CN113201672B

CN113201672B - Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment and preparation method thereof

Info

Publication number: CN113201672B
Application number: CN202110425080.5A
Authority: CN
Inventors: 张济山; 龚文源
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2021-04-20
Filing date: 2021-04-20
Publication date: 2022-06-14
Anticipated expiration: 2041-04-20
Also published as: CN113201672A

Abstract

An Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment and a preparation method thereof, belonging to the field of aluminum alloy and preparation thereof. The alloy comprises the following chemical components, by mass, 1.2-1.6 wt% of Mg, 1.2-1.6 wt% of Si, 0.1-0.3 wt% of Cu, 2.0-4.0 wt% of Zn, 0.1-0.5 wt% of Fe, 0.5-0.8 wt% of Mn, less than or equal to 0.03 wt% of Ti, less than or equal to 0.04 wt% of Ni, and the balance of Al. The main elements of the alloy exceed the range of the traditional alloy system, and further, after the pre-aging process is optimized, the deterioration effect of natural aging is effectively inhibited, the hardening increment of baking finish is greatly improved, and the hardening increment of the alloy baking finish reaches 180MPa. Through high baking varnish hardening increment, the yield strength of the alloy after baking varnish is improved, so that the dent resistance is enhanced, meanwhile, the T4P alloy has good plasticity, the elongation of the alloy in the T4P state reaches 25%, and the strength is greatly improved on the basis of high formability.

Description

Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment and preparation method thereof

Technical Field

The invention belongs to the field of aluminum alloy and preparation thereof, and relates to an Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment for an automobile and a preparation method thereof.

Background

With the attention of people on energy conservation, the light weight of automobiles has become a global trend. Aluminum alloys have received much attention as important lightweight materials. In particular, 6xxx (Al-Mg-Si) alloys are widely used because they possess excellent formability (meeting the requirement of press forming) before baking finish (T4P state), and yield strength is significantly increased during baking finish to meet the requirement of dent resistance during subsequent use. However, the low temperature (160-185 ℃) and short time (20-30 min) of the baking finish process can not fully exert the potential baking finish hardening capacity, so that the yield strength of the baking finish hardened alloy is far lower than that of the alloy in a peak aging state. The 6xxx (Al-Mg-Si) alloy is 6016 and 6111 alloy which are widely applied to the field of automobiles at present, but after the simulated paint baking process of keeping the temperature at 185 ℃ for 20min, the paint baking hardening increment of the two alloys is 85 MPa and 90MPa respectively, and the yield strength of the two alloys in a paint baking state is only about 300MPa and is far lower than that of steel. Therefore, increasing the hardening increment of the baking varnish has become a research hotspot.

The alloy must be shipped and stored for a period of time before use, and natural aging during this period of time will significantly reduce the bake hardening increment. This is because the clusters formed at room temperature after the solution treatment do not act as nuclei for the precipitation of the β ″ phase during the baking process, but are redissolved and waste baking time. In order to weaken the deterioration effect of natural aging, researchers introduce a pre-aging process and carry out a great deal of research, and the fact that the pre-aging process is added immediately after solid solution quenching can effectively inhibit the deterioration effect of natural aging and improve the hardening increment of baking finish.

In addition, researches show that the aging response speed of the alloy can be improved by adding Cu, and the baking varnish hardening increment of the alloy can be improved by adding Zn, so that an Al-Mg-Si-Cu-Zn alloy system is developed. However, due to the consideration of the severe influence of the relative plasticity of the latticed micron-sized precipitates, the solution treatment is often carried out in the Al single-phase region to achieve the purpose of completely dissolving the micron-sized precipitates, but the content of the alloy is also limited, and the content of Mg and Si elements is strictly limited within 1.2 wt%. Even if the alloy is subjected to baking finish treatment, the alloy yield strength still does not meet the application requirement, the indentation resistance requirement in the use process is difficult to meet, the baking finish hardening increment still needs to be further improved, if the alloy composition limitation can be broken through, the serious influence of relative plasticity of latticed micron-sized precipitation is solved by utilizing deformation and a heat treatment process while the alloy is subjected to high alloying, and the alloy baking finish hardening increment and the yield strength are expected to be greatly improved on the basis of keeping the high formability of the alloy so as to meet the use requirement. However, this strategy is far from fully understood and little work has been reported.

Disclosure of Invention

The invention provides an improved 6xxx (Al-Mg-Si) alloy material, which is characterized in that Cu and Zn are added on the basis of Al-Mg-Si to form an Al-Mg-Si-Cu-Zn alloy system, the content of main elements (Mg, Si and Zn) exceeds the range of the 6xxx alloy based on a high alloying concept, the shape and the size of a micron-sized precipitate are improved by utilizing deformation and heat treatment processes, the deterioration effect of the micron-sized precipitate on plasticity is solved, multi-scale particles (micron-scale and nano-scale) are formed at the same time, and the grains are refined by using the principle that coarse particles promote nucleation and fine particles to hinder the growth of the grains. Compared with the standard 6016 and 6111 alloys, the mechanical property of the alloy is greatly improved on the premise that the alloy forming property is basically unchanged, so that the problems of insufficient hardening property and dent resistance of 6xxx alloy baking finish are solved.

