Linear friction welding (LFW) is a solid-state joining process that poses significant challenges for numerical simulation due to intense frictional heating, contact, and localised, extreme plastic deformation. Accurate modelling requires capturing thermomechanical coupling and resolving fine details of the deformation close to the weld line, which distorts the underlying finite element mesh. Typically, these simulations require the use of remeshing and/or Adaptive Lagrangian Eulerian (ALE) techniques, which involve transferring the plastic history between different meshes. While conventional approaches to computational plasticity can be used in this context, they do not inherently provide a consistent mechanism for mapping the plastic history, due to the plastic internal variables being stored at the integration-point level. To address these challenges, we present an extension of the multifield plasticity formulation [1] to problems in LFW. The approximation of plastic variables as fields on the finite element mesh facilitates the consistent mapping of plastic history during remeshing. A novel regularisation of the Fischer-Burmeister complementarity function, used to handle the Karush-Kuhn-Tucker conditions, enhances the robustness of the method. The adopted multifield plasticity approach and a mixed formulation for the thermal problem, ensures stability and allows all thermoplastic state variables (plastic multiplier, plastic strain, and the temperature) to be approximated in L2. Mesh quality is ensured using a combination of mesh quality indicators and modifications to the mesh topology (edge flipping, edge splitting, and vertex merging). Projection of the plastic state variables between meshes is facilitated by their functional setting. The block structure of the resulting system of equations provide a scalable model suitable for both CPU and emerging GPU architectures, as well as implicit-explicit time integration strategies. The formulation is implemented in the open-source finite element software MoFEM [2]. Simulations of LFW will demonstrate capabilities of the formulation. [1] Lewandowski K. et al., Multifield finite strain plasticity: Theory and numerics. Computer Methods in Applied Mechanics and Engineering, Vol. 414, p. 116101, 2023. [2] Kaczmarczyk Ł. et al., MoFEM: An open source, parallel finite element library, The Journal of Open Source Software, Vol. 5 (45), 2020.