Heat treatment process of 7xxx series aluminum alloy with ultrahigh fatigue strength and high Zn content

Abstract

The invention relates to the technical field of aluminum alloy thermal deformation and thermal treatment, in particular to a thermal treatment process for a 7xxx series aluminum alloy with ultrahigh fatigue strength and high Zn content. According to the technology, heat treatment is conducted on the powder metallurgy high-Zn-content 7xxx series aluminum alloy, two-stage solution treatment is conducted firstly, quenching treatment is conducted, and then two-stage aging treatment is conducted on the aluminum alloy to obtain the ultrahigh-strength anti-fatigue aluminum alloy. The content of eutectic structures generated in the solidification process is reduced through two-stage solid solution, and the material uniformity is effectively improved; the two-stage overaging process can improve the precipitation effect of the precipitated phase, and effectively improve the strength and fatigue strength of the material.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum alloy thermal deformation and heat treatment, in particular to a heat treatment process for an ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy. Background Art

[0002] Al-Zn-Mg-Cu alloys are widely used in aerospace and military fields due to their advantages such as high strength, low density and good processing properties. However, the rapid development of the aerospace industry has put forward higher requirements on the fatigue properties of aluminum alloys. The increase in the content of alloying elements can effectively improve the strength of 7xxx series aluminum alloys, but a large number of micron-level precipitation phases are produced during the casting process. Local stress and strain concentration is likely to occur inside these grains during subsequent deformation or fatigue processes, causing cracks, which seriously limits the elongation and fatigue properties of the material. Therefore, it is urgent to develop a heat treatment process for 7xxx series aluminum alloys with ultra-high fatigue strength and high Zn content. Summary of the invention

[0003] In order to solve the problem of low fatigue strength of ultra-high strength aluminum alloys, the object of the present invention is to provide a heat treatment process for ultra-high fatigue strength and high Zn content 7xxx series aluminum alloys.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is as follows: A heat treatment process for an ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy, wherein the process is for heat treatment of a powder metallurgy high Zn content 7xxx series aluminum alloy, firstly performing a two-stage solid solution treatment, then performing a quenching treatment, and then performing a two-stage aging treatment to obtain an ultra-high strength and fatigue resistant aluminum alloy; The two-stage solution treatment is a first-stage solution treatment and a second-stage solution treatment performed sequentially, wherein: the first-stage solution treatment temperature is 400-450°C, and the holding time is 1-4 hours; the second-stage solution treatment temperature is 450-500°C, and the holding time is 2-10 hours; The double-stage aging treatment includes a first-stage aging treatment and a second-stage aging treatment, wherein: the first-stage aging temperature is 50-100° C., and the insulation time is 3-12 hours; the second-stage aging temperature is 100-150° C., and the insulation time is 16-20 hours.

[0005] The heat treatment process of the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy, the two-stage solid solution treatment includes first performing a short-term low-temperature solid solution on the aluminum alloy and then slowly heating it to a high temperature and performing a long-term secondary solid solution. The heating rate between the first-stage solid solution and the second-stage solid solution temperature does not exceed 0.5°C / min.

[0006] In the heat treatment process of the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy, the transfer time from the end of the solution treatment to the quenching treatment is 15 seconds, and the quenching medium is room temperature water.

[0007] In the heat treatment process of the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy, the transfer time from the end of quenching treatment to aging treatment is 0-2h, the aging treatment equipment is a heat-collecting constant temperature heating magnetic stirrer, and air cooling to room temperature is performed after the second stage aging treatment.

[0008] The heat treatment process of the ultra-high fatigue strength high Zn content 7xxx series aluminum alloy is as follows: the chemical composition of the high Zn content 7xxx series aluminum alloy is as follows, by weight percentage: Zn 7.6~10%, Mg 1.8~3.2%, Cu 0.8~2.6%, Zr 0.08~0.3%, and the balance is Al.

[0009] The heat treatment process of the ultra-high fatigue strength high Zn content 7xxx series aluminum alloy is that the high Zn content 7xxx series aluminum alloy is prepared by powder metallurgy and is heat treated after hot extrusion densification treatment.

[0010] The heat treatment process of the ultra-high fatigue strength high Zn content 7xxx series aluminum alloy is as follows: the high Zn content 7xxx series aluminum alloy is prepared by a powder metallurgy process and then subjected to hot extrusion densification treatment with an extrusion ratio of 9:1 to 25:1, an extrusion temperature of 300°C to 400°C, and an extrusion pressure of 80MPa to 200MPa.

