Semi-solid plastic forming method for 6xxx aluminum alloy

Abstract

The invention discloses a semi-solid plastic forming method for 6xxx aluminum alloy, and belongs to the technical field of semi-solid forming. The method comprises the steps that an original aluminum alloy ingot is smelted to obtain liquid aluminum alloy, and pouring is started to prepare semi-solid slurry when the temperature is 20-50 DEG C higher than a liquidus; transferring the semi-solid slurry into a preheated mold on an extruder for extrusion casting to obtain a semi-solid cylindrical blank; and the semi-solid blank is preheated and subjected to heat preservation, the preheated semi-solid blank is transferred into a preheated extrusion container, aluminum alloy profile extrusion is conducted, and finally the high-strength and high-plasticity aluminum alloy profile is obtained. According to the semi-solid rheological squeeze casting technology, grains and a second phase are refined, a semi-solid cast ingot which is more uniform in structure and better in compactness is obtained, and follow-up profile extrusion of the semi-solid cast ingot is facilitated; and finally, the aluminum alloy profile with higher compactness and better mechanical property is obtained through profile extrusion.

Description

Technical Field

[0001] The invention relates to a method for semi-solid plastic forming of a 6xxx aluminum alloy, and belongs to the technical field of semi-solid forming. Background Art

[0002] Semi-solid forming (SSF) technology is a metal semi-solid forming method discovered and proposed by Fledglings et al. in the United States in the last century. Semi-solid metal forming refers to a forming method that utilizes the rheological properties of metal during the transformation from liquid to solid or from solid to liquid (i.e., liquid-solid coexistence); the semi-solid state exists between the solidus and liquidus temperatures of the metal, and has the advantages of complex castings, low shrinkage, low forming temperature, high productivity, low porosity, and low energy consumption; semi-solid forming technology is generally divided into two types: rheoforming and thixoforming; thixoforming requires the production of billets first, and when necessary, these billets are reheated to the semi-solid range before use; rheoforming is to directly inject semi-solid slurry into the mold for molding, which has a shorter process and lower cost in actual industrial production.

[0003] 6xxx aluminum alloy belongs to AlMgSi series aluminum alloy, which is a widely used deformable aluminum alloy and can be strengthened by heat treatment. It has medium strength, and the characteristics of magnesium and silicon are more, which makes it have very good processing performance, good corrosion resistance, high toughness and invariance after processing, excellent welding characteristics, easy color film, good oxidation effect and other characteristics; it is widely used in building materials and general structural materials, and also has broad application prospects in aerospace, automotive industry, power electronics and other fields; when using traditional casting technology, turbulence, oxidation and air entrainment are prone to occur during the casting process, resulting in defects such as cracks, pores and oxidation inclusions, as well as coarse primary α-Al dendrites, which will have a great impact on the surface quality and mechanical properties of the casting, and its practical application is greatly limited; the main strengthening phase Mg 2 The morphological distribution of Si also has a great influence on the mechanical properties; therefore, how to use advanced preparation technology to refine the organization and improve the second phase Mg 2 In recent years, the research focus has been on the Si morphology, size and distribution, improving its comprehensive performance, and ultimately preparing 6xxx aluminum alloy materials with excellent comprehensive performance.

[0004] There are many methods to refine the structure of aluminum alloys, the most common ones are chemical modification, spray deposition, rapid solidification, and semi-solid treatment; alloying has certain effects and the process is simple, but it is easy to introduce secondary pollution and make the material properties unstable; there are many physical field methods, and good results have been achieved, but there are generally problems such as too long process flow or too high cost. Summary of the invention

[0005] The present invention provides a method for semi-solid plastic forming of Al-Mg-Si aluminum alloy, which aims to prepare a high-strength and high-plasticity aluminum alloy by combining semi-solid rheological casting and extrusion; the method comprises the following specific steps:

[0006] (1) By operating the central console of the extruder, the slider is controlled to drive the upper mold of the cylindrical mold upward to open the mold, clean the residues in the upper and lower mold cavities, and evenly spray the release agent. The mold is closed and the mold is heated to 200-400℃ through a heating rod and kept warm for 1-3 hours.

