Preparation method of large aluminum alloy bar for inhibiting generation of coarse grain rings

Abstract

The invention discloses a preparation method of a large aluminum alloy bar for inhibiting generation of coarse grain rings, which comprises the steps of component design, smelting, casting, homogenization treatment, saw cutting and turning, extrusion, pre-stretching, solution treatment, aging treatment and the like, and the aluminum alloy bar with the diameter of more than or equal to 300mm is obtained. The purpose of eliminating the coarse grain ring of the 6082 large aluminum alloy bar is achieved through comprehensive measures such as optimizing alloy components, improving an extrusion method and adopting graded heat treatment, and therefore the mechanical property of the bar is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aluminum alloy processing and preparation, in particular to a preparation method of an aluminum alloy large rod for inhibiting the generation of coarse grain rings. BACKGROUND

[0002] 6082 aluminum alloy belongs to 6XXX (Al-Mg-Si) series aluminum alloy, is a kind of heat treatable strengthening aluminum alloy, has medium strength, good weldability, formability and machinability, high impact toughness and good corrosion resistance, is an ideal material for replacing steel to manufacture high-speed components and high-load structural components in modern industry. In the prior art, the production process of aluminum alloy rod is roughly as follows: batching, melting and casting, ingot homogenization, extrusion, rod quenching, stretching and cutting, and artificial aging. Excellent performance depends on the optimization of each link, such as suitable composition design, heat treatment process and forming method.

[0003] Coarse grain ring is a ring-shaped coarse grain area formed on the cross-section periphery of an aluminum alloy extruded product, and is one of the main defects of the aluminum alloy extruded product. The generation of coarse grain ring is related to factors such as chemical composition of the alloy, homogenization system, extrusion method and process parameters, solid solution treatment system and aging treatment system. When the cross-section forms a large proportion of coarse grain ring area, the mechanical properties and fatigue strength of the aluminum alloy material are significantly reduced, the coarse grain area has a large crack tendency, and is harmful to further processing of the blank. The 6082 aluminum alloy extruded rod is prone to coarse grain ring during forming.

[0004] At present, many achievements have been made in optimizing the process method of conventional size 6082 aluminum alloy rod, but there are few studies on the performance optimization process method of large 6082 aluminum alloy rod (diameter ≥ 300 mm). This kind of rod is usually used in medium-sized hydraulic cylinder, building support column and ship structure, etc. Due to the large size and working condition requirements, the heat treatment difficulty and internal organization uniformity requirement of this kind of rod are significantly improved. SUMMARY

[0005] In view of the above problems existing in the prior art, the purpose of the present application is to provide a preparation method of an aluminum alloy large rod for inhibiting the generation of coarse grain rings. The present application optimizes the alloy composition, improves the extrusion method, and adopts comprehensive measures such as staged heat treatment to eliminate the coarse grain ring of 6082 large aluminum alloy rod, and finally improves the mechanical properties of the rod, thereby solving the problem of lacking related process for eliminating coarse grain ring in large 6082 aluminum alloy rod in the prior art.

[0006] In order to solve the above technical problems, the present application adopts the following technical scheme:

[0007] A preparation method of an aluminum alloy large rod for inhibiting the generation of coarse grain rings, specifically comprising the following steps:

[0008] Step 1: prepare raw materials of aluminum alloy according to the element composition of 6082 aluminum alloy; wherein, Sc element is added to 6082 alloy, and the amount of Sc added is 0.08%-0.15% according to weight percentage;

[0009] Step 2: heat the raw materials prepared in step 1 to complete melting, then add a refining agent for refining, and then filter the molten aluminum liquid;

[0010] Step 3: prepare the aluminum liquid described in step 2 into an ingot by using a semi-continuous casting method; wherein, the diameter of the ingot is >300mm;

[0011] Step 4: perform two-stage homogenization treatment on the ingot obtained in step 3; wherein, the temperature of the first-stage homogenization treatment is 350℃-420℃, the holding time is 4h-8h, and the heating time is controlled to be 4h-6h; the temperature of the second-stage homogenization treatment is 550℃-570℃, the holding time is 20h-30h, and the heating time is controlled to be 0.5h-1.5h;

