanism on the workpiece. With this objective several variants of FSW have been incessantly developed over the past few years.
FSW of harder aluminum alloys, high strength, high hardness, high melting point and high conductivity materials such as ferrous, titanium, nickel and copper alloys, dissimilar materials and other metal matrix composites is feasible only if the localized frictional heat generation is sufficiently high to plasticize the workpiece near the weld region. This necessitates imposing high downward axial force and spindle torque on the tool that leads to poor tool performance and failure. The rapid tool wear causes premature tool failure that results in poor weld quality and high production cost. This issue also applies to the aluminum alloys with intermediate hardness such as the precipitation hardened aluminum alloys of class 2xxx, 6xxx and 7xxx. Furthermore, the variations in mechanical, thermal and chemical properties of the tool material during the FSW process affect the tool performance, durability and weld quality. Even the highly durable tools based on the polycrystalline cubic boron nitride (PCBN), tungsten alloys, Si3N4 and molybdenum are susceptible to tool wear that is governed by diffusion, abrasion and chemical affinity of the tool material for oxide and nitride formation. Also a great amount of tool wear takes place during the plunging stage as the work piece material is cold at this time [12]. Another issue of the FSW process is the heavy load requirement that leads to large and sophisticated design of the FSW welding equipment.
Although, the travel speeds have been an issue, the travel speeds are now competitive with other fusion joining processes, such as gas metal arc welding (GMAW). Weld speeds in FSW are slower which can lead to time-consuming joining process. Therefore, the use of standard FSW machines runs into high capital cost requirements and relatively poor productivity.
Chapter 1