[0007] (2) placing the aluminum ingot in a crucible for heating and smelting, heating the alloy melt to 730-830° C. and keeping the temperature for 20-30 min, and using hexachloroethane for degassing and deslagging; measuring the temperature, waiting for it to drop to 600-680° C., keeping the temperature for 5-15 s, and obtaining a semi-solid slurry.

[0008] (3) The semi-solid slurry in step (3) is transferred into a barrel, and the extruder is operated to close the mold. The extruder ejector extrudes the semi-solid slurry in the barrel into the mold cavity at a speed of 19-24 mm / s and a pressure of 60-120 MPa. After the extrusion is completed, the pressure is maintained for 5-20 seconds and the temperature is kept warm for 2-3 minutes. After the casting is formed, the upper mold is moved upward and the casting remaining in the lower mold is taken out by the thrust of the ejector, thereby obtaining an aluminum alloy cylindrical ingot.

[0009] (4) By operating the extruder, the extrusion barrel is moved backward to remove the mold, the extrusion mold is opened for cleaning and then closed, and the die base and extrusion barrel of the extruder are opened in advance for preheating.

[0010] (5) The aluminum alloy ingot obtained in step (3) is processed, and then the ingot is placed in a furnace for heating at a temperature of 400-500° C. for a holding time of 2-5 h.

[0011] (6) The preheated ingot is transferred to a preheated extrusion barrel, and finally an extruder is operated to extrude the ingot into a mold cavity to produce an aluminum alloy extruded profile.

[0012] As a preferred embodiment of the present invention, in step (1) of the present invention, the Al-Mg-Si aluminum alloy used contains the following elements and their percentages: Mg 1.07%, Si 0.72%, Cu 0.29%, Fe 0.49%, Cr 0.21%, Mn 0.01%, Ti 0.06%, Zn 0.02%, and Al is a matrix element.

[0013] As a preferred embodiment of the present invention, in step (1) of the present invention, the cylindrical ingot obtained by semi-solid rheological casting has a height of 100-160 mm and a diameter of Φ90.

[0014] The diameter of the aluminum alloy ingot is designed to meet the size of the extrusion barrel of the extruder, and the height is verified through multiple experiments to better save and utilize materials while ensuring good product quality and moderate length.

[0015] As a preferred embodiment of the present invention, the preheating temperature of the extrusion barrel of the extruder is 380-520°C, and the preheating temperature of the extrusion die is 400-520°C.

[0016] The purpose of preheating the extrusion barrel and the die is to keep the temperature consistent with the preheated ingot temperature to prevent the temperature of the extruded ingot from dropping when it contacts the extrusion barrel and the die, which will reduce the metal fluidity and affect the uniformity of the organizational morphology, resulting in the actual extrusion temperature being inconsistent with the set temperature; after a large number of experiments and analyses, when the preheating temperature of the extrusion barrel is set to 400-520°C and the extrusion speed is 3-8mm / s, the microstructure and mechanical properties of the product are better; when the extrusion speed is too low, the production efficiency is reduced, and when the extrusion speed is too low, below 3mm / s, the surface of the extruded part is not smooth and the surface is rough; when the extrusion speed is too high, although the elongation is improved, the tensile strength of the product is reduced, resulting in the product having a tendency to thermal cracking.

[0017] In step (5), the aluminum alloy ingot needs to be processed before preheating to remove the surface oxide layer and cut off the two ends of the ingot.

