3.2 Deficiencies investigation of investment casting
A precision casting process makes the traditional cable cutter. However, due to the difference in geometrical thickness of the product (aspect ratio > 100), it is easy to cause defects such as pore cracking. It will result in the product's poor mechanical properties, and the entire processing process takes a long time, and the production cost is high. The related dewaxing will cause many defects, as shown in Fig. 2(a), and the defect microstructure will be observed in Fig. 2(b) – 2(d). As shown in Fig. 2(b), there are many different size hole defects and cracks in the red region. This is the pore cracking caused by dewaxing casting, as shown in Fig. 2(c). Furthermore, Fig. 2(d) shows that the blue region has a defect of ~ 3mm. The crack has been produced when the dewaxing is completed and will not be affected by the production's advancement to increase the number of cracks.
The actual process of precision toothed cable clamp and precision hand tool forging tooth is mainly precision casting. The dewaxing process will cause many defects, mainly in the form of pores. It also has a significant impact on subsequent heat treatment and related mechanical cutting processes. After surface polishing and bottom milling 2mm (to eliminate the flash and show the cutting edge), the workpiece is shown in Fig. 3(a). Even after the surface polishing and bottom milling, the workpiece still has defects in the blue region and red region, as shown in Fig. 3(b). The defect microstructure can be observed by electron microscopy, as shown in Fig. 3(c) – 3(d). They have 4mm defects and 3.5mm defects in the blue region and redregion,respectively.
The instrument construction used for the last forging processes and production stages of the adjustable blade's near net shape extrusion-forging process was shown in Fig. 4. The die of gear tooth forging the adjustable blade and comparing it with the numerical model was shown in Fig. 4(a). Besides, Fig. 4(b) compares the flattening forging die with the numerical model. The forging dies were built by a collaborative company and fitted on a 350kN hydraulic testing device. At the near net shape extrusion-forging process of the multicore cable cutter gear, the forming dies were heated up to 200 ± 20℃ in advance, and the high-temperature lubrication was water-based graphite lubricant. The billet was heated up to 1000 ± 10℃ forging to the adjustable blade by forging dies. The experimentally forged commodities are matched with the simulation analysis outcomes to confirm the precision and workability. Figure 4(c) – 4(d) shows a matching of the experimental and simulated outcomes. Figure 4(c) explains the experimental results matched with the preform simulation results.
The comparison explains that the experimental results' gear tooth is very close to the simulation analysis results. After forming experiments, the components were chilled to room temperature in the air.
Figure 5 presents the tooling forging structure and production stages of the near net shape extrusion-forging process design of the fixed blade. Concerning the ease of the mold release and efficacy of the subsequent process, machining allowances at the flash areas are 2 mm thickness. Similarly, Fig. 5(a) shows the fixed blade forging die and compares it with the numerical model. Moreover, Fig. 5(b) compares the flattening forging die with the numerical model. In order to construct a dimensional model, SolidWorks software was being used. Design factors including forming shape, pattern sketch, inside and outside fillet, forming tolerance, finish allowance, Etc. That needs to be considered for the forging design. Furthermore, according to the forging shape, the forming die was designed.. Figure 5(c) shows the numerical (left) and experimental (right) outcomes of the laser cutting preform of a fixed blade. Numerical simulation was performed by using the commercial FE code, QForm. QForm forging process simulation analysis software covers almost the forging process of metal volume forming. Comparing the experimental results and simulation results of finish forging is shown in Fig. 5(d), which shows a striking resemblance both in geometrical material filling and the flash location. The flash part reveals the actual positions, while the three protrusions marked the positions for knock-out areas. The carbon steel S45C and JIS SKD61 die steel is used for all the workpiece and near net shape extrusion-forging dies, respectively.
In this study, three different preforms for the forging workpieces were proposed, as shown in Fig. 6. Three different preforms of multicore cable cutter gear with Preform A of extrusion process (a), Preform B with the initial gear tooth of 2mm thick on forging workpiece (b), and Preform C without gear tooth on forging workpiece (c). Besides, numerical simulation improves and optimizes mold design by simulating issues that happen during the forming process. Preform A is the traditional forging perform as shown in Fig. 6(a). It can be seen that the gear tooth is extruded from the forging workpiece. Preform B is the novel extrusion-forging teeth perform with the gear tooth on forging workpiece as shown in Fig. 6(b). Finally, the preform C forging method is the same as preform B, but there is no gear tooth on the forging workpiece, as shown in Fig. 6(c).
The forming load results in the different preforms illustrated in Fig. 7. The differences in the forging loads are apparent under the three different forging preforms. It can be observed that Preform A has the most considerable forging load, and the maximum forging load is up to 6.06MN. The forming loads of Preform B and Preform C are 4.05 and 3.97, respectively. Preform A's forming load is much greater than the other two preforms compared with Preform B and C. Therefore, Preform A's design is not included in the follow-up discussion. The differences in the simulation analysis of Preform B and C will be discussed behind. Finally, a preform that is best suited for the manufacture of gear tooth will be selected.