[0012] Step 5: machine the ingot obtained in step 4 into an extrusion billet, and extrude the extrusion billet into an aluminum alloy rod by using a reverse extrusion method;

[0013] Step 6: perform pre-stretching treatment on the aluminum alloy rod obtained in step 5; wherein, the stretching amount is 0.5%-1% according to the length of the rod;

[0014] Step 7: perform two-stage solid solution treatment on the aluminum alloy rod obtained in step 6; wherein, the first-stage solid solution temperature is 350℃-480℃, the holding time is 2h-4h, and the heating time is controlled to be 2.5h-3.5h; the second-stage solid solution temperature is 530℃-560℃, the holding time is 0.5h-2h, and the heating time is controlled to be 15min-45min;

[0015] Step 8: perform two-stage aging treatment on the aluminum alloy rod within 3h after step 7 is completed; or, after the rod is naturally placed for at least 48h after step 7 is completed, perform single-stage aging treatment;

[0016] wherein, the two-stage aging treatment steps are as follows: the first-stage aging temperature is 110℃-130℃, the holding time is 1h-3h, and the heating time is controlled to be 0.5h-1h; the second-stage aging temperature is 160℃-170℃, the holding time is 4h-6h, and the heating time is controlled to be within 15min; the single-stage aging treatment steps are as follows: the aging temperature is 170℃-185℃, and the holding time is 8h-10h;

[0017] Step 9: perform stretching, straightening and sawing on the rod after step 8 to obtain the rod.

[0018] Preferably, the aluminum alloy comprises the following elements in percentage by weight: Si: 0.9-1.1%; Fe≤0.3%; Cu: 0.05-0.1%; Mn: 0.4-0.7%; Mg: 0.6-1.2%; Cr: 0.1-0.25%; Zn≤0.2%; Ti: 0.02-0.05%; Zr: 0.08-0.15%; Sc: 0.08-0.15%; the balance being Al; and unavoidable impurities, the total content of which is not more than 0.15%.

[0019] Preferably, the mass ratio of Sc and Zr is 1:1.

[0020] Preferably, in step 2:

[0021] Step 2-1: first put the aluminum ingot and Si into the smelting furnace, heat and melt, and control the melt temperature at 740-760°C; after complete melting, add the intermediate alloy containing Cu, Mn, Cr and Zr; finally add the intermediate alloy containing Mg until complete melting;

[0022] Step 2-2: reduce the temperature of the melt prepared in step 2-1 to the range of 720-740°C and keep it, perform 2-3 times of refining treatment on the melt, and control the refining time at 30-40 min each time; after refining, perform slagging on the melt and stand for 30-40 min; after standing, send the Al-Sc alloy into the melt; perform degassing on the melt by gas blowing method, and after degassing, perform filtration on the aluminum liquid by double-stage filtration method.

[0023] Preferably, in step 3, the aluminum liquid described in step 2 is prepared into an ingot by using a hot-top casting crystallizer, and the casting temperature is 680-720°C.

[0024] Preferably, in step 4, after double-stage homogenization treatment, the ingot is cooled by cooling water, and the cooling rate is 300-500°C / h.

[0025] Preferably, in step 5, during the reverse extrusion process, the billet is gradiently heated, the preheating temperature of the head of the billet is 480-495°C, the preheating temperature of the tail of the billet is 495-510°C; the preheating temperature of the mold is 450-470°C; the preheating temperature of the extrusion cylinder is 430-450°C; and the extrusion speed is 2-3.5 m / min.

[0026] Preferably, in step 7, after each stage of solid solution treatment, the rod is quenched by placing it in cooling water, and the transfer time of the rod in the cooling water after solid solution treatment is not more than 25 s.

[0027] Compared with the prior art, the present application has the following beneficial effects:

[0028] 1、The present application optimizes the traditional 6082 aluminum alloy composition, limits the content of strengthening phase elements, and limits the content of transition group elements and impurity elements such as Fe, Mn, Cr, Ti, etc., so that sufficient dispersion phase can be precipitated during heat treatment to inhibit the formation of coarse grains while preventing the formation of harmful phases; In particular, rare earth element Sc is introduced into the alloy composition, and the content of Zr is controlled to make Sc:Zr close to 1:1, which optimizes the alloy organization and performance while reducing the cost, and takes into account the economic benefits.