The alloy comprises, by weight, 1.2-1.6 wt% of Mg, 1.2-1.6 wt% of Si, 0.1-0.3 wt% of Cu, 2.0-4.0 wt% of Zn, 0.1-0.5 wt% of Fe, 0.5-0.8 wt% of Mn, less than or equal to 0.03 wt% of Ti, less than or equal to 0.04 wt% of Ni, and the balance of Al.

The preferable composition range of the alloy of the invention is that the weight percentage of Mg is 1.2-1.4 wt%, Si is 1.4-1.6 wt%, Cu is 0.1-0.2 wt%, Zn is 2.0-3.0 wt%, Fe is 0.2-0.4 wt%, Mn is 0.5-0.6 wt%, Ti is 0.01-0.02 wt%, Ni is 0.02-0.03 wt%, and the rest is Al.

The preparation process of the alloy comprises the following steps: and smelting the Al-Mg-Si-Cu-Zn alloy according to the range of the alloy components, performing two-stage homogenization treatment on the obtained cast ingot, performing hot rolling, cold rolling and annealing treatment, performing solution treatment and water quenching on the annealed cold-rolled sheet, and immediately performing pre-aging treatment, namely putting the cold-rolled sheet into an isothermal aging furnace at the temperature of 80-185 ℃ for heat preservation for 4 min-15 h to obtain the Al-Mg-Si-Cu-Zn alloy with high baking finish hardening increment.

And the two-stage homogenization treatment is to heat the mixture to 510-520 ℃ at a speed of 20-40 ℃/h and preserve the heat for 6-10 h, and then heat the mixture to 540-560 ℃ at a speed of 20-40 ℃/h and preserve the heat for 15-20 h.

The hot rolling, cold rolling and annealing treatment process comprises the steps of carrying out cold rolling at the hot rolling start temperature of 520-540 ℃ and the total deformation of 90-93%, then carrying out cold rolling with the total deformation of 40-60%, carrying out intermediate annealing at the temperature of 380-420 ℃ for 0.5-1.5 h, and carrying out cold rolling with the total deformation of 50-70% on a plate after annealing.

The solid solution treatment is to carry out 1-10 min on the cold-rolled sheet in a salt bath furnace at 540-560 ℃.

The preferable pre-aging process is to keep the temperature at 140-180 ℃ for 4-8 min.

The baking varnish hardening increment of the prepared Al-Mg-Si-Cu-Zn alloy is 130-180 Mpa, the elongation at T4P state is 23-26% after natural placement for 14 days, the yield strength is 330-370 MPa after natural placement for 14 days and aging treatment (simulated baking varnish), and the tensile strength is 380-410 MPa.

According to the invention, Cu and Zn are added on the basis of 6xxx (Al-Mg-Si) alloy, so that the content of main elements is increased, the pre-aging process is optimized, the deterioration effect of natural aging is effectively inhibited, the hardening increment of baking finish is greatly increased, the yield strength of the alloy after baking finish is increased through the high hardening increment of baking finish, so that the dent resistance is enhanced, meanwhile, the T4P alloy has good plasticity, the elongation of the alloy in the T4P state reaches 25%, the alloy is the same as 6016 alloy and higher than 6111 alloy, but the hardening increment of baking finish reaches 180MPa and is more than twice of 6016 and 6111 alloys. Therefore, the alloy disclosed by the invention not only has the advantage that the 6xxx (Al-Mg-Si) alloy has high plasticity and can be integrally formed by stamping, but also greatly improves the baking varnish hardening increment, so that the problem that the yield strength of the 6xxx (Al-Mg-Si) alloy is too low to meet the dent resistance is solved. The alloy of the invention has great industrial application value and can be applied to automobile outer plates.

Drawings

FIG. 15 is a graph of natural age hardening of alloy # after different pre-aging processes.

FIG. 25 is a schematic representation of the grain size of the alloy # alloy in the solution quenched state.

Detailed Description

The invention is further supplemented and explained below with reference to specific embodiments.

The raw materials used in the present invention were 99.9 wt% pure metals (Al, Mg and Zn) and master alloys (Al-20 wt% Si, Al-50 wt% Cu, Al-10 wt% Mn, and Al-20 wt% Fe). The specific chemical composition of the alloy is shown in Table 1. Wherein, the No. 1 is commercial alloy 6016, the No. 2 is commercial alloy 6111, and the No. 1 and the No. 2 are reference alloys; 3#, 4#, 5#, 6#, and 7# are alloy compositions designed by the invention.

