Motivated by the undesired microstructure and unsatisfactory weld properties during keyhole tungsten inert gas (K-TIG) welding process, the use of filler materials was introduced for the first time in K-TIG to modify the weld microstructure and improve weld properties. Single pass full penetration was achieved on 6.2 mm armour steel plate at a welding speed of 350 mm/min. The metal transfer behaviour, microstructure and mechanical properties of weld joint were analysed in detail. The results showed that the introduction of filler material was very effective in modifying microstructure and improving joint properties. The wire feeding speed can reach up to 300 cm/min without compromising full penetration, providing a great scope for microstructure modification. The applied austenitic filler material can significantly change the weld microstructure with just 6.7% dilution, along with much increased weld metal hardness and joint efficiency, matching the welds produced by using conventional arc welding processes with V-joint preparation and matching filler metal. The introduction of filler material dramatically expands the capability of K-TIG process and also shows great potential to produce high performance armour steel joint with high productivity.