[0029] 2、The present application optimizes the heat treatment process, adopts double-stage homogenization treatment, double-stage solid solution treatment and double-stage aging treatment for 6082 aluminum alloy bars; In the double-stage homogenization treatment, the first stage treatment optimizes the distribution of alloying elements, improves the precipitation kinetics, reduces the width of the precipitate-free zone (PFZ) near the grain boundary, and at the same time promotes the uniform precipitation of transition group element compounds, achieving the effect of inhibiting recrystallization, and the second stage treatment makes the eutectic strengthening phase (Mg2Si, etc.) distributed in the grain boundary fully dissolve, in this process, Mg, Si and other elements are more evenly distributed in the matrix, effectively eliminating dendritic segregation and intragranular segregation, so that the strengthening phase can be uniformly precipitated in the subsequent heat treatment process, thereby obtaining more excellent and stable mechanical properties; In the double-stage solid solution treatment, the first stage treatment is carried out at a lower temperature (lower than the recrystallization temperature) to release the strain energy after extrusion forming, reduce the recrystallization driving force, prevent grain coarsening, and at the same time prevent the low-melting-point eutectic phase from melting and dissolving into the matrix, and the second stage treatment temperature is increased to the solid solution temperature of the strengthening phase, so that the second phase re-dissolves into the matrix to form a single-phase solid solution, improving the effect of subsequent aging strengthening; In the double-stage aging treatment, the first stage treatment is carried out at a lower temperature for a short time, which is to promote the precipitation of solute elements and form a large number of uniform and dispersed GP zones and atomic clusters, and the second stage treatment is carried out at a higher temperature for a long time, which is to convert the GP zones and atomic clusters into metastable phases coherent with the matrix, thereby improving the strength and hardness of the alloy; Compared with the traditional single-stage heat treatment, this heat treatment process reduces the probability of coarse grain formation from the principles of crystallization thermodynamics and kinetics, and better controls the size and distribution of precipitated phases by adjusting the temperature and time of each stage, and is more conducive to improving the stability of the bar organization.

[0030] 3、The present application uses a hot top casting device to prepare a cast ingot by a semi-continuous casting method, which effectively reduces the crystallization height, increases the cooling speed of the cast ingot, reduces the liquid cavity of the cast ingot, and narrows the transition zone, thereby improving the density of the cast ingot and the uniformity of the chemical composition distribution along the cross section of the cast ingot, and inhibiting the formation of original coarse grain organization.

[0031] 4、The present application is a process for preparing a rod by reverse extrusion, in which the metal flows uniformly, the structure and mechanical properties along the length of the rod are basically uniform, the required extrusion force is small, it is suitable for the extrusion forming of large ingots, there is no relative movement between the ingot and the extrusion cylinder, i.e. no external friction, the risk of coarse grain ring is greatly reduced, and it has good prospects for industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 Technical roadmap of the method for preparing an aluminum alloy large rod with inhibited coarse grain ring according to the present application. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by a person of ordinary skill in the art based on the present application belong to the scope of protection of the present application.

[0034] Unless otherwise specified in specific cases, the numerical ranges listed in the present application include the upper and lower limits, and all integers and fractions within the range, rather than the specific values listed in the limited range.

[0035] One, a method for preparing an aluminum alloy large rod with inhibited coarse grain ring

[0036] Step 1: Prepare the raw materials of the aluminum alloy according to the element composition in the 6082 aluminum alloy; wherein Sc element is added to the 6082 alloy, and the amount of Sc added is 0.08%-0.15% according to the weight percentage;

[0037] Step 2: Heat the raw materials prepared in step 1 to complete melting, then add a refining agent for refining, and then filter the molten aluminum liquid;

[0038] Step 3: Prepare the ingot by using a semi-continuous casting method for the aluminum liquid described in step 2; wherein the diameter of the ingot is > 300 mm;

