A method for improving the microstructure uniformity of large 7075 aluminum alloy forgings

Abstract

The present application belongs to the technical field of hot working of aluminum alloy, and relates to a method for improving the uniformity of the structure of a large 7075 aluminum alloy forge piece, comprising the following steps: determining the forging heating temperature of a bar or a blank according to the total content of Fe+Cr+Mn elements; determining whether the bar needs to be pre-deformed before the roughing process according to the total content of Fe+Cr+Mn elements; pre-deforming the bar to obtain a blank in the case of needing pre-deformation; roughing the bar or the blank to obtain a rough shape; finally forging the rough shape to obtain a required forge piece; and heat treating the forge piece. The method solves the problems of coarse grains, uneven structure and poor ultrasonic level of the large 7075 aluminum alloy forge piece caused by the fluctuation or uneven distribution of impurity element content.

Description

Technical Field

[0001] This invention belongs to the field of hot working technology of aluminum alloys, and relates to a method for improving the uniformity of microstructure of large 7075 aluminum alloy forgings. Background Technology

[0002] 7075 aluminum alloy belongs to the 7-series Al-Zn-Mg-Cu series of ultra-high-strength aluminum alloys. This alloy was first applied to the aircraft manufacturing industry in the late 1940s and remains widely used in the aerospace industry today. Its characteristics include good plasticity after solution treatment, excellent strengthening effect after heat treatment, high strength below 150℃, and particularly good low-temperature strength. It is especially suitable for manufacturing aircraft structures and other high-stress structural components requiring high strength and corrosion resistance.

[0003] Large 7075 aluminum alloy forgings, such as annular housing forgings, are typically produced using a combination of upsetting, punching, and reaming, with ring rolling as the final forging process. After heat treatment, these forgings often exhibit coarse grains, poor uniformity, and fail ultrasonic testing. Once these issues arise, they cannot be corrected through heat treatment or other methods and must be scrapped. Research indicates that this phenomenon is related to the improper content of certain components in the 7075 alloy bar and the inappropriate formulation of the processing methods. For example, the addition of high-melting-point impurity elements such as Fe, Cr, and Mn forms a certain number of intermetallic compounds with Al atoms, which can increase the alloy's strength. However, these compounds have high melting points and are difficult to homogenize. This firstly leads to an uneven microstructure in the bar, and secondly, if the process parameters are not properly set in subsequent forging processes, the forging will also fail to achieve a uniform microstructure. Depending on the content of high-melting-point impurity elements such as Fe, Cr, and Mn, we must set the forging heating temperature appropriately and pre-deform the bar stock before forging to improve the matrix structure and the morphology of intermetallic compounds. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the uniformity of microstructure in large 7075 aluminum alloy forgings, in order to solve problems such as coarse grains, uneven microstructure, and poor ultrasonic levels caused by fluctuations or uneven distribution of impurity element content in large 7075 aluminum alloy forgings.

[0005] The technical solution of this invention:

[0006] A method for improving the microstructure uniformity of large 7075 aluminum alloy forgings, comprising:

[0007] The forging heating temperature of bars or billets is determined based on the total content of Fe, Cr, and Mn elements.

[0008] Whether the bar stock needs to be pre-deformed before the roughing process is determined based on the total content of Fe, Cr and Mn elements.

[0009] When pre-deformation is required, the bar stock is pre-deformed to obtain the billet;

[0010] The bar stock or billet is shaped into a rough shape;

[0011] The rough shape is then forged to obtain the desired forging.

[0012] Heat treatment is performed on the forgings.

[0013] Furthermore, the forging heating temperature of the bar or billet is determined based on the total content of Fe, Cr, and Mn elements; and whether the bar should be pre-deformed before the roughing process is determined based on the total content of Fe, Cr, and Mn elements, specifically including:

[0014] For bars with a total Fe+Cr+Mn content of <0.30%, the forging heating temperature is 410-420℃, and the roughing and final forging are carried out directly.

[0015] For bars with a total Fe+Cr+Mn content ≥0.30%, the forging heating temperature is 420~435℃, and additional pre-deformation is performed on the bars before roughing. The pre-deformation process specifically includes:

[0016] The pre-deformation heating temperature is 410~435℃, and the free forging and upsetting / drawing alternating deformation method is adopted. The deformation amount of a single step in the upsetting / drawing process should be ≥25%, but not more than 60%. The air hammer or electro-hydraulic hammer impact deformation equipment is used, and the final forging temperature is ≥380℃, with 2~4 heats of pre-deformation.