[0011] The design concept of the present invention is: The existing heat treatment process is difficult to achieve the ultra-high fatigue strength of 7xxx aluminum alloys, and it is urgent to develop a new heat treatment process to break through the fatigue strength limit. Therefore, the original intention of the design of the present invention is to improve the distribution of aging precipitation phases and the refinement of grain boundary non-precipitation phases. First, it is hoped that the supersaturated solid solubility of the matrix will be improved through a two-stage solid solution process to provide a higher aging driving force for subsequent aging precipitation. Secondly, it is hoped that the distribution of precipitation phases and the distribution of grain boundary non-precipitation zones will be improved through a two-stage aging process. The principle is to preferentially precipitate atomic clusters through the first-stage aging, reduce the difficulty of subsequent second-stage precipitation and improve the aging effect.

[0012] The present invention solves the problem of fatigue performance degradation of high-Zn aluminum alloys due to coarsening of precipitation phases through the synergistic effect of a two-stage solid solution + two-stage aging combination with specific process parameters, significantly reduces the width of the grain boundary non-precipitation zone, and improves the uniformity of precipitation phase distribution. In addition, the present invention avoids grain coarsening caused by high-temperature regression through a low-temperature long-aging process, and combines a two-stage solid solution to improve matrix supersaturation, and the fatigue performance far exceeds that of traditional processes.

[0013] Therefore, the present invention significantly optimizes the distribution of precipitated phases and grain boundary structure by combining two-stage solution treatment (low temperature short time + high temperature long time, controlling the heating rate) with two-stage aging treatment (low temperature pre-aging + high temperature long aging), combined with powder metallurgy preparation + hot extrusion process, and achieves a breakthrough improvement in the fatigue strength of 7xxx series aluminum alloys (in the number of cycles N f For 10 7 The fatigue strength at this time is 300~350MPa).

[0014] Compared with the prior art, the advantages and beneficial effects of the present invention are: 1. After the double-stage solid solution treatment, the eutectic structure produced in the solidification process of the alloy can be fully dissolved back, thereby improving the solid solution effect of the Zn-rich aluminum alloy and increasing the precipitation kinetics of the aging process. In addition, the first-stage solid solution can effectively release the residual stress generated by extrusion, dissolve the low-melting point phase, increase the supersaturated solid solubility of the matrix, optimize the solid solution effect, and effectively improve the uniformity of the material. In addition, the double-stage aging process can improve the precipitation effect of the precipitated phase. The first-stage aging process preferentially precipitates atomic clusters, reducing the incoherent MgZn in the crystal during the subsequent second-stage aging process. 2 It reduces the difficulty of phase precipitation and limits the width of the grain boundary non-precipitation zone, improves aging efficiency, and effectively improves the strength and fatigue strength of the material.

[0015] 2. In the prior art, the micron-scale precipitation phase generated in the casting process of the high-Zn 7xxx aluminum alloys easily leads to local stress and strain concentration in the subsequent deformation or fatigue process, which limits the elongation and fatigue performance of the material. In view of this problem, the present invention optimizes the heat treatment process, effectively improves the microstructure and mechanical properties of the alloy, solves the problem of the difficulty in achieving ultra-high fatigue strength in the prior art, and achieves unexpected technical effects.

[0016] 3. The present invention not only innovates in the heat treatment process, but also specifically designs the chemical composition of the high Zn content 7xxx series aluminum alloy, and adopts a preparation method combining powder metallurgy and hot extrusion densification treatment. The synergy of this composition design, preparation process and heat treatment process further improves the comprehensive performance of the alloy.

[0017] Therefore, the two-stage solid solution and two-stage aging treatment process proposed in the present invention effectively improves the solid solution effect and aging precipitation effect of high Zn content 7xxx series aluminum alloys through specific temperature, time and heating rate control, especially strict control of the heating rate in the two-stage solid solution treatment and specific parameter setting of the two-stage aging treatment, and significantly improves the fatigue strength of the alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1The microstructure diagram of 7055 aluminum alloy prepared after extrusion of Example 1, Example 2 and Comparative Example 1, Comparative Example 2.

[0019] Figure 2 The microstructure diagram of 7055 aluminum alloy prepared after solution treatment in Example 1, Example 2 and Comparative Example 1, Comparative Example 2.

[0020] Figure 3 The microstructure diagram of 7055 aluminum alloy prepared after solution treatment and aging heat treatment of Example 1, Example 2 and Comparative Example 1, Comparative Example 2.

[0021] Figure 4 These are the tensile curves of the 7055 aluminum alloy prepared in Example 1, Example 2 and Comparative Example 1, Comparative Example 2.