[0039] Step 4: Perform two-stage homogenization treatment on the ingot obtained in step 3; wherein the temperature of the first-stage homogenization treatment is 350-420°C, the holding time is 4-8h, and the heating time is controlled to be 4-6h; the temperature of the second-stage homogenization treatment is 550-570°C, the holding time is 20-30h, and the heating time is controlled to be 0.5-1.5h;

[0040] Step 5: Mechanically process the ingot obtained in step 4 into an extrusion blank, and extrude the extrusion blank into an aluminum alloy rod by using a reverse extrusion method;

[0041] Step 6: pre-stretching the aluminum alloy rod obtained in step 5; wherein the stretching amount is 0.5%-1% according to the length of the rod;

[0042] Step 7: double-stage solid solution treatment is performed on the aluminum alloy rod obtained in step 6; wherein the first-stage solid solution temperature is 350-480℃, the holding time is 2-4h, and the heating time is controlled to be 2.5-3.5h; the second-stage solid solution temperature is 530-560℃, the holding time is 0.5-2h, and the heating time is controlled to be 15-45min;

[0043] Step 8: within 3h after step 7 is completed, double-stage aging treatment is performed on the aluminum alloy rod; or after the rod is naturally placed for at least 48h after step 7 is completed, single-stage aging treatment is performed;

[0044] wherein the double-stage aging treatment steps are as follows: the first-stage aging temperature is 110-130℃, the holding time is 1-3h, and the heating time is controlled to be 0.5-1h; the second-stage aging temperature is 160-170℃, the holding time is 4-6h, and the heating time is controlled to be less than 15min; the single-stage aging treatment steps are as follows: the aging temperature is 170-185℃, and the holding time is 8-10h;

[0045] Step 9: stretching, straightening and sawing are performed on the rod after step 8 to obtain the rod.

[0046] The present application has found that, for how to eliminate the coarse grain ring in the aluminum alloy extruded rod, in the field of aluminum alloy rods of conventional size (diameter < 300mm), the prior art has made many achievements, and can well solve the problem of coarse grain ring in aluminum alloy rods of conventional size. But for large 6082 aluminum alloy rods (diameter ≥ 300mm), the process optimized for aluminum alloy rods of conventional size is difficult to eliminate the coarse grain ring. Therefore, the present application considers optimizing alloy composition, improving extrusion method, adopting staged heat treatment and other comprehensive measures to eliminate the coarse grain ring of 6082 large aluminum alloy rods, so as to improve the mechanical properties of the rod.

[0047] In some embodiments of the present application, the aluminum alloy contains the following elements in percentage by weight: Si: 0.9-1.1%; Fe≤0.3%; Cu: 0.05-0.1%; Mn: 0.4-0.7%; Mg: 0.6-1.2%; Cr: 0.1-0.25%; Zn≤0.2%; Ti: 0.02-0.05%; Zr: 0.08-0.15%; Sc: 0.08-0.15%; balance of Al; and unavoidable impurities, the total content of which is not more than 0.15%. Among them, the mass ratio of Sc and Zr is 1:1. Compared with the 6082 aluminum alloy standard GB / T 3190-2020, the present application appropriately relaxes the limitation on the content of Fe, because when the content of Fe is low, the damage of Fe to the mechanical properties of the aluminum alloy is negligible; in addition, when the content of Fe is low, the Al-Fe phase (such as Al3Fe) can act as a heterogeneous nucleation core of α-Al, playing a role in refining the as-cast grains, and the small Al-Fe phase has the effect of pinning the grain boundary and inhibiting the growth of recrystallized grains during extrusion, thereby to some extent inhibiting the generation of coarse grain rings. At the same time, through the melting purification process, the addition of a certain amount of Mn can avoid the harm caused by excessive Fe. Ensuring the addition of a small amount of Cu can play a solid solution strengthening effect, and is also conducive to eliminating the tendency of uneven recrystallization temperature of each part of the matrix due to the segregation of Mn on the grain boundary, thereby inhibiting the generation of coarse grains. For aluminum alloys, Sc is a very strong grain refiner and recrystallization inhibitor. The dispersion strengthening effect produced by adding a small amount of Sc can enable the aluminum alloy to still maintain a stable non-recrystallized structure under conventional hot working conditions, thereby inhibiting the generation of coarse grains. However, the addition of Sc needs to be controlled within a suitable range. If it is too low, it cannot play a refining role, but if it is too high, it will have adverse effects. The addition of Sc can greatly improve the strength, hardness, corrosion resistance and weldability of the aluminum alloy bar, but due to the scarcity of Sc and its high price, the combination of Sc and Zr in a ratio close to 1:1 can precipitate ternary coherent Al3(Sc, Zr) particles with a lattice constant difference of 1.5% from α(Al), which can act as a heterogeneous nucleation core during the solidification of α(Al), greatly improving the nucleation rate, reducing costs while ensuring grain refinement and recrystallization inhibition.

