Titanium alloy has extremely excellent properties, such as small density, corrosion resistance, high strength, high yield ratio, etc., and has a wide range of applications in aviation, ships, power stations and medical fields. In recent years, the use of titanium alloy plate has increased year by year, and the demand in various fields has increased significantly. Due to the high deformation impedance of titanium alloy sheet and the narrow temperature range of hot working, it is easy to produce cracks during hot processing, which increases the difficulty of manufacturing. Small-sized titanium alloy plates generally refer to plates with a thickness of less than 6mm, which are larger in the aerospace and medical market. In the case of a large market, customers have relatively strict requirements for the organizational performance of their products, therefore, the hot processing link must have a suitable process to obtain better organizational performance, and the production process route of upsetting forging slab heating and rolling is often used in the production of small-gauge plates, but the organization is often not particularly optimized, and only the forging process will actually bring great differences. Titanium alloy is a difficult deformable metal, and forging is particularly important in the hot processing of all titanium materials, so it must be paid attention to in the research and development of slab forging process.
On the other hand, the thermal parameters such as temperature range, deformation degree and deformation speed of forging also have a great impact on the structure and properties of titanium alloy forgings [1-2]. Casting forging methods are generally divided into three types: free forging, die forging and upsetting forging, in which die forging is a forging method that uses a die to form a blank on a special die forging equipment to obtain forgings, and is not suitable for the production of slabs.
The purpose of this paper is to compare the two methods of free forging and upsetting forging to produce TC4 titanium alloy small-size plates, so as to obtain the optimal production process.
1. Experimental process
The selected ingredients are in accordance with GB T36201-2007 [3] TC4 titanium alloy ingots were produced in the production experiment, and its composition is shown in Table 1, and its phase transition point is 970. The 3150t hydraulic press is used for forging, and the 2800mm four-high reversible hot rolling mill is used for rolling with an 800mm four-high reversible hot rolling mill to produce small-sized plates.
Process plan: 30kg ingots are slitted in the middle, forged in two ways, and processed to the size of the slab to be rolled (70 180) mm 1000 100mm 1500 100mm (thickness, width, length) before rolling production. The free forging process is: 1150 one-fire forging, 1050 two-fire forging and slab shaping to the corresponding size.
The upsetting forging process is: 1150 one-fire upsetting forging (three upsetting and three drawing), 1050 two-fire upsetting forging (three upsetting and three drawing), depending on the cracking of the slab for 1050 return furnace insulation, shaping, forging for the size of the slab, wherein, upsetting slab according to 0 0 (where 0, 0 are the height and diameter before upsetting) 15～2.0 times the upsetting ratio to perform [3-].
The selection of the above process route is to ensure the effectiveness of the comparison, because the conventional free forging two fires are enough to complete, so after the calculation of the upsetting ratio, the upsetting is also operated according to the two fires, the temperature selection is carried out in accordance with the cooling gradient summarized by many years of production accumulation, the first fire is above the phase transition point, and the second fire is the near phase change point.
The rolling is carried out using the same process to ensure the effectiveness of the contrast. Using a 2800mm four-high reversible hot rolling mill, a slab with a thickness of 170 180mm is rolled at 9 0 to a slab with a thickness of 50 60mm.
Under 920, a slab with a thickness of 50 60mm is rolled into a slab with a thickness of 20mm (reversing rolling). Under 920, a slab with a thickness of 20mm is rolled into a slab with a thickness of 10mm. Using an 800mm four-high reversible hot rolling mill, a slab with a thickness of 10mm is rolled at 900 to a thickness of 36mm finished sheets. The results of the transverse and longitudinal metallographic and performance of the finished plate were cross-compared, and the differences between the two different forging methods under the same subsequent process were compared [4-5].
2. Experimental results and analysis
2.1. Comparison of metallographic structure of forged slabs.
By comparing the structure obtained by the two forging methods, it can be found (Fig. 1) that there is basically no difference in the transverse and longitudinal aspects of the obtained structure in either free forging or upsetting forging, which is due to the fact that the ingot forging slab is different from the unidirectional elongation forging, and the metal flow is always in a certain area in the microscopic state, unlike the free flow end in unidirectional elongated forging, which will cause obvious differences.
From the slab structure obtained by free forging (Fig. 1 and Fig. 1), it can be found that the organization in the two directions is basically the same, and the microstructure and morphology are more complex, and there is a Wechsleigh microstructure and local basket structure.
