Titanium alloy is one of the materials with the highest specific strength among the currently used materials. It has a series of excellent properties such as strong corrosion resistance, high strength, low density, and stable medium temperature performance. It is widely used in aerospace industry, biomedicine, petroleum industry and atomic energy. It is widely used in high-tech fields.
In the hot working process of titanium and titanium alloys, free forging is one of the main processing methods. Taking titanium and titanium alloy bars as an example, free forging can process finished bars and blanks in intermediate processes. As an intermediate blank, it is a material that provides a certain shape, a certain size, a certain structure and properties for subsequent processing such as forging and rolling. For the finished product, not only the organization and performance of the material should be strictly controlled, but also the size and surface quality should meet the requirements, and any one of them cannot be delivered. For example, a bar with a fixed length of Φ152+3/-0×2000mm cannot be delivered just because one crack is repaired and ground out of tolerance by 2mm. Although it can be used for other purposes, it affects the delivery time. In the actual production process, such examples are not uncommon. Therefore, how to adopt the correct process route and reasonable forging method to ensure the microstructure, properties and surface quality of the material, especially to prevent the forgings from substandard performance due to the microstructure or the appearance of deep cracks and folds on the surface resulting in excessive size, has become an engineering technology. one of the subjects of research.
Free forging equipment and main methods
In the current actual production, the commonly used heating furnaces for free forging include coal furnace, natural gas furnace, electric furnace and so on. The first two heating furnaces are commonly used for ingot opening. Because the heating temperature of the electric furnace is easy to control, the accuracy is high (generally ±10 °C), and the degree of pollution is small, the heating before the finished product generally uses an electric furnace. Forging equipment includes forging hammer, hydraulic press, fast forging machine, etc. The basic methods of free forging titanium and titanium alloy bars are drawing and upsetting, or a combination of drawing and upsetting. Of course, for small-sized bars such as Φ12, Φ20, etc., rolling is required, which will not be introduced here.
1. Pull out
Plastic deformation of metals follows the rule of volume invariance and the rule of least resistance. When drawing, the length of the blank becomes longer and the cross-sectional area becomes smaller. The process is roughly divided into three steps: drawing, chamfering, and rounding. When drawing, the feeding amount should not be too large, and generally should be less than the width of the blank. At this time, the longitudinal flow of the metal along the blank is greater than the lateral flow. On the contrary, the lateral flow of the metal along the blank is greater than the longitudinal flow, which reduces the drawing efficiency. At the same time, the unilateral pressing amount should be equal to or less than the feeding amount, otherwise folds will occur. Moreover, the edges of the upper and lower anvils of the forging hammer should be rounded, otherwise folding will also occur. When chamfering, the striking force of the forging hammer should be lighter to avoid cracks in the center and end of the billet, and the chamfering should be timely, otherwise, cracks may easily occur at the edges and corners due to the rapid reduction of the edge temperature. When the forging starts to be forged, the temperature drops due to the turnover of the furnace. At this time, the hammer should be lightly and fast. After the temperature rises, the heavy hammer should be beaten slowly. In the later stage of forging, due to the large temperature drop, the hammer should be lightly and fast. Otherwise, the forging should be beaten fast. The surface is easy to crack, and even internal cracks occur.
In actual production, the forgeability of the material can also be changed by changing the stress state. If the flat anvil is used to pull the length, the lower anvil can be changed to a V-shaped anvil. Due to the effect of the compressive stress on the side of the blank, the tensile stress at the core of the blank can be reduced and cracks can be avoided. Baoji Xinglong Titanium Industry Co., Ltd. uses V-shaped anvils to forge finished and semi-finished bars of different specifications in the lower anvil of its 1600T fast forging machine, and the yield of materials has been effectively improved.