[0048] In some embodiments of the present application, in step 2:

[0049] Step 2-1: First, aluminum ingots and Si materials are put into a melting furnace, and the molten temperature is controlled at 740-760°C; after complete melting, the intermediate alloy containing Cu, Mn, Cr, and Zr is added to ensure rapid melting and uniformity; finally, the intermediate alloy containing Mg is added, and the cover cage is used to immerse it below the molten surface until complete melting to prevent burning loss of Mg and inclusion of oxides. After each feeding is completed, more than two times of mechanical stirring or electromagnetic stirring is required to make the intermediate alloy melt rapidly and uniformly.

[0050] Step 2-2: The melt prepared in step 2-1 is transferred to a static furnace, the melt temperature is reduced to 720-740°C and kept, the granular sodium-free refining agent is selected as the refining agent to prevent the "sodium brittleness" phenomenon, and the melt is refined for 2-3 times with pure argon or high-purity argon (GB / T 4842-2015), and the refining time is controlled between 30-40 min each time; after refining, the melt is skimmed and allowed to stand for 30-40 min to make the impurities fully float and sink to crystallize the melt; after the standing is completed, Al-Sc alloy wire is fed into the melt for online refinement, and the addition step of Sc should be set after the refining treatment is completed, on the one hand to prevent the reaction of Sc with the refining slag to cause loss, and on the other hand to ensure that the time of adding Sc to the casting is controlled within the best invalidation time range; after online refinement, the melt is degassed by gas blowing method, the inert gas used is pure argon or high-purity argon (GB / T 4842-2015), and the inert gas is introduced by a rotating rotor degassing device; after degassing, the aluminum liquid is filtered by a two-stage filtering method, and the filter screen combination is selected as 20PPi / 40PPi or 30PPi / 50PPi.

[0051] In some embodiments of the present application, in step 3, the aluminum liquid prepared in step 2 is prepared into an ingot by using a hot top casting crystallizer semi-continuous casting method, and the casting temperature is kept in the range of 680-720°C; since the diameter of the ingot is greater than 300mm, an integrated crystallizer is used; the height of the crystallizer is selected in the range of 150-200mm; during the casting process, the crystallizer continuously and online lubricates and cools the ingot with circulating cooling water; the circulating cooling water temperature is controlled at 25-27°C; the casting speed is controlled at 80-100mm / min; the ingot requires smooth surface and dense internal structure.

[0052] In some embodiments of the present application, in step 4, the ingot described in step 3 is subjected to two-stage homogenization treatment, the first stage homogenization treatment is at a temperature range of 350-420°C, the holding time is 4-8h, and the temperature rising time is controlled to be 4-6h, the purpose is to optimize the distribution of alloying elements, improve the precipitation kinetics, reduce the width of the precipitate free zone (PFZ) near the grain boundary, and promote the uniform precipitation of transition element compounds, thereby achieving the effect of inhibiting recrystallization; the second stage homogenization treatment is at a temperature range of 550-570°C, the holding time is 20-30h, and the temperature rising time is controlled to be 0.5-1.5h, the purpose of the second stage homogenization treatment is to make the eutectic strengthening phase (Mg2Si, etc.) distributed in the grain boundary fully dissolve, in this process, Mg, Si and other elements are more uniformly distributed in the matrix, effectively eliminating the dendritic segregation and intragranular segregation, so that the strengthening phase can be uniformly precipitated in the subsequent heat treatment process, thereby obtaining more excellent and stable mechanical properties. After homogenization treatment, the ingot is cooled by cooling water, and the cooling rate is 300-500°C / h.