[0018] As a preferred embodiment of the present invention, in step (6) of the present invention, the extrusion ratio is 10-40 and the extrusion speed is 3-8 mm / s.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention adopts semi-solid rheological technology to prepare semi-solid slurry, so that the alloy is in the semi-solid range, and the iron-rich phase in the alloy liquid is also refined while the primary α-Al is refined; then the bottom injection extrusion technology is adopted to obtain the aluminum alloy ingot, and the extrusion casting reduces the pore defects formed by air entrainment during the filling process, reduces the segregation of elements, improves the microstructure of the alloy material, improves its density and uniformity of the structure, and obtains a better product in the subsequent plastic deformation.

[0021] After plastic deformation, the ingot obtained by semi-solid treatment has a more uniform structure and significantly finer grains than the as-cast ingot after deformation, which is beneficial to the improvement of grain boundary strengthening and plasticity; the existing iron-rich phase is broken and refined in the extrusion direction after plastic deformation, which reduces the stress concentration at the tip of the iron-rich phase, enhances the coordination of deformation, and is beneficial to the improvement of alloy strength and plasticity; at the same time, the M 2The Si phase is dispersed after homogenization and plastic deformation, which helps to improve its mechanical properties. The high-strength and high-plasticity aluminum alloy prepared by the present invention has the characteristics of simple process, good organizational uniformity, excellent mechanical properties, stable quality, strong controllability, and suitability for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a flow chart of the method for preparing the high-strength and high-plasticity aluminum alloy of the present invention;

[0023] Figure 2 This is the microstructure diagram of the semi-solid billet after rheological squeeze casting;

[0024] Figure 3 This is the microstructure diagram of the billet produced by conventional liquid squeeze casting;

[0025] Figure 4 This is the microstructure diagram of the gravity cast semi-solid billet;

[0026] Figure 5 This is the microstructure diagram after hot extrusion in Example 2;

[0027] Figure 6 This is the microstructure diagram of the hot extrusion in Comparative Example 2. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the contents described above.

[0029] Example 1

[0030] An embodiment of a semi-solid plastic forming method of an Al-Mg-Si aluminum alloy according to the present invention comprises the following specific steps:

[0031] (1) Weigh 2.5 kg of aluminum alloy raw material and melt it in a medium frequency induction furnace at a melting temperature of 750°C for 20 min. Degas and deslag the melt with hexachloroethane. When the alloy liquid drops to 690°C, keep it for 10 s and pour the semi-solid slurry.

[0032] (2) The parameters of the semi-solid slurry preparation device are as follows: pouring temperature of 690°C, water flow rate of the upper and lower cooling water channels of 200 L / h, inclination angle of 45°, channel length of 300 mm, and height of 5 mm; a graphite crucible with a preheating temperature of 650°C was used to collect the aluminum alloy semi-solid slurry. The purpose of preheating the slurry is to perform quasi-isothermal treatment on the semi-solid slurry to make the temperature field of the slurry more uniform, and then it is immediately poured into the bottom injection extruder.

[0033] (3) The obtained aluminum alloy semi-solid slurry is transferred to the lower mold barrel on the extruder, and extruded to obtain a semi-solid billet with a diameter of Φ90; in the rheological extrusion casting process, the extruder parameters are set as a mold temperature of 250°C, a forming pressure of 90 MPa, and an extrusion rate of 19 mm / s. After the extrusion forming is completed, the piece is taken out after being kept warm in the mold for 2 minutes and cooled at room temperature to obtain a semi-solid cylindrical billet with a diameter of Φ90.

[0034] (4) The billet is homogenized and kept at 460°C for 3 h. The preheating temperature of the extrusion die is 460°C, and the preheating temperature of the extrusion cylinder and the extrusion pad is 460°C.

[0035] (5) placing the preheated mold into the mold cavity, and quickly placing the extrusion pad and the semi-solid billet into the extrusion barrel for extrusion, with an extrusion ratio of 40:1 and an extrusion rate of 6 mm / s; the obtained extruded part is cooled at room temperature.

