Eﬀects of cold tandem rolling on microstructures and mechanical properes of TC16 tanium alloy wire for fastener use

High-precision dimension controlling and special requirement of microstructures and mechanical for TC16 Alloy wire was obtained by cold rolling. In this paper, the eﬀects of cold tandem rolling on M and STA mechanical of TC16 Alloy wire were invesgated through Comparave study TC16 Alloy wire by cold tandem rolling and hot-draw. The results show that the M state tensile and yield strength of TC16 Alloy wire obtained by cold tandem was lower, while the duclity was higher. And this beneﬁts the subsequent Cold Heading formability. The result of solid soluon for TC16 Alloy wire by cold tandem rolling was evidenced, the tensile strength has been increased and the duclity has been reduced more drascally. To break down elongated α of Vercal microstructure ability and drasc machining deformaon have signiﬁcant contribuons to the annealed duclity and soluon strengthening. It is very posive to TC16 tanium alloy use for fastener of heat treatment strength with the cold tandem rolling technology applicaon.


Introduction
Cold rolled wire not only has good surface quality and high dimensional accuracy, but also has be�er microstructure and proper es. The reasonable coordina on of cold rolling deforma on and heat treatment can not only easily meet the requirements of users for processing quality and comprehensive performance, but also be par cularly beneficial to the produc on of some important products requiring special structure and performance [1] .
TC16 tanium alloy containing a small amount of Al, the more stable β elements (Mo, V), β stability coefficient K β is as high as 0.8, which has high specific strength, good plas city, the least sensi vity to notch stress concentra on and torsional stress concentra on at solid solu on aging condi on. It is a typical solid solu on aging for fied α+β tanium alloy, be widely used in avia on, aerospace fasteners and parts [2][3][4] .TC16 tanium alloy has good deforma on ability at room temperature and can be directly formed by cold upse ng a�er annealing. Cold upse ng forming is adopted for fasteners, which has the advantages of high machining precision, good surface quality, high produc on efficiency, easy opera on condi ons and low processing cost. It is the preferred technological direc on for the engineering produc on of TC16 tanium alloy fasteners in China. However, the annealed microstructure and proper es of TC16 alloy wires must be strictly controlled to realize cold upse ng processing stably, and the proper es of the fasteners a�er cold upse ng forming must be improved obviously a�er solu on aging strengthening, so as to realize the processing and applica on of heat treatment strengthening cold upse ng TC16 tanium alloy fasteners [5][6][7] .
The microstructure and proper es of TC16 tanium alloy formed by cold upse ng were comprehensively analyzed by domes c scholars and research ins tu ons through heat treatment process, phase structure and deforma on-induced martensite [8][9] . Based on the Johnson-cook cons tu ve model, Yangyang [10] et al. obtained the parameters of the cons tu ve model of TC16 alloy by fi ng experimental data, which provided a reference for the cold upse ng deforma on of TC16 alloy. Zhang zhengang [11] et al. described the quan ta ve changes of microstructure of TC16 tanium alloy under different heat treatment processes, and the reasonable heat treatment process can significantly improve the strengthening effect of TC16 alloy for cold upse ng. However, due to the limita on of equipment condi on, the current domes c processing and prepara on of TC16 tanium alloy wire limited to hot rolling, hot drawing or both the processing way of combining. Eventually, the proper es of TC16 alloy wire which was obtained cannot meet the stability requirements of cold upse ng for TC16 fasteners, while to adopt the new cold rolling process of the prepara on of cold upse ng TC16 tanium alloy wire is used to study less. Therefore, in this paper, the effects of the cold tandem rolling process on the microstructure and proper es of TC16 tanium alloy wires for cold heading were studied by comparing the annealing and solu on aging proper es of the TC16 tanium alloy wires prepared by the cold tandem rolling method and the tradi onal hot drawing method.
Cold tandem rolling is a kind of processing method which adopts imported 90D cold tandem rolling machine. Its working principle is to obtain special-shaped wire with 12-side shape in cross section through 10 pass rolling of cold tandem rolling, and then that special-shaped wire was rolled to Φ4.0mm wire by adding two cold-rolling gauge circle units of the frame equivalently to be finishing rolling. Hot drawing processing is drawing Φ8.5mm to Φ4.0mm under 700℃ heat through 8 pass drawing. Samples were obtained from Φ4.0mm wire which were prepared in the above two ways respectively. Table 1 shows the heat-treatment for the sample with the two processing methods.

