Optimization of Forming Scheme for 1Cr17Ni2 Stainless Steel Bolt Forgings for Aircraft

Optimization of Forming Scheme for 1Cr17Ni2 Stainless Steel Bolt Forgings for Aircraft

To solve the problems of poor surface quality, multiple forging processes, and low die life of 1Cr17Ni2 stainless steel bolt forgings for aircraft, the forging process of 1Cr17Ni2 stainless steel bolt forgings for aircraft was studied based on DEFROM numerical simulation software. The results show that by increasing pre-forging, the wear of the forging on the final forging cavity is reduced, and the service life of the die is improved. Using a holding and cutting die for edge cutting ensures the quality of edge cutting, and one-time forging is achieved. The trial production results using the optimized process are in good agreement with the numerical simulation results, the mold life has been significantly improved, and the various indicators of the product meet the requirements.

1Cr17Ni2 stainless steel belongs to martensitic stainless steel, generally used at 400 ℃ after quenching and tempering, and has excellent mechanical properties, corrosion resistance, and machining performance. Domestic and foreign scholars have researched the composition control, corrosion resistance, and forging process of 1Cr17Ni2 stainless steel. The results show that 1Cr17Ni2 stainless steel has good plasticity at temperatures above 900 ℃, and its plasticity is determined by the degree of dynamic recrystallization completion and volume fraction. The more fully dynamic recrystallization is performed, the better the plasticity of 1Cr17Ni2 stainless steel is.

1. Traditional forging process for 1Cr17Ni2 stainless steel bolts

As can be seen from Figure 1 and Figure 2, 1Cr17Ni2 stainless steel bolt forgings belong to long shaft forgings, which are widely used in various military and civil aircraft, and the number of applications on each aircraft is huge. The forging shape is the intersection of two cylinders, ϕ The 6.8 mm cylindrical surface is non-machined, and the dimensional tolerance requirements for forgings are high. The traditional production process is shown in Figure 3. It can be seen that there are many processes in the forging process, resulting in unstable process control; On the other hand, there are many production processes and a large number of products, resulting in a sharp rise in the production cost of forgings.

Figure.1 Bolt Forging Drawing

Figure.2 Three-dimensional Modeling of Bolts

Figure.3 Traditional Production Processes

The 1Cr17Ni2 stainless steel bolt forgings produced using traditional forging processes have high scrap rates, long production cycles, and short mold life. Through statistical analysis of data such as the qualification rate, production cycles, and mold life of the bolt forgings produced in the previous company, it can be seen that using traditional processes to produce 1Cr17Ni2 stainless steel bolt forgings will cause a large amount of cost waste.

2. Optimized forging process plan

Based on the analysis of traditional forging process schemes for 1Cr17Ni2 stainless steel bolt forgings, it can be seen that the main factors causing various production processes are the following: after forging, the forging has a large amount of under pressure, and the forging does not meet the requirements of the drawing; After forging, there is a large amount of burring in the trimming, which requires manual polishing to remove the burr, affecting production efficiency and increasing labor production costs; During the forging process, the mold is extremely prone to failure, and the shape and size of the forging do not meet the requirements; The trimming process causes warpage of the forging, and it is necessary to add a correction process to eliminate the warpage of the forging; The heating process in the correction process is prone to produce oxide scales that are difficult to remove, affecting the surface quality of the forging.

It can be seen that the most fundamental issues mainly focus on: first, the service life of the forging mold and the size of the forging after one heat; Secondly, when trimming forgings, long rod forgings are extremely prone to warpage, and correction procedures have to be added. The forging mold and trimming mold are now optimized, and the optimized process is simulated based on the DEFORM software. The mold size is further optimized through the simulation results.

2.1 Optimization of the mold structure

Optimize the mold structure and design a pre-forging cavity. The main structure of the pre-forging cavity is similar to that of the final forging, mainly optimizing the burr structure of the pre-forging cavity and the concave corners of the forging, increasing the concave corners, and avoiding the reduction of the mold life caused by the violent flow of metal. The main functions of the pre-forging cavity are: to deform the main deformation on the pre-forging cavity, reduce the wear of the final forging cavity, and ensure the service life of the forging; The excess metal is discharged through pre-forging, and finally, it is pressed against the final forging cavity to ensure that the under-pressure amount of the forging is within a controllable range.