[0022] Figure 5 This is the fatigue curve of the 7055 aluminum alloy prepared in Example 1 after high extrusion ratio extrusion and two-stage solution aging heat treatment.

[0023] Figure 6 Fatigue curve of 7055 aluminum alloy prepared in comparative example 1 after high extrusion ratio extrusion and single-stage solution aging heat treatment.

[0024] Figure 7 This is the fatigue curve of the 7055 aluminum alloy prepared in Example 2 after small extrusion ratio extrusion and two-stage solution aging heat treatment.

[0025] Figure 8 This is the fatigue curve of the 7055 aluminum alloy prepared in Comparative Example 2 after small extrusion ratio extrusion and single-stage solution aging heat treatment.

[0026] Fig. 9 The tensile and fatigue properties of the 7055 aluminum alloy prepared in Example 1, Example 2 and Comparative Example 1, Comparative Example 2 are compared with the properties of other alloys. DETAILED DESCRIPTION

[0027] In the specific implementation process, the present invention proposes a heat treatment process for ultra-high fatigue strength and high Zn content 7xxx series aluminum alloys. The process is for heat treatment of high Zn content 7xxx series aluminum alloys after extrusion densification treatment. The heat treatment process is: first, a two-stage solid solution treatment is performed, and after quenching treatment, a two-stage aging treatment is performed to obtain an ultra-high strength and fatigue-resistant aluminum alloy.

[0028] The two-stage solution treatment is the first-stage solution treatment and the second-stage solution treatment carried out in sequence, wherein: the first-stage solution treatment temperature is 400~450℃, and the holding time is 1~4h; the second-stage solution treatment temperature is 450~500℃, and the holding time is 2~10h; the heating rate between the first-stage solution treatment and the second-stage solution treatment temperature does not exceed 0.5℃ / min. The transfer time from the end of the solution treatment to the quenching treatment is 15s, and the quenching medium is room temperature water. The transfer time from the end of the quenching treatment to the aging treatment is 0~2h, and the aging treatment equipment is a DF-101S collector-type constant temperature heating magnetic stirrer, and the manufacturer is Shanghai Qiuzuo Scientific Instrument Co., Ltd.

[0029] The double-stage aging treatment includes the first-stage aging treatment and the second-stage aging treatment, wherein: the first-stage aging temperature is 50~100℃, and the holding time is 3~12h; the second-stage aging temperature is 100~150℃, and the holding time is 16~20h.

[0030] The chemical composition of the high Zn content 7xxx series aluminum alloy is as follows by weight percentage: Zn: 7.6~8.4%, Mg: 1.8~3.0%, Cu: 2.0~2.6%, Zr: 0.08~0.25%, and the rest is Al. The high Zn content 7xxx series aluminum alloy is prepared by powder metallurgy and densified by hot extrusion with an extrusion ratio of 9:1~25:1. The extrusion ratio is the ratio of the cross-sectional area of ​​the billet before extrusion to the cross-sectional area of ​​the product after extrusion.

[0031] In order to further understand the present invention, the present invention is described below in conjunction with examples, but the examples are only for further elaboration of the features and advantages of the present invention, rather than for limiting the claims of the present invention.

[0032] The following examples are directed to heat treatment of a 7xxx series aluminum alloy with a high Zn content, which is prepared by powder metallurgy and then heat treated after hot extrusion.

[0033] Embodiment 1:

[0034] In this embodiment, the chemical composition of the 7055 aluminum alloy is (wt%): Zn: 8.2%, Mg: 3.1%, Cu: 2.3%, Zr: 0.17%, and the rest is Al.

[0035] For the 7055 aluminum alloy prepared by powder metallurgy and 16:1 extrusion ratio (extrusion temperature of 350°C and extrusion pressure of 80MPa), the heat treatment process is as follows: First, after the solution furnace is heated to 450°C, the 7055 aluminum alloy with an extrusion ratio of 16:1 is placed in the solution furnace for 3 hours; then it is heated to 500°C, the heating rate is 0.5°C / min, the holding time is 8 hours, and then quenching is performed. The transfer time from the end of solution treatment to quenching treatment is 15s, and the quenching medium is room temperature water. After quenching to room temperature, the material is transferred to an oil bath stirring furnace within 1 hour, and the temperature is raised to 80°C with the furnace and kept for 6 hours; then it is heated to 120°C, kept for 20 hours, and air-cooled to room temperature.

[0036] Embodiment 2:

[0037] In this embodiment, the chemical composition of the 7055 aluminum alloy is (wt%): Zn: 8.2%, Mg: 3.1%, Cu: 2.3%, Zr: 0.17%, and the rest is Al.