[0017] Further, the bar stock or billet is processed into a rough shape to obtain a rough form, specifically including:

[0018] For bars with a total Fe+Cr+Mn content of <0.30%, the rolling heating temperature is 410-420℃; for bars with a total Fe+Cr+Mn content of ≥0.30%, the rolling heating temperature is 420-435℃, the rolling deformation is ≥25%, the forging forming method is free forging or die forging, the final forging temperature is ≥380℃, and the bars are air-cooled after forging.

[0019] Furthermore, the rough shape is then subjected to final forging to obtain the required forgings, specifically including:

[0020] For the rough shape of bar stock with a total Fe+Cr+Mn content of <0.30%, the final forging heating temperature is 410-420℃; for the rough shape of bar stock with a total Fe+Cr+Mn content of ≥0.30%, the heating temperature is 420-435℃, the final forging deformation is ≥30%, and free forging or die forging is adopted. The final forging temperature is ≥380℃, and the bar stock is air-cooled after forging.

[0021] Furthermore, the final forging undergoes heat treatment, specifically including:

[0022] Solution treatment, first aging, and second aging.

[0023] Furthermore, the solution treatment temperature is 460–470℃, the heat retention coefficient is 1.5–2 min / mm, and then it is naturally cooled to room temperature by air or water.

[0024] Furthermore, the first aging temperature is 100-115℃, and the temperature is maintained for 6-7 hours, followed by natural air cooling to room temperature.

[0025] Furthermore, the second aging temperature is 170-185℃, and the temperature is maintained for 8-9 hours, followed by natural air cooling to room temperature.

[0026] The beneficial effects of this invention are:

[0027] Large 7075 aluminum alloy forgings, such as annular casing forgings, are typically produced by a combination of upsetting, punching, and reaming, with ring rolling reaming as the final forging process. After heat treatment, these forgings often exhibit coarse grains, poor uniformity in the low-magnification microstructure, and fail ultrasonic testing. Determining the forging heating temperature of the bar or billet based on the total Fe+Cr+Mn element content, and adding an appropriate pre-deformation process before the main bar forging process based on the total Fe+Cr+Mn element content, can effectively solve the problems of coarse grains, uneven microstructure, and poor ultrasonic testing levels caused by fluctuations or uneven distribution of impurity element content in large 7075 aluminum alloy forgings. This can significantly improve the overall performance and market competitiveness of the product. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention.

[0029] Firstly, through comparative research, the applicant found that the total Fe+Cr+Mn content of imported 7075 aluminum alloys is generally less than 0.29%, while the total Fe+Cr+Mn content of domestically produced 7075 aluminum alloys is often higher, with the highest recorded total content being 0.391%. Under the same forging heating temperature and deformation steps, billets with lower total Fe+Cr+Mn content exhibit higher recrystallization and better equiaxed grain structure, while those with higher total Fe+Cr+Mn content show poorer recrystallization and more severe grain elongation in the deformation direction. We know that Fe, Cr, and Mn elements in 7075 aluminum alloys form (CrFe)Al7 and (CrMn)Al... 12 Intermetallic compounds, such as Fe, Cr, and Mn, can improve the strength of alloys to some extent, but they also hinder the nucleation and growth of recrystallization. These intermetallic compounds are formed during the solidification of steel during metal melting. They break up and string together during subsequent forging deformation, but cannot be dissolved in the subsequent solution heat treatment. The amount of intermetallic compounds is determined by the content of Fe, Cr, and Mn elements; the more intermetallic compounds, the more difficult it is for grain recrystallization. Experiments show that when the total content of Fe, Cr, and Mn elements is <0.30%, sufficient microstructure recrystallization can occur above the critical deformation temperature when the forging heating temperature reaches 410℃~420℃. As the total content of Fe, Cr, and Mn elements increases, the recrystallization temperature of the alloy will inevitably increase. If the original forging heating and deformation parameters are still used, the alloy will inevitably fail to complete dynamic recrystallization, and the microstructure will mainly be characterized by some coarse and elongated grains. In this case, we can raise the forging heating temperature to a higher temperature of 420°C or above to ensure that the billet is forged above the recrystallization temperature, thus ensuring that the deformed structure is fine and uniform.

[0030] Secondly, the degree of dynamic recrystallization can be improved by increasing the degree of deformation to lower the recrystallization temperature. However, when the degree or amount of deformation in the final forging is fixed and cannot be adjusted, additional pre-deformation must be added before the final forging. This can also promote the dynamic recrystallization of the billet and refine and homogenize the microstructure. When the total content of Fe+Cr+Mn elements is <0.30%, pre-deformation is not required. However, when the total content of Fe+Cr+Mn elements is ≥0.30%, it is recommended to increase the pre-deformation of the billet before the final forging, depending on the specific parameters of the forging. During billet pre-deformation, the degree or amount of deformation can be appropriately increased, and the heating temperature can be selected within a wide temperature range of 410℃ to 435℃.