2.2 Structural Design of Holding and Cutting Die

The holding and cutting die is designed according to the structural characteristics of 1Cr17Ni2 stainless steel bolt forgings. According to the blanking and punching composite die in the stamping manual and the actual situation of the forgings, the specific structure of the holding and cutting die for 1Cr17Ni2 stainless steel bolt forgings is shown in Figure 4.

Working principle: After final forging, place the forging (with burrs) on the lower top core (the top core is manufactured according to the shape of the forging to ensure the stability of the forging), and start the trimming press; The male mold first contacts the forging, and after contact, the male mold continues to move downward. The male mold and the female mold shear the rough edges of the forging to remove the rough edges the forging; When the equipment reaches the lower dead center, the male mold and the female mold are trimmed, and the equipment moves up with the male mold. The forging pushes out the female mold under the restoring force of the lower spring. Finally, use pliers to clamp the forging and the rough edges. The main function of the holding and cutting die is to control the warpage of 1Cr17Ni2 stainless steel bolt forgings and avoid adding correction procedures.

Figure.4 Assembly Drawing of the First Designed Holding and Cutting Die

1-lower mold base, 2-guide pillar, 3-lower mold fixing plate, 4-female mold, 5-guide sleeve, 6-upper mold base, 7-male mold fixing plate, 8-base plate, 9-male mold, 10-upper positioning pin, 11-lower top core, 12-spring fixing rod, 13-lower positioning pin, 14-spring

2.3 Numerical simulation results

According to the above problem analysis, adding a pre-forging cavity reduces the wear of the final forging cavity and ensures product quality. Based on the DEFORM finite element numerical simulation software, simulate the wear of the final forging cavity, and compare the wear depth of direct forming molds with pre-forging and final forging molds.

Table.1 General Forging Process Parameters

Forging process parameters	Set value
Forging equipment	300t friction press
Friction coefficient	0.4
Heating temperature	1160°C
Thermal conductivity	11N/(s.mm.°C)
Meshing	Minimum 0.5mm, scale 2
Mold material	H13
Mold hardness	52HRC
Material Science	1Cr17Ni2

Figure 5 shows the wear depth distribution of the upper and lower mold cavities for direct final forging. The figure shows that the maximum wear depth of the upper mold is 0.00614mm, and the maximum wear depth of the lower mold is 0.00622mm. The wear depth of the lower mold is higher than the upper mold because the lower mold first contacts the blank, resulting in a higher temperature of the lower mold cavity than the upper mold. During the forming process, the lower mold is more prone to wear.

Figure 6 shows the wear depth distribution of the upper and lower mold cavities for pre-forging and final forging. The figure shows that the maximum wear depth of the upper mold is 0.00373mm, and the maximum wear depth of the lower mold is 0.00370mm. The wear depth is smaller than that of the directly formed mold.

From the above analysis, it can be seen that the reasonable selection of pre-forging and final forging forming molds has significantly improved the mold life, providing a stable guarantee for the subsequent batch production of 1Cr17Ni2 stainless steel bolt forgings.

Figure.5 Distribution of Wear Depth of Direct Final Forging Dies

Figure.6 Distribution of Wear Depth of Performing and Final Forging Dies

Figure.7 Optimized Process Production Procedure

Figure.8 Forgings Produced after Process Optimization

2.4 Final optimized forging process flow

According to the numerical simulation results, the mold structure and production process are improved and optimized. The optimized bolt forging process flow is shown in Figure 7. By adding pre-forging and increasing the design of the holding and cutting die, a large amount of grinding work after forging and increased cost waste in the correction process can be reduced, effectively reducing production capacity waste and improving production efficiency.

3. Batch production verification after optimization

Conduct sample trial production of the optimized process plan and the trial production product is shown in Figure 8. The optimized process’s rough edges, surface quality, and forging quality meet the requirements. At the same time, mold life has also been significantly improved.

4. Conclusion

• (1) Based on the numerical simulation software DEFORM, the mold wear was analyzed, and after production trial production, the trial production results were consistent with the simulation results. The actual production verified the accuracy of the simulation results.
• (2) Using the optimized process plan to produce multiple batches of forgings, it can be seen from the results that the optimized forging process produces good surface quality forgings, reducing a lot of grinding work. The remaining indicators also meet the standard requirements. The optimized process plan is reasonable and feasible, which can provide design ideas for the forging process of the same type of forgings.

Author: Cao Zhenjun, Han Zhifei, Shi Wenpeng

Source: Bolt Forgings Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)