[0038] For the 7055 aluminum alloy prepared by powder metallurgy and 9:1 extrusion ratio (extrusion temperature of 350°C and extrusion pressure of 150MPa), the heat treatment process is as follows: First, after the solution furnace is heated to 450°C, the 7055 aluminum alloy with an extrusion ratio of 9:1 is placed in the solution furnace for 3 hours; then it is heated to 500°C, the heating rate is 0.5°C / min, the holding time is 8 hours, and then quenching is performed. The transfer time from the end of solution treatment to quenching treatment is 15s, and the quenching medium is room temperature water. After quenching to room temperature, the material is transferred to an oil bath stirring furnace within 1 hour, and the temperature is raised to 80°C with the furnace and kept for 6 hours; then it is heated to 120°C, kept for 20 hours, and air-cooled to room temperature.

[0039] The aluminum alloy materials after heat treatment in Example 1 and Example 2 were subjected to microstructure observation, room temperature tensile properties and fatigue properties tests (tensile and compressive symmetrical fatigue test, R = -1, fatigue strength taken from 10 -7 cycle times).

[0040] The powder metallurgy material in Example 1 was subjected to conventional single-stage solution treatment and single-stage T6 peak aging process treatment (solution treatment at 480°C for 2 hours, quenching to room temperature and then aging treatment at 120°C for 20 hours) as Comparative Example 1, and microstructure observation and performance testing were performed.

[0041] The powder metallurgy material in Example 2 was subjected to conventional single-stage solid solution and single-stage T6 peak aging process treatment (solution treatment at 480°C for 2h, quenching to room temperature and then aging treatment at 120°C for 20h) as Comparative Example 2, and microstructure observation and performance testing were performed.

[0042] like Figure 1As shown, from the microstructure diagrams of the 7055 aluminum alloys prepared in Example 1, Example 2 and Comparative Example 1, Comparative Example 2 after extrusion, it can be seen that the grain size of the 7055 aluminum alloy prepared in Example 1 and Comparative Example 1 after large extrusion heat ratio extrusion is small, and the average grain diameter is only about 0.25 μm; while the grain size of the 7055 aluminum alloy prepared in Example 2 and Comparative Example 2 after small extrusion heat ratio extrusion is relatively large, and the average grain diameter is about 0.5 μm.

[0043] like Figure 2 As shown, from the microstructure diagrams of the 7055 aluminum alloys prepared after solution treatment in Example 1, Example 2 and Comparative Example 1, Comparative Example 2, it can be seen that the 7055 aluminum alloys prepared by Example 1 and Example 2 after double-stage solution treatment have a lower content of micron-scale undissolved phases and a better solution effect; while the 7055 aluminum alloys prepared by Comparative Example 1 and Comparative Example 2 after traditional single-stage solution treatment have a higher content of micron-scale undissolved phases and a poor solution effect. Therefore, it can be considered that the present invention has a better improvement effect on the solution process.

[0044] like Figure 3 As shown, from the microstructure diagrams of 7055 aluminum alloys prepared after solution treatment and aging heat treatment in Example 1, Example 2 and Comparative Example 1, Comparative Example 2, it can be seen that the 7055 aluminum alloy grain boundary no precipitation zone width prepared by Example 1 and Example 2 after double-stage solution treatment and double-stage aging heat treatment is smaller, and the aging precipitation effect is better; while the 7055 aluminum alloy grain boundary no precipitation zone width prepared by Comparative Example 1 and Comparative Example 2 after traditional single-stage solution treatment and single-stage aging is larger, and the aging precipitation effect is poor. Therefore, it can be considered that the present invention has a better improvement effect on aging precipitation.

[0045] like Figure 4 As shown, from the tensile curves of the 7055 aluminum alloy prepared by Example 1, Example 2 and Comparative Example 1, Comparative Example 2, it can be seen that, compared with Comparative Example 1 and Comparative Example 2, the 7055 aluminum alloy prepared by Example 1 and Example 2 through double-stage solid solution and double-stage aging heat treatment has higher grain boundary tensile properties; while the 7055 aluminum alloy prepared by Comparative Example 1 and Comparative Example 2 through traditional single-stage solid solution and single-stage aging has lower tensile properties. Therefore, it can be considered that the present invention is very effective in improving tensile properties.

[0046] like Figure 5 As shown in the fatigue curve of the 7055 aluminum alloy prepared in Example 1 after high extrusion ratio extrusion and double-stage solid solution aging heat treatment, it can be seen that the alloy 7 The fatigue limit under the number of cycles is 331MPa.