[0036] from Figure 2 and Figure 3 It can be seen that the structure of the billet after semi-solid treatment is more uniform and refined. Compared with the castings of traditional liquid squeeze casting, there is no element enrichment phenomenon; Figure 5 As shown, after extrusion, the second phase is broken and distributed along the extrusion direction; in addition, after extrusion, casting defects such as pores, shrinkage cavities and shrinkage are reduced or even disappear; the mechanical properties of the semi-solid aluminum alloy after extrusion are greatly improved compared with those after cast extrusion.

[0037] Example 2

[0038] An embodiment of a semi-solid plastic forming method of an Al-Mg-Si aluminum alloy according to the present invention comprises the following specific steps:

[0039] (1) Weigh 2 kg of aluminum alloy raw material and melt it in a medium frequency induction furnace at a melting temperature of 780°C for 30 min. Degas and deslag the melt with hexachloroethane. When the alloy liquid drops to 690°C, pour the semi-solid slurry.

[0040] (2) The parameters of the semi-solid slurry preparation device are as follows: pouring temperature of 690°C, water flow rate of the upper and lower cooling water channels of 200 L / h, inclination angle of 45°, channel length of 300 mm, and height of 5 mm; a graphite crucible with a preheating temperature of 650°C was used to collect the aluminum alloy semi-solid slurry. The purpose of preheating the slurry is to perform quasi-isothermal treatment on the semi-solid slurry to make the temperature field of the slurry more uniform, and then it is immediately poured into the bottom injection extruder.

[0041] (3) The obtained aluminum alloy semi-solid slurry is transferred to the lower mold barrel on the extruder, and extruded to obtain a semi-solid billet with a diameter of Φ90; in the rheological extrusion casting process, the extruder parameters are set as a mold temperature of 250°C, a forming pressure of 90 MPa, and an extrusion rate of 20 mm / s. After the extrusion forming is completed, the piece is taken out after being kept warm in the mold for 2.5 minutes and air-cooled to obtain a semi-solid cylindrical billet with a diameter of Φ90.

[0042] (4) The billet is homogenized and kept at 420°C for 4 h. The extrusion die and extrusion pad are preheated to 430°C, and the extrusion barrel is preheated to 420.

[0043] (5) placing the preheated mold into the mold cavity, and quickly placing the extrusion pad and the semi-solid billet into the extrusion barrel for extrusion, with an extrusion ratio of 40:1 and an extrusion rate of 6 mm / s; the obtained extruded part is water-cooled.

[0044] like Figure 5 As shown in the figure, the second phase in the microstructure of the extruded part after extrusion and water cooling at 420°C is dispersed, especially the iron-rich phase can be refined; water cooling can avoid subsequent static recrystallization and its growth, so that the grains are refined and the performance is greatly improved; under this condition, the tensile strength reaches 257.5Mpa and the elongation reaches 30.22%.

[0045] Comparative Example 1

[0046] This comparative example adopts the method of aluminum alloy gravity casting to prepare a semi-solid billet with a diameter of Φ90, and the specific steps are as follows:

[0047] (1) The gravity casting mold is kept at 250°C for 2 h.

[0048] (2) The methods for smelting and preparing semi-solid slurry are the same as steps (1) and (2) in Example 1, respectively.

[0049] (3) Pour the semi-solid slurry into a preheated gravity casting mold and wait for the melt to cool.

[0050] (4) The semi-solid billet is taken out to obtain a cylindrical billet with a diameter of Φ90.

[0051] Depend on Figure 2 and Figure 4 By comparison, it can be seen that there are defects in the billet under gravity casting, both macroscopically and microscopically. Figure 4 The black color indicates defects such as holes. Under the action of the extrusion force, the solidification shrinkage process of the molten metal is reduced, and it is directly formed under pressure, so that its performance is improved after hot extrusion.

[0052] Comparative Example 2

[0053] The Al-Mg-Si aluminum alloy plastic forming method described in this comparative example has the following specific steps:

[0054] (1) Weigh 2 kg of aluminum alloy raw material and melt it in a medium frequency induction furnace at a melting temperature of 750°C for 20 min. Degas and deslag the melt with hexachloroethane, and pour the alloy liquid when the temperature drops to 690°C.