2. Upsetting
During upsetting, the height of the blank becomes smaller and the cross-sectional area becomes larger. When the amount of deformation is large, the defects such as dendrites and segregation in the center of the billet can be broken more than the elongation, so as to achieve the purpose of improving the structure. During upsetting, the forgings are subjected to axial compressive stress, but there is a maximum shear stress at an angle of 45 degrees to the axis, so oblique cracks are prone to occur along this direction. Occasionally, longitudinal cracks can also occur due to tensile stress. When the upsetting ratio H0/D0 (that is, the ratio of the height to the diameter of the blank) = 3, if the hammering force is insufficient, the two ends of the blank will have a double drum shape. At this time, it should be rounded first. . Sometimes, in order to make the end face flush, the blank should rotate properly in the horizontal direction while the hammer head is pressed down. When H0/D0>3, there will be longitudinal bending, which should be straightened first and then upset. In general, in the upsetting process, H0/D0<3, and it is best to be in the range of 2-2.2.
Influence of Free Forging on Microstructure and Properties
In the actual production process, the diameter of the bar varies from small to large, as small as about 10mm, and as large as 300mm or even larger. Materials of different specifications determine different processing routes. For materials with larger diameters and higher performance standards, if the ingot is single-stretched to a bar with the required diameter, the deformation of the material will be extremely uneven and insufficient, resulting in insufficient fragmentation of the as-cast structure. The structure is uneven, the grains are coarse, and the performance of the material is not up to standard. Therefore, when processing large-sized bars, in order to make the material fully deformed, the structure is uniform, and the grain size is refined, so as to improve the comprehensive performance of the material, the combination of drawing and upsetting is often used repeatedly, and the upsetting is often reversed.
Deformation temperature, deformation speed and deformation degree are very important parameters in the forging process, which play a decisive role in the microstructure and properties of the material. The deformation temperature includes two aspects. It is necessary to ensure an appropriate heating temperature, that is, to ensure heating and forging in a certain phase region to ensure the structure after forging. At the same time, the final forging temperature should also be strictly controlled. Forging cracks, even internal cracks are formed, and the flaw detection is not satisfactory and is scrapped; the final forging temperature is too high, due to static recrystallization, it will cause coarse structure and reduce performance. For example, for TC4 titanium alloy finished bars, the one-fire to two-fire forging before the finished product must be heated and forged in the α+β two-phase region. , known as the three-state structure, which determines the comprehensive properties of the material such as strength and plasticity. When forging small forgings with a small forging hammer, the temperature rise caused by the excessive deformation speed is worth noting. If the temperature rises during the forging process or is too close to the heating body during heating, the local temperature exceeds α+β/phase transition point , it will deteriorate the structure, as shown in Figure β b, α is straight, β grains are coarse, and there is grain boundary α, which is called superheated structure, which greatly reduces the performance of the material. The deformation speed is also important. For example, the deformation speed is very slow during forging with a hydraulic press, and dynamic crystallization will occur during the forging process, which is beneficial to improve the processing plasticity. However, when forging with a forging hammer, the deformation speed is faster. At high deformation speed, dynamic recrystallization is often too late to carry out, which will lead to an increase in deformation resistance. Appropriate deformation speed will neither make the temperature of the forgings rise too high to prevent the deterioration of the structure, nor will the temperature decrease a lot, which requires a reasonable control of the number of times and the weight of the forging hammer.
The size of the deformation also has a very important influence on the structure and properties of the forging. When the ingot is opened, the amount of deformation must be increased, generally 70%-80%, in order to break and improve the casting structure (dendrites, columnar crystals, inclusions, segregation, pores, looseness, etc.). When the finished product is forged, the deformation should not be too small, generally not less than 50%, otherwise the fine-grained structure will not be obtained, and it should not be too large, otherwise the surface quality of the forging will be deteriorated. Of course, if the deformation amount of the last fire is not enough, the temperature can be appropriately lowered, and a better structure can also be obtained.
Process determines organization, and organization determines performance. In actual production, various factors must be comprehensively considered to ensure that the forging material (or forging) that has undergone a certain amount of deformation can have the required structure and properties.
Summarize
1) During the free forging process, the forging ratio, the reduction amount, the feed amount, the initial forging temperature, the final forging temperature and the frequency and weight of the hammer are better controlled, which can effectively prevent the occurrence of cracks and folds;
2) In the required phase region, the β structure with a certain amount of equiaxed primary α plus transformation can be obtained with a suitable deformation rate within the specified deformation range, and the comprehensive performance is good.
Contact us for more information. Thank you
Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
Cuntry:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
Cel:+86 13369210920
Gmail:nicole@jmyunti.com
Website:www.jm-titanium.com
