Study On Microstructure And Mechanical Properties Of Friction Stir Welded In-situ TiB₂/Al-Si-Mg Composite

Particles of in-situ TiB₂ reinforced aluminum composite are nano-sized or submicron-sized and their morphologies are regular and round.The interface bonding of TiB₂ particles and aluminum matrix is good.These characteristics make TiB₂/Al composite widely concerned.With the rapid development of particle reinforced aluminum matrix composite?PRAMCs?in the aviation and aerospace,military and automotive industries,welding properties become a significant factor to influence its application range.Friction stir welding?FSW?is a new solid state joining process and effectively avoid the pores and particle segregation which are common in conventional fusion welding process.Sub-micron and regular particles can increase material flowability and microstructural stability during FSW.Accordingly,this research focuses on the effects of FSW on in-situ TiB₂reinforced aluminum matrix composite.The main findings are as follows:?1?Rotation speed and traverse speed have crucial effects on TiB₂particles and grains in nugget zone?NZ?.Increasing rotation speed and decreasing welding speed make the size of particles and grains uniformly decreased and the particles more dispersed and rounder.Grain size and recrystallized fraction are influenced by particle distribution.More uniform distribution of TiB₂ particles leads to finer aluminum grains and higher recrystallization degree.By contrasting the mechanical properties of various parameter combinations,the optimum FSW parameters for this material are rotation speed of 900 rpm and traverse speed of 180 mm/min.?2?Mechanical properties of NZ are all enhanced than that of base composite.Grain refinement and Orowan strengthening are the main reasons for the increased properties of NZ.Dynamic recrystallization is attributed to high dislocation density and elevated temperature.Meanwhile,the movement of grain boundaries are restricted by TiB₂,so grain growth is inhibited.Orowan strengthening is mainly due to fine and uniformly distributed TiB₂ particles.?3?Mechanical properties of heat affected zone?HAZ?are all enhanced than that of base composite.Peak temperature of HAZ are over 400°C,Mg and Si atoms are dissolved which assist the coarsening of?''-Mg₂Si.The coarsened Mg₂Si leads to the decreasing of mechanical properties of HAZ.?4?Grain sizes of NZ and HAZ have little change after T6 treatment,which is due to the“Zener pinning”effect caused by nano-sized particles.Precipitation in NZ and HAZ are influenced by T6 treatment.The precipitation of HAZ is?'',the same as base composite.And the precipitation of NZ is?',which is over-aged.?5?T6 treatment can increase the mechanical properties of the whole joint without distinct softening zone.The strength of FSW-T6 joint can be95.5%of base composite.