Comparison of annealed microstructure and properties
Transverse and Longitudinal microstructure of sample of Φ8.5mm billet which were heated treating for 2 hour at 780℃ then cooling with furnace temperature to 550℃were observed. The transverse microstructure of Φ8.5mm billet is small near-basket microstructure, and the longitudinal microstructure is elongated microstructure in the rolling direction, as shown in FIG.1. a transverse microstructure, 1000× b longitudinal microstructure, 1000× Fig.1 Microstructures of Dia. 8.5mm TC16 alloy annealed wire This Φ4.0mm wire was obtained by Φ8.5mm billet through 8 pass drawing. FIG.2 shows the transverse and longitudinal microstructure of the Φ4.0mm wire sample after heated for 2 hour at 780℃ then cooling with furnace temperature to 550℃. Although dynamic recovery exists in the process of hot drawing at the two-phase temperature, the drawing deformation is a gradual transition from the edge to the center of the wire, and the amount of inter-pass deformation is small, so the original longitudinal elongation of phase structure of the wire billet cannot be broken. At the same time, during the drawing process, part of phase will be elongated along the drawing direction, resulting in obvious fibrous tissue, as shown in FIG. 2 (b). Therefore, the texture of longitudinal rolling after hot drawing at two-phase temperature is elongated to a structure in which both alpha phase and equiaxed alpha coexist. a transverse, 1000× b longitudinal, 1000× Fig.2 Microstructures for Dia. 4.0mm TC16 alloy as annealed by Hot-draw This Φ4.0mm wire was obtained by Φ8.5mm billet through 10 pass rolling and 2 pass finishing rolling. FIG.3 shows the transverse and longitudinal microstructure of the Φ4.0mm wire sample after heated for 2 hour at 780℃ thencooling with furnace temperature to to 550℃. Compared with hot drawing, cold tandem rolling has more severe deformation. The original longitudinal elongation of phase microstructure can be effectively broken at room temperature, and the obtained longitudinal microstructure after annealing has an equiaxed phase state, as shown in FIG. 3 (b). The tensile proper es of TC16 tanium alloy Φ4.0mm wire, which were processed by cold tandem rolling and hot drawing methods respec vely a�er same annealing were analyzed. The contrast shows that TC16 tanium alloy wires tensile strength and yield strength of cold tandem rolling are lower than that of hot drawing, but the plas city is be�er than that of hot drawing.

Cold heading forming proper es of wire in two processing methods
The TC16 tanium alloy Φ4.0mm wire, which were processed by cold tandem rolling and hot drawing methods respec vely a�er same annealing were carried out in accordance with the HB 8015-2002(Hexagon head bolts in tanium alloy TC16 with tolerance f9) to cold heading Φ4mmx20 mm fasteners, eventually cold heading fasteners, as shown TC16 tanium alloy such as Dia.4mm wires with equal axial α phase in transverse and longitudinal direc on obtained by cold con nuous rolling have low strength and high plas city. That op mum process performance makes the fastener deform well in the process of cold heading. Thus, the surface of the cold heading fastener is free from cracks, strains and other defects, as shown in FIG. 5 (a). While, the horizontal direc on is equiaxed α phase and the longitudinal direc on is strip α phase of the TC16 tanium alloy Dia.4mm wires which was obtained by hot drawing that have low strength and high plas city. That Poor performance prevents the fasteners from deforming well during the cold heading forming process. Thus, the surface of the cold heading fastener is cracked and the rod is scratched, as shown in FIG. 5 (b).
The cold heading process of fasteners is a composite forming method in which upse ng and extrusion are combined. Whether cold heading can be successfully achieved requires not only the plas city of the material at room temperature, but also the longitudinal microstructure of the material [12][13][14] .
The longitudinal elongated microstructure is less compa ble with the longitudinal basket microstructure and the longitudinal isoaxial microstructure during cold heading deforma on. Compared with the longitudinal basket and the longitudinal equiaxed microstructure, the lengthwise elongated microstructure is less changeable in the process of cold heading deforma on, Therefore, for TC16 tanium alloy wire used in cold heading, the longitudinal wire with elongated microstructure is more likely to crack during cold heading forming, while the fine equiaxed microstructure and net basket microstructure are conducive to cold heading forming [15,16] .

Comparison of microstructure and properties in solution aging state
For two kinds of processing methods of TC16 titanium alloy wire sample according to the 800℃/2h, WQ+560℃/8h, AC respectively at the same time to carry on the solid solution aging treatment. The horizontal and vertical microstructures of the two groups were compared and observed, as shown in FIG.6.  a transverse, hot-draw,1000×;  A�er the solu on aging, the transverse and longitudinal microstructure of hot-drawn wire retained the annealed microstructure, and the transverse microstructure was composed of isoaxial α phase, and the longitudinal microstructure was composed of elongated α phase and isoaxial α phase. But the overall phase content was less than that of cold tandem rolling. The transverse and longitudinal microstructure of cold con nuous rolling wire a�er solu on aging were all composed of equiaxed α phase. A�er the solu on aging treatment, the tensile strength and yield strength of TC16 wires processed by hot drawing and cold tandem rolling were significantly improved, and the plas city was decreased. However, the strengthening effect of TC16 wire processed by cold tandem rolling is more obvious a�er the solid solu on aging treatment, especially the tensile strength, yield strength and shear strength of the wire are higher than that of hot drawing processing.
Compared with hot drawing process, TC16 alloy wire produced by cold tandem rolling process has lower tensile and yield strength a�er common annealing, and its superior plas city provides a good basis for cold heading process of TC16 alloy fasteners. TC16 alloy wire produced by cold tandem rolling process has high tensile and yield strength and high shear proper es a�er solid solu on aging, which can improve the performance of fasteners a�er cold heading processing.