TABLE 1 specific chemical composition of alloy (mass%; wt%)

	Mg	Si	Cu	Zn	Fe	Mn	Ti	Ni	Al
										1#	0.6	1.1	0.2	0.2	0.4	0.2	0.02	/	Balance of
2#	0.9	0.7	0.6	0.15	0.4	0.5	0.02	/	Balance of
										3#	1.2	1.2	0.2	3.0	0.4	0.5	0.02	0.03	Balance of
4#	1.2	1.4	0.2	3.0	0.4	0.5	0.02	0.03	Balance of
										5#	1.2	1.6	0.2	3.0	0.4	0.5	0.02	0.03	Balance of
6#	1.4	1.2	0.2	3.0	0.4	0.5	0.02	0.03	Balance of
										7#	1.6	1.2	0.2	3.0	0.4	0.5	0.02	0.03	Balance of

Smelting the Al-Mg-Si-Cu-Zn alloy; carrying out two-stage homogenization treatment on the obtained cast ingot, namely heating to 520 ℃ at the speed of 30 ℃/h, preserving heat for 8h, and then heating to 550 ℃ at the speed of 30 ℃/h, preserving heat for 16 h; carrying out hot rolling after homogenization, wherein the initial rolling temperature is 540 ℃, and the total deformation is 90-93%; then cold rolling with total deformation of 50 percent is carried out; after cold rolling, intermediate annealing is carried out for heat preservation for 1h at 400 ℃; after annealing, the sheet was cold rolled to a total deformation of 67%. Finally obtaining the 1mm cold-rolled sheet of the 1# -7 # alloy.

Comparative example 1

A1 mm cold-rolled sheet of the 1# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And putting the quenched sample into an aging furnace for pre-aging treatment at the temperature of 80 ℃/12 h. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated stamping) and then aged by 185 deg.C/20 min (simulated paint-baking). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Comparative example 2

A1 mm cold-rolled sheet of the 2# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And putting the quenched sample into an aging furnace for pre-aging treatment at the temperature of 80 ℃/12 h. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Comparative example 3

A1 mm cold-rolled sheet of the 5# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. Then left at room temperature for 14 days (state T4). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thus, the elongation rate of the T4P state and the baking varnish hardening increment are obtained.

Example 1

A1 mm cold-rolled sheet of the 5# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And putting the quenched sample into an aging furnace for pre-aging treatment at the temperature of 80 ℃/12 h. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 2

A1 mm cold-rolled sheet of the 5# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 140 ℃/0.5 h. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 3

A1 mm cold-rolled sheet of the 5# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 160 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated stamping) and then aged by 185 deg.C/20 min (simulated paint-baking). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 4

A1 mm cold-rolled sheet of the 5# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 185 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thus, the elongation rate of the T4P state and the baking varnish hardening increment are obtained.

Example 5

A1 mm cold-rolled sheet of the 3# alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 160 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 6

A1 mm cold-rolled 4# alloy sheet obtained by the method is sampled, placed into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 160 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 7

A1 mm cold-rolled sheet of the No. 6 alloy obtained by the method is sampled, put into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched by water. And placing the quenched sample into an aging furnace for pre-aging treatment at 160 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

Example 8

A1 mm cold-rolled 7# alloy sheet obtained by the method is sampled, placed into a salt bath furnace for solution treatment at 550 ℃ for 8min, and then rapidly quenched with water. And placing the quenched sample into an aging furnace for pre-aging treatment at 160 ℃/6 min. Then left at room temperature for 14 days (T4P state). One group is directly stretched; the other group was pre-stretched by 2% (simulated press forming) and then aged at 185 deg.C/20 min (simulated paint bake). Thereby obtaining the elongation rate of the T4P state and the baking varnish hardening increment.

TABLE 2 mechanical properties of alloy samples of different compositions and states before and after 185 deg.C/20 min (simulated paint baking) treatment

As can be seen from Table 2, on the basis of the comparative alloy No. 1 (6016) and No. 2 (6111), the content of Mg, Si and Zn elements is increased to exceed the element range of the traditional 6xxx series aluminum alloy based on a high alloying concept, and the 3# to 7# alloy is obviously higher than the comparative alloy in terms of the strength after baking finish and the hardening increment of baking finish; and the elongation in the T4P state is not reduced and is even higher than that of the 2# (6111) alloy. Comparing comparative example 1 and examples 1-4, it can be seen that the pre-aging process can effectively inhibit the natural aging deterioration effect and significantly improve the hardening increment of the baking varnish. In particular, the baking varnish of example 5 has an increase in hardening twice or more as high as that of comparative example 1 and comparative example 2, and the elongation at the T4P state is higher than that of comparative example 2 as in comparative example 1. It can be seen from fig. 1 that the pre-aging process at 160 deg.c/6 min suppresses the deteriorating effect of natural aging most significantly. Other alloy embodiments employ this process. As can be seen from figure 2, the alloy has fine grain size, the average grain size is only 12um, because the multi-scale (nano-micron) precipitated phase in the alloy promotes nucleation through coarse particles, and the fine case hinders recrystallization growth effect to refine grains, thereby playing a role of fine grain strengthening.

