The Linear Friction Welding Process

The LFW is a solid-state joining process for complex component assemblies such as Blade Integrated Disks (Blisks)  in gas turbines or aircraft engines. The two workpieces are brought together and jointed thanks to their reciprocal motion and compression pressure applied on the sample to ensure the contact between the workpieces. It is a solid state joining technique in contrast to the conventional welding where the two faces to-be-welded are melted.

The aeronautic industry is using this technology for the manufacturing and the repair of the Blisks. Bladed disks are assembled with mechanical junctions such as dovetail which are prone to erosion, cracking and fatigue. Blisks limit these issues as the blades and the disks make only one piece. They can be made by casting or Computer Numerical Control (CNC) milling which is highly expensive and complex to manufacture. However, those production methods may not allow the repair of damaged pieces. LFW can be used to joint the blade to the disk, avoiding complex casting and precision machining to create the mechanical junction. It also provides the ability to repair the damage blades during a maintenance check up by removing the faulty ones and linearly friction weld new ones.

The main characteristic of the LFW joining process is that it never reaches the melting point of the material. It reduces the interactions with atmospheric gas, making an excellent method to weld titanium alloys. These latter are highly reactive with the atmospheric oxygen at high temperature which requires the use of shielding gas like argon to prevent oxidation during the weld. The oscillating motion removes the oxide layers present at the welding surface and the pressure applied to the workpiece bring new metallic atoms together and creates the bond. So, no filler material is required in LFW compared to a TIG or MIG techniques.

LFW reduces the manufacturing cost as it does not require any consumable and, furthermore, it reduces the component weight as less material is needed to join the blade to the disk compared to a mechanical assembly. As it is a high-added-value process, it is used on costly and critical assemblies (ex: aircraft turbines) that numerical simulations allow designers to test various process configurations and optimize the assembly before its production.

Thermo-Mechanical Analysis

The process is composed of two workpieces and dies holding the workpieces in position. This study has chosen a vertical mounting (Figure 1.2) as a case study as it is commonly reported in the literature: Vairis & Frost (2000); Wanjara & Jahazi (2005); Dalgaard (2011); Yang et al. (2015); McAndrew et al. (2017). Usually, the lower workpiece oscillates (Equation 1.1) with (a) the oscillation amplitude and (f) the frequency. u(t) = asin(2π ft) (1.1)

A contact force (F) is applied on top of the upper workpiece to insure the contact and generate friction between the two workpieces. The amplitude (a), the frequency (f), and the contact force (Fc) or its equivalent pressure (pc) are the process parameters of the LFW.

Process Characteristics

The LFW is a self-regulating process (Dalgaard (2011); McAndrew et al. (2017)). Once, sufficient heat has been provided to soften the weld line, the viscoplastic layer formed at the latter is flowing due to the process loading – the applied force, the high temperature, and the oscillations – and behaves like a fluid. The weld line cannot sustain the constraints it is subjected to, and so, collapses and flows. Therefore, the friction between the two workpieces is less than it used to be, as a result, the temperature at the weld line decreases. While the viscoplastic layer is being expelled, new matter is brought to the weld line due to the axial shortening. Since the weld line is colder, it hardens itself and the friction between the two workpieces increases. As a consequence, the heat power input increases and soften the new matter which will collapse when the mechanical threshold is reached. Thus, multiple mechanisms are involved in establishing and keeping that cycle .

Heat Affected and Thermo-Mechanical Affected Zones

A consequence of the temperature increase and the severe deformation at the weld line is the formation of a Heat Affected Zone (HAZ) and a Thermo-Mechanical Affected Zone (TMAZ). First, the Heat Affected Zone (HAZ) is due to the temperature increase which alters the material properties such as the Young’s modulus and the phase transformations (e.g. α to β phase for titanium alloys). The literature barely provides a value when the HAZ starts, however, it is usually taken when the changes in the material properties become important: for titanium alloy, it can be assumed that the HAZ temperature is before the β transus temperature. The HAZ expands at the beginning of the process due to the heat diffusion and reaches a limit (xHAZ).

Schröder et al. (2015) also investigated the correlation between the process parameters and the flash morphology , and reported that each parameter has a visible influence on the flash morphology:

– A low frequency will create a compact flash whereas a high frequency will produce a long and wavy flash.

– A low amplitude will give a long and thick flash whereas a high amplitude will end with a chaotic flash shape.

– A low pressure will form a short and wavy flash whereas a high pressure will generate a straight and long flash.

The purpose of qualifying flash morphology is to determine visual criteria that characterize a sound-joint.