[0047] like Figure 6As shown in the figure, the fatigue curve of the 7055 aluminum alloy prepared in comparative example 1 after large extrusion ratio extrusion and single-stage solid solution aging heat treatment shows that the alloy 7 The fatigue limit under the number of cycles is 294MPa.

[0048] like Figure 7 As shown in the fatigue curve of the 7055 aluminum alloy prepared in Example 2 after small extrusion ratio extrusion and double-stage solid solution aging heat treatment, it can be seen that the alloy 7 The fatigue limit under the number of cycles is 317MPa.

[0049] like Figure 8 As shown in the figure, the fatigue curve of the 7055 aluminum alloy prepared in comparative example 2 after small extrusion ratio extrusion and single-stage solution aging heat treatment shows that the alloy 7 The fatigue limit under the number of cycles is 256MPa.

[0050] like Fig. 9 As shown in the figure, the tensile and fatigue properties of the 7055 aluminum alloy prepared by Example 1, Example 2 and Comparative Example 1, Comparative Example 2 and other alloys are compared. It can be seen that the 7055 aluminum alloy prepared by Example 1 and Example 2 after double-stage solid solution and double-stage aging heat treatment has higher grain boundary fatigue performance; while the 7055 aluminum alloy prepared by Comparative Example 1 and Comparative Example 2 after traditional single-stage solid solution and single-stage aging has lower fatigue performance. It can be considered that the present invention has a better improvement effect on fatigue performance. In addition, after the heat treatment process of the present invention, the fatigue performance of the alloy in Example 1 and Example 2 far exceeds the fatigue performance of traditional aluminum alloys, achieving a major breakthrough in the fatigue strength of aluminum alloys.

[0051] The implementation results show that the two-stage solid solution of the present invention reduces the content of eutectic structure produced during solidification and effectively improves the uniformity of the material; the two-stage over-aging process can improve the precipitation effect of the precipitate phase and effectively improve the strength and fatigue strength of the material. In the present invention, the performance indicators of the high Zn content 7xxx series aluminum alloy are as follows: the tensile strength is 650~700MPa, and the cycle number N f For 10 7 The fatigue strength at this time is 300~350MPa.

Claims

1. A heat treatment process for ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy, characterized in that: This process is for heat treatment of powder metallurgy high Zn content 7xxx series aluminum alloy, firstly, double-stage solid solution treatment is carried out, then after quenching treatment, double-stage aging treatment is carried out to obtain ultra-high strength fatigue-resistant aluminum alloy; The two-stage solution treatment is a first-stage solution treatment and a second-stage solution treatment performed sequentially, wherein: the first-stage solution treatment temperature is 400-450°C, and the holding time is 1-4 hours; the second-stage solution treatment temperature is 450-500°C, and the holding time is 2-10 hours; The double-stage aging treatment includes a first-stage aging treatment and a second-stage aging treatment, wherein: the first-stage aging temperature is 50-100° C., and the insulation time is 3-12 hours; the second-stage aging temperature is 100-150° C., and the insulation time is 16-20 hours.

2. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 1, characterized in that: The double-stage solution treatment includes firstly subjecting the aluminum alloy to a short-term low-temperature solution treatment and then slowly heating it to a high temperature and subjecting it to a long-term secondary solution treatment. The heating rate between the first-stage solution treatment and the second-stage solution treatment does not exceed 0.5°C / min.

3. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 1, characterized in that: The transfer time from the end of solution treatment to quenching treatment is 15 s, and the quenching medium is room temperature water.

4. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 1, characterized in that: The transfer time from the end of quenching treatment to aging treatment is 0~2h. The aging treatment equipment is a heat-collecting constant temperature heating magnetic stirrer. After the second stage aging treatment, it is air-cooled to room temperature.

5. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 1, characterized in that: Calculated by weight percentage, the chemical composition of high Zn content 7xxx series aluminum alloy is: Zn 7.6~10%, Mg 1.8~3.2%, Cu 0.8~2.6%, Zr 0.08~0.3%, and the balance is Al.

6. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 1, characterized in that: The high Zn content 7xxx series aluminum alloy is prepared by powder metallurgy and is heat treated after hot extrusion densification.

7. The heat treatment process for the ultra-high fatigue strength and high Zn content 7xxx series aluminum alloy according to claim 6, characterized in that: The high Zn content 7xxx series aluminum alloy is prepared by powder metallurgy process and then subjected to hot extrusion densification treatment with an extrusion ratio of 9:1 to 25:1, an extrusion temperature of 300°C to 400°C, and an extrusion pressure of 80MPa to 200MPa.

Images