[0031] This invention determines the forging heating temperature of bars or billets based on the total content of Fe, Cr, and Mn elements, and adds an appropriate pre-deformation process before the main forging process of bars by combining the total content of Fe, Cr, and Mn elements. This can effectively solve the problems of coarse grains, uneven microstructure, and poor ultrasonic level caused by fluctuations or uneven distribution of high melting point impurity element content in large 7075 aluminum alloy forgings.

[0032] The present invention provides a method for improving the microstructure uniformity of large 7075 aluminum alloy forgings, comprising the following steps:

[0033] The forging heating temperature of bars or billets is determined based on the total content of Fe, Cr, and Mn elements.

[0034] The determination of whether a pre-deformation process should be added before the main forging process of the bar stock is based on the total content of Fe, Cr, and Mn elements.

[0035] As needed, the bar stock is pre-deformed before the main forging process to fully crush the intermetallic compounds and obtain the billet.

[0036] The pre-deformed billet is then roughened to obtain a rough shape.

[0037] The rough shape is then forged to obtain the desired forging.

[0038] Heat treatment is performed on the forgings.

[0039] Specifically, for bars with a total Fe+Cr+Mn content < 0.30%, the heating temperature is 410–420℃, and conventional roughing and final forging can be carried out directly. For bars with a total Fe+Cr+Mn content ≥ 0.30%, the heating temperature is 420–435℃, and additional pre-deformation should be performed on the bars before roughing and final forging to fully break down the intermetallic compounds and matrix structure, thereby obtaining a more uniform microstructure. The pre-deformation process specifically includes:

[0040] The pre-deformation heating temperature is 410-435℃, which is 10-15℃ higher than the subsequent roughing and final forging temperatures. The free forging and alternating upsetting / drawing deformation method is adopted. The theoretical deformation amount of a single step in the upsetting / drawing process should be ≥35%, but not more than 60%. Impact deformation equipment such as air hammers or electro-hydraulic hammers should be used as much as possible. The final forging temperature is ≥380℃. Multiple pre-deformation cycles can be performed, with 2-4 cycles recommended.

[0041] The conventional roughing process is performed on the bar stock or pre-deformed billet to obtain the rough shape, specifically including:

[0042] For bars or billets with a total Fe+Cr+Mn content of <0.30%, the rolling temperature is 410-420℃; for bars or billets with a total Fe+Cr+Mn content of ≥0.30%, the rolling temperature is 420-435℃, the rolling deformation is ≥25%, conventional forging is used, the final forging temperature is ≥380℃, and air cooling is performed after forging.

[0043] The final forging of the rough shape yields the required forgings, specifically including:

[0044] For rough dies with a total Fe+Cr+Mn content of <0.30%, the final forging heating temperature is 410-420℃; for rough dies with a total Fe+Cr+Mn content of ≥0.30%, the heating temperature is 420-435℃, the final forging deformation is ≥30%, conventional forging methods are used, the final forging temperature is ≥380℃, and air cooling is performed after forging.

[0045] The final forging undergoes heat treatment, specifically including:

[0046] Solution treatment, first aging, and second aging;

[0047] The solution treatment temperature is 460–470℃, the heat retention coefficient is 1.5–2 min / mm, and then it is naturally cooled to room temperature by air or water.

[0048] The first aging temperature is 100-115℃, and the temperature is maintained for 6-7 hours, followed by natural air cooling to room temperature;

[0049] The second aging temperature is 170-185℃, and the temperature is maintained for 8-9 hours, followed by natural air cooling to room temperature.

[0050] The forging heating temperature of bars or billets is determined based on the total content of Fe, Cr, and Mn elements. By adding an appropriate pre-deformation process before the main forging process of bars, the problems of coarse grains, uneven microstructure, and poor ultrasonic level caused by fluctuations or uneven distribution of impurity element content in large 7075 aluminum alloy forgings can be effectively solved.

[0051] Example:

[0052] The following section describes the production and development of a 7075 upper casing ring forging for a certain type of helicopter, and provides a detailed explanation of the process steps of this invention.

[0053] A method for improving the microstructure uniformity of large 7075 aluminum alloy forgings is provided, comprising the following steps:

[0054] Bar blanking dimensions: The total Fe+Cr+Mn element content of the furnace bar is 0.39%, and pre-deformation is required before the main forging process.

[0055] Step 1: First firing of pre-deformation: Upsetting to Stretch it to Heating temperature 425~430℃, heat preservation coefficient 1.8~2.0min / mm, final forging temperature ≥380℃, and air cooling after forging.

