Research On Heat Treatment And Forming Process Of Ti5523Titanium Alloy For Fastener

With the development of aerospace industry, fastener using high-strength titaniumalloy instead of high-strength steel became research hotspot. Ti5523alloy, developedby China, is a high-strength titanium alloy. In this paper, the relationship between heattreatment and microstructures, microstructures and mechanical properties, and theforming process for this alloy under different microstructures have been investigated.The hot forging of screw head, thread rolling, and cold extrusion of the joint at screwhead and rod were focused and the process of fastener production has been drawn up.These results would provide the theoretical and practical guidance for fastenersproduction using Ti5523titanium alloy.As the solution temperature increased, the volume percent of primary phasedecreased. The tensile strength reduced, while the elongation firstly decreased, passedthrough the minimum at800℃, and then increased. This was owing to the formationof lamellae in the grains. As cooling rate increased, the grain size significantlyrefined, and correspondingly the strength increased. Heat Treatment regimes in labwere verified in Oriental Bluesky Titanium Technology Co.,Ltd.(OBTC) using thelarge quenching furnace and aging furnace. The results indicated that the combinedmechanical properties can be achieved after solution treatment at750-770℃andaging at475-485℃.Hot forging die for hexagon head bolt was designed. As the heating time for9seconds, holding time for1second, and heating current for35amperes were applied,overheating phenomenon does not occur in the fastener head, and the forging flowlines were continuous. Crack was also not found in fastener head. The microstructureconsisted of primary and transformed structure.As the fastener rod was rolled, the chamfer angle was designed to20°±5°. The arcradius at the transition is0.9mm, and thread guide is3.7mm in diameter, which caneffectively prevent the cracking at the onset of thread rolling and prolong the servicelife of rolling wheels. During the rolling, as the feed rate for0.3mm/rev, spindle speed for10RPM, cycle time for3seconds and pressure for13kN were applied, cracks,folding and other defects are not found.When the rolling pressure for900N, rolling time for2seconds, rolling speed at500RPM was carried out, cold-extrusion rounded corner can be successfully achieved.The grain became elongated along the direction of deformation. Fatigue life can keepover130,000cycle times without breaking of screw.