Although embodiments of the present invention have been shown and described, it should be noted that various changes, modifications, substitutions and alterations can be made herein by those skilled in the art without departing from the principles and concepts of the invention, the scope of which is defined by the appended claims and equivalents.

Claims

1. The Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment is characterized in that the alloy comprises 1.2-1.6 wt% of Mg, 1.2-1.6 wt% of Si, 0.1-0.3 wt% of Cu, 3.0 wt% of Zn, 0.4 wt% of Fe, 0.5-0.8 wt% of Mn, less than or equal to 0.03 wt% of Ti, less than or equal to 0.04 wt% of Ni and the balance of Al in percentage by weight;

the preparation method of the Al-Mg-Si-Cu-Zn alloy with the high baking varnish hardening increment comprises the steps of smelting the Al-Mg-Si-Cu-Zn alloy, carrying out two-stage homogenization treatment on the obtained cast ingot, then carrying out hot rolling, cold rolling and annealing treatment, carrying out solution treatment and water quenching on the annealed cold-rolled sheet, and then immediately carrying out pre-aging treatment, namely putting the cold-rolled sheet into an isothermal aging furnace at the temperature of 80-185 ℃ for heat preservation for 4 min-15 h to prepare the Al-Mg-Si-Cu-Zn alloy with the high baking varnish hardening increment.

2. The Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment as recited in claim 1, wherein the alloy comprises, in weight percent, 1.2 to 1.4 wt% of Mg, 1.4 to 1.6 wt% of Si, 0.1 to 0.2 wt% of Cu, 3.0 wt% of Zn, 0.4 wt% of Fe, 0.5 to 0.6 wt% of Mn, 0.01 to 0.02 wt% of Ti, 0.02 to 0.03 wt% of Ni, and the balance of Al.

3. The preparation method of the Al-Mg-Si-Cu-Zn alloy with the high baking finish hardening increment as recited in claim 1 or 2, characterized in that the Al-Mg-Si-Cu-Zn alloy is smelted, the obtained cast ingot is subjected to two-stage homogenization treatment, then hot rolling, cold rolling and annealing treatment are carried out, the annealed cold-rolled plate is subjected to solution treatment and water quenching, and then is subjected to pre-aging treatment immediately, namely the cold-rolled plate is placed into an isothermal aging furnace at 80-185 ℃ for heat preservation for 4 min-15 h, so that the Al-Mg-Si-Cu-Zn alloy with the high baking finish hardening increment is prepared.

4. The method for preparing the Al-Mg-Si-Cu-Zn alloy with high baking varnish hardening increment according to claim 3, wherein the two-stage homogenization treatment comprises the steps of heating to 510-520 ℃ at a speed of 20-40 ℃/h and preserving heat for 6-10 h, and then heating to 540-560 ℃ at a speed of 20-40 ℃/h and preserving heat for 15-20 h.

5. The method for preparing Al-Mg-Si-Cu-Zn alloy with high baking finish hardening increment according to claim 3, wherein the hot rolling, the cold rolling and the annealing treatment process comprise the steps of carrying out cold rolling at the hot rolling start rolling temperature of 520-540 ℃ and the total deformation of 90-93%, then carrying out cold rolling with the total deformation of 40-60%, carrying out intermediate annealing at the temperature of 380-420 ℃ for 0.5-1.5 h, and carrying out cold rolling with the total deformation of 50-70% after annealing.

6. The method for preparing the Al-Mg-Si-Cu-Zn alloy with high baking finish hardening increment according to claim 3, wherein the solution treatment is carried out on the cold-rolled sheet in a salt bath furnace at 540-560 ℃ for 1-10 min.

7. The method for preparing the Al-Mg-Si-Cu-Zn alloy with high baking finish hardening increment according to claim 3, wherein the pre-aging process is carried out at 140-180 ℃ for 4-8 min.

8. The method for preparing the Al-Mg-Si-Cu-Zn alloy with high baking finish hardening increment according to claim 3, wherein the baking finish hardening increment of the prepared Al-Mg-Si-Cu-Zn alloy is 130-180 Mpa, the elongation at T4P state is 23-26% after the Al-Mg-Si-Cu-Zn alloy is naturally placed for 14 days, the yield strength is 330-370 MPa after the Al-Mg-Si-Cu-Zn alloy is naturally placed for 14 days and subjected to aging treatment by adding simulated baking finish, and the tensile strength is 380-410M Pa.