[0055] (2) In the traditional extrusion casting process, the extruder parameters are set as a mold temperature of 250°C, a forming pressure of 90 MPa, and an extrusion rate of 20 mm / s. After the extrusion forming is completed, the mold is kept warm for 2.5 min and then the piece is taken out and air-cooled to obtain a cylindrical billet with a diameter of Φ90.

[0056] (3) The plastic deformation process is the same as steps (4) and (5) in Example 2.

[0057] Comparing the microstructures of Example 2 with Comparative Example 2, the semi-solid billet has a uniform structure and reduced element segregation, so that the second phase is refined in the subsequent hot extrusion process, while the second phase of the liquid billet becomes coarser after hot extrusion, and the iron-rich phase is severely split in the matrix, which reduces its performance and affects the surface quality of the extruded part. Figure 5 and Figure 6 The white phase in the sample is an iron-rich phase; under the conditions of this comparative example, the tensile strength is only 223.6 and the elongation is 27.08%.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for semi-solid plastic forming of a 6xxx aluminum alloy, characterized in that: The specific steps include: (1) By operating the central console of the extruder, the slider is controlled to drive the upper mold of the cylindrical mold upward to open the mold, clean the residues in the upper and lower mold cavities, and evenly spray the release agent. The mold is closed and the mold is heated to 200-400°C through a heating rod and kept warm for 1-3 hours; (2) placing the aluminum ingot in a crucible for heating and melting, measuring the temperature, and waiting for the temperature to drop to 600-680° C., keeping the temperature for 5-15 seconds to obtain a semi-solid slurry; (3) transferring the semi-solid slurry in step (2) into a barrel, operating an extruder to close the mold, and extruding the semi-solid slurry in the barrel into the mold cavity of the mold at a speed of 19-24 mm / s and a pressure of 60-120 MPa by a push rod of the extruder. After the extrusion is completed, the pressure is maintained for 5-20 seconds and the temperature is kept for 2-3 minutes. After the casting is formed, the upper mold is moved upward and the casting remaining in the lower mold is taken out by the thrust of the push rod to obtain an aluminum alloy cylindrical billet; (4) By operating the extruder, the extrusion barrel is moved backward to remove the mold, the extrusion mold is opened for cleaning and then the mold is closed, and the extrusion mold and the extrusion barrel are opened in advance for preheating; (5) placing the aluminum alloy ingot obtained in step (3) into a furnace and heating it at a temperature of 400-500° C. for a holding time of 2-6 h; (6) The preheated billet is transferred to a preheated extrusion barrel, and finally the extruder is operated to extrude the ingot into a mold cavity to produce an aluminum alloy extruded profile.

2. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1, characterized in that: The Al-Mg-Si aluminum alloy used contains the following elements and their percentages: Mg 1.07%, Si 0.72%, Cu 0.29%, Fe 0.49%, Cr 0.21%, Mn 0.01%, Ti 0.06%, Zn 0.02%, and Al is a matrix element.

3. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1, characterized in that: The heating and smelting conditions in step (2) are as follows: the alloy melt is heated to 730-830° C. and kept warm for 20-30 minutes, and degassing and deslagging are performed using hexachloroethane.

4. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1, characterized in that: The cylindrical ingot obtained by semi-solid rheological casting in step (3) has a height of 100-160 mm and a diameter of Φ90.

5. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1, characterized in that: The preheating temperature of the extrusion barrel of the extruder is 380-520℃, and the preheating temperature of the extrusion die is 400-520℃.

6. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1 or 3, characterized in that: In step (5), the aluminum alloy ingot needs to be processed before preheating to remove the surface oxide layer and cut off the two ends of the ingot.

7. The method for semi-solid plastic forming of 6xxx aluminum alloy according to claim 1 or 4, characterized in that: The extrusion ratio is 10-40 and the extrusion speed is 3-8 mm / s.

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