[0056] Step 2: Pre-deformation second firing: Upsetting to Stretch it to Heating temperature 425~430℃, heat preservation coefficient 1.8~2.0min / mm, final forging temperature ≥380℃, and air cooling after forging.

[0057] Step 3: First fire for clearing the wasteland: Upsetting to Heating temperature 430~435℃, heat preservation coefficient 1.8~2.0min / mm, final forging temperature ≥380℃, and air cooling after forging.

[0058] Step 4: Second fire for clearing wasteland: Upsetting and punching to Heating temperature 430~435℃, heat preservation coefficient 1.8~2.0min / mm, final forging temperature ≥380℃, and air cooling after forging.

[0059] Step 5: First enlargement pass: Enlarge the hole to... Heating temperature 430~435℃, heat preservation coefficient 1.8~2.0min / mm, final forging temperature ≥380℃, and air cooling after forging.

[0060] Step 6: The heat treatment solution temperature is 460-470℃, the heat holding coefficient is 1.5-2 min / mm, and then it is naturally air-cooled or water-cooled to room temperature;

[0061] Step 7: The first aging temperature of the heat treatment is 100-115℃, and the temperature is held for 6-7 hours, and then it is naturally cooled to room temperature.

[0062] Step 8: The second aging temperature of the heat treatment is 170-185℃, and the temperature is held for 8-9 hours, and then the mixture is naturally cooled to room temperature.

[0063] The forging heating temperature of bars or billets is determined based on the total content of Fe, Cr, and Mn elements. By adding an appropriate pre-deformation process before the main forging process of bars, the problems of coarse grains, uneven microstructure, and poor ultrasonic level caused by fluctuations or uneven distribution of impurity element content in large 7075 aluminum alloy forgings can be effectively solved.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for improving the microstructure uniformity of large 7075 aluminum alloy forgings, characterized in that, include: The forging heating temperature of bars or billets is determined based on the total Fe+Cr+Mn element content. Whether pre-deformation of the bars before the roughing process is also determined based on the total Fe+Cr+Mn element content. Specifically, for bars with a total Fe+Cr+Mn element content < 0.30%, the forging heating temperature is 410~420℃, and roughing and final forging are carried out directly. For bars with a total Fe+Cr+Mn element content ≥ 0.30%, the forging heating temperature is 420~435℃, and additional pre-deformation is performed before roughing. The pre-deformation process specifically includes: a pre-deformation heating temperature of 410~435℃, using a free forging and upsetting / drawing alternating deformation method, with a single-step deformation amount of ≥ 25% but not exceeding 60% during the upsetting / drawing process, using an air hammer or electro-hydraulic hammer impact deformation device, and a final forging temperature ≥ 380℃, with 2~4 heats of pre-deformation. When pre-deformation is required, the bar stock is pre-deformed to obtain the billet; The bar stock or billet is shaped into a rough shape; The rough shape is then forged to obtain the desired forging. Heat treatment is performed on the forgings.

2. The method according to claim 1, characterized in that, The process of shaping bars or billets into rough shapes includes: For bars with a total Fe+Cr+Mn content of <0.30%, the rolling heating temperature is 410~420℃; for bars with a total Fe+Cr+Mn content of ≥0.30%, the rolling heating temperature is 420~435℃, the rolling deformation is ≥25%, the forging forming method is free forging or die forging, the final forging temperature is ≥380℃, and the bars are air-cooled after forging.

3. The method according to claim 1, characterized in that, The final forging of the rough shape yields the required forgings, specifically including: For the rough shape of bar stock with a total Fe+Cr+Mn content of <0.30%, the final forging heating temperature is 410~420℃; for the rough shape of bar stock with a total Fe+Cr+Mn content of ≥0.30%, the heating temperature is 420~435℃, the final forging deformation is ≥30%, and the forging forming method is free forging or die forging, with a final forging temperature of ≥380℃, followed by air cooling.

4. The method according to claim 1, characterized in that, The final forging undergoes heat treatment, specifically including: Solution treatment, first aging, and second aging.

5. The method according to claim 4, characterized in that, The solution treatment temperature is 460~470℃, the heat preservation coefficient is 1.5~2min / mm, and then it is naturally cooled to room temperature by air or water.

6. The method according to claim 4, characterized in that, The first aging temperature is 100~115℃, and the temperature is maintained for 6~7 hours, followed by natural air cooling to room temperature.

7. The method according to claim 4, characterized in that, The second aging temperature is 170~185℃, and the temperature is maintained for 8~9 hours, followed by natural air cooling to room temperature.