The composition of this morphology is due to: the forging temperature of the second flame is above the phase transition point, and the free forging is mainly formed, so the processing speed is faster, resulting in the terminal temperature between the phase transition point and the non-uniformity, the local Weissen structure is formed above the phase transition point, and a primitive grain boundary is formed at a slower cooling rate from the transition point above, and the grain is composed of small or small sheets, and there are coarse bundles, long and straightThe local basket structure is the structure obtained after the deformation is terminated below the phase transition point, which is a piece or small piece obtained by the fragmentation of the original grain boundaries, which is short, crooked, and has a small aspect ratio.
From the slab structure obtained by upsetting forging (Fig. 1 and Fig. 1), it can be found that because upsetting forging is non-directional, the material is forged in multiple directions, so the horizontal and longitudinal consistency is very good.
The overall morphology of the tissue showed a bimorphic organization, and there were secondary phases in the local area. This is due to the long time of the upsetting forging process, the temperature has dropped to the temperature range of the two-phase zone when the forging is completed, and part of the forging process is completed in the two-phase zone. The appearance of the secondary phase is due to the decomposition of some phases during the cooling process to produce the phase [5]. According to the comparison of the tissues obtained by the above two forging methods, the tissues obtained by upsetting forging are better and more stable [6].
2.2. Comparison of metallographic structure of rolled slabs.
Fig. 2 shows the metallographic structure obtained by rolling the slab forged by two methods. The change in thickness dimensions is 600mm-->20.0mm-->3.6mm, the crushing equiaxial situation of the free forging organization as a whole, with the progress of rolling, the closer to the finished product, the better the structure morphology, the horizontal and longitudinal structure has been completely broken equiaxial, and the transverse structure is better than the longitudinal structure, which is because of the rolling direction of rolling, resulting in the difference in the structure of the two directions.
As can be seen from Figure 3, the overall change of the upsetting forging structure is small, and the equiaxed grain becomes smaller with the processing size, but the overall degree of crushing is not enough, which is caused by the stability of the phase, and the transformation structure basically disappears completely, and there is basically no difference between the horizontal and longitudinal directions, which is the advantage of upsetting. In general, the microstructure of free-forged plates is better than that of upset plates [7-9].
2.3. Comparison of the performance results of the finished plate.
Table 2 shows that the performance of the finished plate produced by the free forging method is better than that of the plate produced by upsetting forging.
The performance comparison results also fully reflect the process of the evolution of metallographic structure, the structure of the free forging slab is still in the early stage because it is formed above the phase transition point, but the upsetting forging slab starts above the two-phase zone and completes the forging process below the phase transition point, and the structure is broken to a certain extent, and the phase state is relatively stable.
In the case of sufficient rolling deformation, after the slab of the two forging methods enters the rolling process, the freely forged slab can be fully broken and deformedOn the contrary, the slab structure of upsetting forging is not sufficient due to the relatively stable phase state, but the subsequent rolling deformation and crushing ground are not sufficient. As a result, the organizational differentiation of the final finished plate is obvious.
2.4. The hardness value of the board surface.
Fig. 4 is the schematic diagram of the stepped hardness sampling of the finished product 1000mm 500mm plate, the size of each sample is 0mm 0mm, the length direction sampling is carried out according to the spacing of every two points 140 160mm, and the width direction sampling is carried out according to the spacing of every two points 40 50mm.
As can be seen from the hardness values in Table 3, the surface hardness of the freely forged sheet is slightly higher than that of the upset forged sheet, and the hardness value results further verify the influence of the forging method of the material on it, corresponding to the metallographic structure and tensile properties results described above [10-12].
3. Conclusion
1) Compared with ordinary free forging, the upsetting forging process improves the slab structure greatly, and the more deformation the upsetting process, the better the structure obtained.
2) In the case of the determination of rolling deformation, compared with upsetting forging, the slab obtained by ordinary free forging is more suitable for the production of TC4 titanium alloy small-gauge sheet, and the microstructure of small-gauge plate is better.
3) When producing large-size thick plates, when the deformation of the rolling mill feeding and finished product size is insufficient, upsetting forging can be considered, so that the obtained structure can reach a certain equiaxial state during upsetting, and the general requirements of large-size thick plates will be slightly lower than that of thin plates.
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