Preforms B and C reached a final deformation state of 100% stroke during simulated forging, as shown in Fig. 8(a). Two different preforms can be observed, all of which can simulate a complete tooth profile. In Fig. 8(b), the effective stress distribution simulation of the Preform B precision-machined forged workpiece reaches 120 MPa or more, and the maximum stress reaches about 300 MPa. Preform C's effective stress distribution is simulated to reach 100 MPa or more, and the maximum stress is about 280 MPa. When the deformation process is completed, the maximum strain of Preform B reaches about 11, and the maximum strain of Preform C reaches about 9.5, as shown in Fig. 8(c). As the above mold progresses, the plastic deformation of the gear teeth (especially the burr area) increases, and since the metal flow resistance in the complex tooth profile is considerable, effective strain accumulates in the gear tooth cavity. During the forging stages, both Preform B and C plastically deformed in the gear corner's vicinity such that the maximum deformation occurs in the flash region of the bottom die. After forging, the workpiece temperature increases with increasing pressure, the maximum temperature for Preform B was up to 1100 ° C, and the maximum temperature for Preform C was up to 1080 ° C, as shown in Fig. 8(d). The workpiece's surface temperature is significantly reduced due to the heat transfer from the lubricant between the workpiece and the mold. Finally, it can be found that the maximum value of stress, strain, and temperature, the Preform C is slightly lower than the Preform B, but there is not much difference in the simulated distribution.
The schematic diagram on the tooth dies (Preform B and Preform C) shows in Fig. 9. Figure 9(b)shows the differences in effective stress for forging processes between Preform B and C. In terms of the stress values, the tooth die requires the maximum force during the forging process. Nevertheless, the maximum effective stress of the two performs similarly, which is more than 1600 MPa. It is because of the high metal flow resistance on this part. Therefore, there is a severe deformation in this area.
Figure 10(a) shows the distributions of the forging temperature of Preform B and C. The workpieces' temperature was increased by Local large deformation. Furthermore, the high temperature will reduce the service life of tooth die. At Fig. 10(a), the maximum average temperature of Preform B is 513.05°C higher than Preform C (the maximum average temperature is about 506.9°C). From these results, Preform B and C's temperatures are roughly the same, which means both preforms have a similar deformation mechanism while the Preform C can be relatively obtainable through the delicate blanking process.
The displacement vector distributions for Preform B and C was displayed in Fig. 10(b). At the gear's summit of the workpiece's displacement vector is evident, which indicates the flash-forming procedure is the last forging moment. The tooth's maximum displacement vector of Preform B is 0.1416 mm, shown in Fig. 10(b), and the flash area's velocity is similar. Likewise, the maximum vector of Preform C is 0.1546 mm. In Fig. 10(b), the average displacement vector of tooth both Preforms B and C is similar, which means there is just a slight deformation at the workpiece's gear. Hence, Preform C was chosen for the near net shaped extrusion-forging process of adjustable gear device because the company will save more expense about laser cutting procedures for Preform B at the initial gear protrusion.
The evaluation index of forming and the resultant of forming quality can be used as the objective functions, particularly in the gear forming section. Figure 11(a) shows the Schematic region on the forging workpiece and schematic location on the gear tooth. The evaluation index of the height filling ratio (HFR) and the width filling ratio (WFR) of the formed gear on preform B and preform C has shown in Fig. 11(b) − 11(c), respectively. The complex formula for calculations of forming index was similarly performed as follows [?].
(1) The height filling ratio . Hs is the height of filling section of the workpiece; Hg is the designed height of gear toothed die.
(2) The width filling ratio filling ratio . Ws is the width of filling section of the workpiece; Wg is the the designed width of gear toothed die.
The HFR and WFR analysis outcomes explain in Fig. 11(B) and Fig. 11(C). The ideal height filling ratio of tooth top of experimental gear blade is Hg ( from 6.3 to 6 mm ), the maximum minus-deviation for Preform B and C is -0.02%, the mean deviation for Preform B is 0.987%, the mean deviation for Preform C is 0.985%. On the other hand, the width filling ratio of tooth top of experimental gear blade is Wg (from 1.25 to 1 mm), the highest minus-deviation is -0.19%, and the mean deviation is -0.693% for Preform B, and the highest minus-deviation is -0.17%. The mean deviation is -0.74% for Preform C. That implies the mean deviation of both HFR and WFR for Preform B and C is essentially identical.
Due to understand the microstructure's forging and the change of the forging line, the workpiece was cut by an electric discharge machining (EDM) wire cutter, and it was cut along the region A ~ F as shown in Fig. 12(a). The workpiece was ground and polished, corroded by the corrosive liquid mixed with 0.7% picric acid and 9.3% alcohol. The corrosion time is controlled to be about 15 ~ 20 seconds. After the corrosion, the surface is rinsed with water and dried to observe the metallographic microstructure and forge a line on the forging surface. Figure 12(b) shows the results of the forging line after simulation. Comparing the actual situation with the simulation results, the two are almost identical, especially in the E and F regions, and it is more evidence that they are similar. No forging line wrinkles were observed in the experiment, and the forging line was continuously continuous in the area where the forging was severely changed, which indicates that the forging process has a good design.
Lastly, An optical micrograph (OM) was used to research the forged product's microstructure analysis. A schematic picture of the forged gear tooth is shown in Fig. 13 (a). The OM micrograph of region I, region II and region III is shown in Fig. 13(b) – 13(d), respectively. Binarization of initial OM images converts grayscale into binary ones. The grain radius was calculated using MATLAB as 14.63, 9.78, and 12.71µm for region I, region II and region III.