Linear Friction Welding

Industrial and aircraft gas turbine engines are high technology products and their manufacturing processes involve various innovative techniques. Technological viability and manufacturing costs are the key factors in the successful development of new processes. In the turbine engines, ″blade″ and ″disk″ (BLISK) can be integrated by machining from a single forged part as a single piece for both compressor and turbine sections. However, this manufacturing technique is expensive for large BLISKs, and is not good for fabricating dissimilar blades and disk. Blades and disks can be attached mechanically (fir tree assembly); although damaged blades are easily replaceable, fatigue cracks initiate from the fir tree, and the weight of the part will increase due to the fir tree root .

Recent developments have led to emerging LFW technology. The major commercial use of LFW is for the joining of aeroengine compressor blades to compressor discs to form blisks. As a result, much of the published literature on the subject has focused on the joining of materials for aeroengine applications. To this end, a range of titanium alloys and Ni-based superalloys have been studied for understanding the influence of LFW process parameters on the microstructural characteristics and mechanical performance. However, the process can be used to join other materials and interest in this aspect is growing. The application of LFW has greatly extended the commercial potential of conventional frictional welding techniques since non round and round parts can be joined with precise angular alignment (Nicholas et Thomas, 1998).

Friction welding is classified as a solid state welding process, in which joining occurs at a temperature below the melting point of the work metal. If incipient melting does occur, there is no evidence in the finished weld, because the metal is worked during the welding stage (Bhamji et al., 2011).

In LFW, an imposed linear reversing motion of the two surfaces to be joined generates frictional heat and plasticizes the material at the weld interface . When adequate heat and metal flow has been reached, the moving part is brought into alignment with the stationary one while the axial load is maintained or increased to finalize the weld sequence. Material, including surface oxides, are expelled from the interface as “flash” and a ThermoMechanically Affected Zone (TMAZ) expands from the interface into the parent material (McAndrew et al., 2018). LFW has three main stages: Conditioning, Frictional and Forge (Bhamji et al., 2011; Vairis et Frost, 2000a).

Conditioning Phase
Oscillation of one of the parts is increased and stabilized over a set period (usually very quickly) and the parts are brought together under a small force for a predetermined time .

Frictional Phase
The compressive force (friction force) is increased to a set level and heat is generated at the interface. The material at the interface becomes plastic and flows out of the weld, as flash, because of the shearing motion between the two parts and the applied force. This loss of material from the weld causes the parts to shorten (or burn-off). This phase usually ends, and the next is triggered, when a predetermined loss of length, or burn-off distance, is reached. However, the next phase can also be triggered after the frictional phase has continued for a predetermined time (burn-off time) or a number of oscillation cycles (burn-off cycles). The LFW process is always carried out under load control, but other parameters also play a role in controlling the welding process. For example, when using a burn-off distance the load is controlled throughout the welding process, however, the burn-off is also monitored (although not controlled) and at a set burn-off distance, the next phase (forge phase) is triggered. Similarly with burn-off time or cycles the load is controlled throughout welding and the amount of time or cycles determines the transition to the next phase.

Forge Phase
The last phase is known as the deceleration or forge phase. The amplitude is decayed to zero over a predetermined time to ensure good alignment (usually very quickly). In other words, the materials are brought to rest after the desired shortening has been attained. Once the materials have been brought to rest and aligned, the axial pressure is increased and held for a set time until the weld is consolidated.

LFW of Superalloys

Very little has been published on LFW of Ni-based superalloys (Chamanfar et al., 2011a; 2012a; Dalgaard et al., 2012; Ola et al., 2011a; Ola et al., 2012). The few reported publications have been focused on LFW of Ti alloys (Dalgaard et al., 2012; McAndrew et al., 2018). While fusion welding is an economical joining technique for the fabrication and repair of service-damaged Ni-based superalloy turbine parts; their poor weldability limits the application of welded components in critical structure locations. The alloys, particularly, those with high-volume fraction of the γ′ phase (>40%) are highly susceptible to intergranular microcracking during conventional welding processes. The fundamental cause of the weldability problem in these alloys is the liquation of grain boundary region, which weakens the material during joining (Rowe, Ishwar et Klarstrom, 2006). Developments in welding research have resulted in the use of solid-state friction joining processes (Henderson et al., 2004; Korsunsky et al., 2009). These include Friction Stir Welding (FSW), friction spot welding, inertia friction welding, continuous drive friction welding and LFW, which are state of- the-art in producing crack-free welds in difficult-to-weld structural alloys. Studies have shown that, in particular, LFW is potentially well-suited for joining highly cracksusceptible Ni based superalloys (Karadge et al., 2008). Currently, there is active on-going research on effective and efficient industrial utilization of this technique in the manufacturing of aero-engine components like turbine discs and blades (Vairis et Frost, 1998).

Microstructure Evolution during LFW

Since the mechanical properties of superalloys strongly depend on their microstructure, and its widespread industrial applications, it is necessary to understand microstructural evolution with time, temperature and deformation during LFW to optimize the friction welding process.

Although the interface temperature during LFW is not expected to exceed the melting temperature of the materials being welded, the peak interface temperature is nevertheless very high, and could be close to the solidus temperature of the material (Mary et Jahazi, 2007a). In all reported cases, this high temperature, along with the applied pressure, cause significant microstructural variations at the weld interface and its neighboring region.