Titanium alloy bars usually have a high reinforcing rib structure, and the shape of the parts is complex and the size is large. The traditional method of titanium alloy bars is mainly forging into simple shapes of blanks, and then combined with modern CNC machining and other methods to process one by one, the processing cost is increased, and the quality and strength of the obtained products are also significantly reduced [1]. Therefore, it is one of the urgent problems to be solved when searching for a process with stable processing performance and improved material utilization [2]. Die forging process is a new titanium alloy bar processing technology, which can effectively reduce the processing cost and improve the utilization rate of materials.
1. Partial loading forming technology of bar hot die
1.1 Overview of local loading forming technology
Bar forgings are complex circular forgings composed of bars and stiffeners, and the web and stiffeners add difficulty to the forming of the forgings. Therefore, the influence of die pressure on forgings must be considered in the forming design process of forgings. In order to effectively prevent the overall loading during the hot die forging process, the metal flows in a large amount along the radial direction and then the phenomenon of eddy current, folding, and turbulence occurs. This time, local loading pads and annular pads are used [3]. In the process of hot die forging forming test, the test blank is placed in the middle of the annular backing plate, and the local loading backing plate is placed between the blank and the upper die, which can effectively ensure that the metal flow deformation is fully filled into the mold cavity, effectively It avoids the problems of eddy current and turbulence along the radial flow.
The hot die forging forming processing technology of titanium bars is usually carried out by the overall loading method. In the overall loading technology, when the reinforcing ribs are filled to a certain extent during the forming process, the filling resistance of the ribs increases significantly, resulting in a large amount of metal along the bar. The center flows around, or flows along the radial direction, which causes turbulence, folding, eddy current and other phenomena, and finally makes the filling of the forgings not in place, making the quality of the forgings not up to standard. If the overall loading method is changed to the local loading and step-by-step forming method during the filling process, the direction of metal flow can be effectively controlled, so that the metal can be fully filled to the bottom of the reinforcing rib, and the metal flow in the radial direction can be avoided to the greatest extent. The probability of occurrence of the above problems can be better avoided.
1.2 Main steps of local loading forming technology
According to the characteristics of the bar forging during the hot die forging process, it is decomposed into two main steps for testing. The first step is the pre-forming stage of the bar workpiece. This process is to axially deform the blank under the influence of the local loading pad evaluation pressure, and preferentially fill the lower die rib and other positions, and put the metal along the The radial movement flows until the annular backing plate is fully filled; due to the small relative contact area between the mold and the blank during the filling process, and the extremely fast metal flow deformation, the heat loss of the blank during the local loading process is small. , the preformed parts of the workpiece can be obtained. On the basis of completing the first step, the pre-formed parts are finally formed, that is, the annular backing plate and the local loading backing plate in the bar forging are taken out, the pre-formed parts are heated to 950 ° C, and placed in The final forming process is carried out in the lower mold cavity. During the later deformation forming process, the thickness of the blank gradually decreases, so the contact area with the mold gradually increases. In addition, the deformation load increases significantly due to the excessively rapid temperature reduction during the final forming process.
1.3 Microstructure characteristics of bars after hot die forging
Until the two important steps of hot die forging of the bar are completed, the forgings formed by hot die forging are annealed, and the microstructure of the forgings is observed. A small amount of primary α-phase structure can be seen in the forgings, which is equiaxed. bimorphic and banded tissue distribution.
Since the annealing temperature is close to the transformation point of the titanium alloy rod, the content of primary α phase in the titanium alloy microstructure is relatively small. Defect analysis of the forgings shows that no obvious deformation defects are found in the forgings, which shows that the quality of the bars obtained by this method is reliable [3].
Using the local loading hot die forging forming technology, compared with the traditional forging process, the use of this technology method significantly improves the utilization rate of raw materials, greatly reduces the material waste rate, and indirectly reduces the forging cost. To sum up, the local loading method and hot die forging technology can obtain reliable quality titanium alloy products whose mechanical properties are far better than the design requirements, and can meet the basic requirements of enterprises for the quality of titanium alloy bars.
2. Introduction to other common traditional bar hot die forging processes
In addition to local loading forming technology, common hot die forging forming technologies include: ① α + β forging technology, which is to make α alloy or α + β alloy at the β transformation temperature under the condition of moderate strain rate This technology has the advantages of low cost and simple operation, and is widely used; this technology is prone to local overheating during the forging process, and it is easy to cause cracks on the surface of the forging, but with the improvement of the forging process of this technology, Gradually successfully forging finished products with high precision and smooth surface; ② β forging technology, this technology completes the forging process of titanium alloy under the high temperature condition of β phase, which has the advantages of high precision forging and small deformation resistance. Improve the life of the forgings; the α phase crystal interface in the microstructure of the titanium alloy is significantly increased, mostly in the form of a basket structure, which improves the creep resistance and strength of the titanium alloy; The forging process is a forging process in which the heat treatment in the phase region and the forging process are completed in the two-phase region. This technology has similar characteristics to the β forging. In the forging process, the boundary α phase is mainly broken first, so that the α phase in the boundary region gradually turns to form. α equiaxed transformation, thereby improving the overall properties of titanium alloys.
3. Conclusion
In summary, titanium alloy bars have broad application prospects in various modern fields due to their superior properties. Strengthening the research on titanium alloy forging technology will help promote the development of titanium alloy materials in more fields. Starting from the hot die forging forming technology of titanium alloy bars, this paper analyzes the advantages of the local loading hot die forging technology from two aspects, and analyzes the mechanical properties and microstructure of the forgings. The loading method and hot die forging forming technology can obtain titanium alloy products with reliable quality, whose mechanical properties are far better than the design requirements, and can meet the basic requirements of enterprises for the quality of titanium alloy bars.
references:
[1] Li Liang, Liu Xilin. Research on hot die forging forming technology of titanium alloy diaphragm forgings [J]. Forging Technology, 2010,35(06):11-13.
[2] Gao Jun, Li Miaoquan. Research progress and development trend of precision forging technology [J]. Precision Forming Engineering, 2015,7(06):37-43+80.
[3] Wu Ruiheng. Computer Simulation of the Forming Process of GH4169 Alloy Hot Die Forging [C]. Chinese Society of Stereology, 2005:59.