[0053] In some embodiments of the present application, in step 5, the ingot obtained in step 4 is machined into an extrusion blank, and the extrusion blank is extruded into an aluminum alloy rod by using a backward extrusion method. In the machining process, the head and tail of the ingot are the most serious parts of defects such as shrinkage, porosity, inclusions, segregation and the like, and should be removed. The head removal length should be 1-1.5 times the diameter of the ingot, and the tail removal length should be 1.5-2 times the head removal length. Preferably, the removal length can be more accurately determined by ultrasonic flaw detection on the head and tail of the ingot, and the waste material is minimized. The surface segregation layer is turned by turning, and the turning depth should be 3-5 mm to ensure that the cross-sectional composition of the blank is uniform. Preferably, the turning depth can be more accurately determined by sampling and characterizing the metallographic structure of the ingot cross-section near the surface, and the waste material is minimized. After surface turning, the surface of the blank should be free of visible cracks, slag inclusions and color difference, and the surface should be smooth and clean to prevent impurities from entering the extrusion cylinder. In the backward extrusion process, the backward extrusion process is characterized in that the metal flow direction is opposite to the movement direction of the extrusion rod, and there is no relative movement between the metal ingot blank and the extrusion cylinder, so there is no external friction. In the backward extrusion process, the metal flows uniformly, the required extrusion force is small, alloys with higher deformation resistance can be extruded, the mechanical properties of the extrusion product length are basically the same, the waste such as excess pressure is significantly reduced, the precision and yield are high, which is beneficial to the formation of large-size aluminum alloy rods. When the metal is backward extruded, the deformation only occurs near the die hole, and there is no obvious difference in metal flow velocity, which effectively reduces the friction between the rod and the extrusion cylinder during extrusion and inhibits the generation of coarse grain rings. In the backward extrusion process, the blank is gradient heated, the preheating temperature of the head of the blank is 480-495℃, the preheating temperature of the tail of the blank is 495-510℃, the preheating temperature of the die is 450-470℃, the preheating temperature of the extrusion cylinder is 430-450℃, and the extrusion speed is 2-3.5 m / min.

[0054] In some embodiments of the present application, in step 6, the rod is pre-stretched, and the stretching amount is 0.5-1%. Pre-stretching can release a part of the internal stress generated during the extrusion process due to non-uniform cooling and severe deformation, reduce the crystallization driving force, and prevent the generation of coarse grains during subsequent solid solution and aging treatment. If the stretching amount is too small, it cannot release stress, and if the stretching amount is too large, it will increase the tendency of coarse grains.

[0055] In some embodiments of the present application, in step 7, the bar processed in step 6 is subjected to two-stage solid solution treatment, the first stage of solid solution treatment is at a temperature range of 350-480°C, the holding time is 2-4h, and the heating time is controlled to be 2.5-3.5h; the first stage of the two-stage solid solution treatment is to be subjected to solid solution treatment at a lower temperature (lower than the recrystallization temperature), the purpose is to release the strain energy after extrusion molding, to reduce the recrystallization driving force, to prevent grain coarsening, and to prevent the low-melting-point eutectic phase from melting and dissolving into the matrix; the second stage of solid solution treatment is at a temperature range of 530-560°C, the holding time is 0.5-2h, and the heating time is controlled to be 15-45min; the second stage of the two-stage solid solution treatment is to increase the temperature to the solid solution temperature of the strengthening phase, so that the second phase is re-dissolved into the matrix to form a single-phase solid solution, and the effect of subsequent aging strengthening is improved; after each stage of solid solution treatment, the bar is quenched in cooling water (water temperature 25-50°C) in a transfer time of not more than 25s. For solid solution treatment, the solid solution temperature should be as high as possible and the solid solution time should be as long as possible under the premise that the microstructure is not overburned, so that the difficult-to-dissolve phase is fully re-dissolved into the matrix to prepare for subsequent aging treatment. In a long-time high-temperature environment, the grains will grow, resulting in a decrease in the mechanical properties of the alloy. The large aluminum bar needs a long solid solution time, and the grain coarsening is reduced by the two-stage solid solution treatment of "low temperature and long time-high temperature and short time".

