Introduction
Linear friction welding (LFW) is a solid-state joining process which works by oscillating one workpiece relative to another while under a large, compressive force; see Figure 1 . The friction between the oscillating surfaces produces heat, causing the interface material to plasticise. The plasticised material is then expelled from the interface causing the workpieces to shorten (burn-off) in the direction of the compressive force. During the burn-off the interface contaminants, such as oxides and foreign particles, which can affect the properties and possibly the service life of a weld, are expelled into the flash. Once free from contaminants, pure metal to metal contact occurs, resulting in a bond.
The LFW process is typically used for joining metals, however, it has been used to join plastics and wood. LFW is particularly effective for joining metals that have good high-temperature properties (compressive yield and shear strength) and low thermal conductivities. This allows the generated heat to remain at the interface causing the interface to rapidly heat and plasticise. This makes titanium alloys particularly suitable for the process, however many similar and dissimilar material combinations have been investigated with varying degrees of success.
Applications
LFW is commercially established as a technology for the fabrication of titanium alloy integrated bladed disks (blisks) in low temperature sections of aero-engines. An example of a linear friction welded blisk is shown in Figure 2 . LFW offers many advantages when used to manufacture blisks. For example, conventionally manufactured titanium alloy bladed disk assemblies are reliant on mechanical fixings and dovetail joints. LFW allows for the blade to be integrally joined to the disk, which significantly reduces the weight of the component. The lack of a mechanical interface between the blades and the disk also eliminates a common source for fatigue crack initiation, which is often the life limiting feature of these parts. Moreover, linear friction welded blisks offer better aerodynamic performance, which helps to lower the overall operating costs for the end user.
Owing to the many benefits of LFW, the process is finding increasing industrial interest for the manufacturing of aircraft structural components. Metallic aircraft components are typically machined from oversized ingots, forgings and extrusions. This is an expensive process due to the proportionally large amount of material that is purchased compared to the amount that remains after machining. For example, buy-to-fly (BTF) ratios of 20:1 are not uncommon. LFW reduces the material required to make a component by joining smaller workpieces to produce a preform, which is subsequently machined to the desired dimensions, as shown in Figure 3. This brings substantial improvements to the BTF ratios, which significantly reduces manufacturing costs.
Microstructure and Mechanical Properties
Linear friction welds are similar in appearance in that they have several distinct zones: a weld centre zone (WCZ), a thermo-mechanically affected zone (TMAZ) and a heat affected zone (HAZ). Technically, the WCZ and TMAZ are both “thermo-mechanically affected zones” but due to the vastly different microstructures they possess they are often considered separately. The WCZ experiences significant dynamic recrystallisation (DRX), the TMAZ does not. The extent and microstructural composition of these zones are very dependent on the material and processing conditions used. A typical example of a weld is shown in Figure 4 .
LFW can produce joints that are superior or similar in strength to the parent material of various titanium alloys, aluminium alloys, nickel-based superalloys and steels, as well as certain dissimilar material combinations. Moreover, recent research at TWI Ltd has shown that the fatigue performance of the weld can also exceed that of the parent material in titanium alloys.
Advantages
Linear friction welding offers many advantages over competing manufacturing processes, for example:
- The weld remains in the solid-state, avoiding many of the defects associated with melting and solidification during fusion welding, such as pores and solidification cracks. The distortion of the welded component is also reduced.
- The process has lower peak temperatures than fusion welding, reducing intermetallic formation and allowing for a range of dissimilar materials to be joined.
- The process does not need a filler metal, flux and shielding gas.
- The process is easily automated, making the process highly repeatable and not dependant on human influence, resulting in very low defect rates.
- LFW reduces the material required to make a component by joining smaller workpieces to produce a preform, which is subsequently machined to the desired dimensions. This brings substantial improvements to the buy-to-fly ratios, which significantly reduces manufacturing costs.
Summary
Linear friction welding is an established technology for the manufacturing of titanium alloy blisks for aero-engines. Owing to the many benefits of the process, it is finding increasing industrial interest for the manufacturing of structural components.
TWI Ltd has developed extensive knowledge of LFW over many years of research and development. If you would like to know more about the process, please contact friction@twi.co.uk.