[0056] In some embodiments of the present application, the bar processed in step 7 is subjected to two-stage aging treatment. The first stage of aging is at a temperature of 110-130°C, the holding time is 1-3h, and the heating time is controlled to be 0.5-1h; the second stage of aging is at a temperature of 160-170°C, the holding time is 4-6h, and the heating time is controlled to be within 15min; after the aging treatment, the bar is air-cooled to room temperature; the delay time (the interval time between the two-stage solid solution treatment and the two-stage aging treatment) of the two-stage aging should be controlled to be within 3h, and with the extension of the delay time, the holding time of the first stage of aging is shortened to prevent the GP zone and atomic clusters from being converted into metastable phases in advance, resulting in excessive coarsening of precipitated phases during the holding process of the second stage of aging, and weakening the aging effect; if the bar cannot be subjected to artificial aging in time after the solid solution treatment, it should be subjected to single-stage aging after being naturally placed for at least 48h, the aging temperature is 170-185°C, and the holding time is 8-10h.

[0057] In some embodiments of the present application, the bar processed in step 8 is subjected to stretching, straightening and sawing according to product requirements to obtain the bar.

[0058] II. Examples and Comparative Examples

[0059] Example 1:

[0060] 6082 aluminum alloy φ300mm extruded bar production, according to Figure 1 the technical route shown, comprising the following steps:

[0061] Step 1: ingredient: alloy element weight percentage is Si: 0.95%; Fe: 0.1%; Cu: 0.05%; Mn: 0.45%; Mg: 0.8%; Cr: 0.15%; Zn: 0.01%; Ti: 0.025%; Zr: 0.1%; Sc: 0.1%; the balance is Al;

[0062] Step 2: smelting: batch feeding is adopted, first, aluminum ingot and Si material are put into the smelting furnace, and the melt temperature is controlled at 740-760℃; after complete melting, intermediate alloy containing Cu, Mn, Cr and Zr is put in; finally, intermediate alloy containing Mg is added, and it is immersed below the melt surface by a cage until complete melting; mechanical stirring or electromagnetic stirring is carried out twice after each feeding; the prepared melt is transferred into a holding furnace, the melt temperature is reduced to 720-740℃ and is kept, the refining agent is selected as granular sodium-free refining agent, and the melt is refined twice by using pure argon or high-purity argon (GB / T 4842-2015), and the refining time is controlled between 30 min each time; after refining, the melt is raked and is kept for 30 min; after keeping, Al-Sc alloy wire is fed into the melt for online refinement; after online refinement, the melt is degassed by gas blowing method, the inert gas used is pure argon or high-purity argon (GB / T 4842-2015), and the inert gas is introduced by rotating rotor degassing; after degassing, the aluminum liquid is filtered by double-stage filtration method, and the filter screen combination is selected as 20PPi / 40PPi;

[0063] Step 3: semi-continuous casting: hot top casting crystallizer is adopted to prepare ingot, the casting temperature is 700℃; the crystallizer height range is selected as 150mm; the crystallizer implements continuous online oil lubrication and circulating cooling water cooling during casting; the circulating cooling water temperature is controlled at 25℃; the casting speed is controlled at 80mm / min; the ingot requires smooth surface and dense internal structure;

[0064] Step 4: two-stage homogenization: the first stage temperature is 385℃, the holding time is 6h, and the heating time is 5h; the second stage temperature is 560℃, the holding time is 25h, and the heating time is 1h; after homogenization treatment, the ingot is cooled by cooling water, and the cooling rate is 400℃ / h;

[0065] Step 5: sawing and turning: the head length of 300mm and the tail length of 600mm are cut off, and the turning depth is 4mm; after surface turning, the blank surface should be free of visible cracks, slag and color difference, and the surface should be smooth and clean, and impurities are prevented from being brought into the extrusion cylinder;

[0066] Step 6 reverse extrusion: the preheating temperature of the head of the blank is 485℃, the preheating temperature of the tail of the blank is 500℃; the preheating temperature of the die is 460℃; the preheating temperature of the extrusion cylinder is 440℃; the extrusion speed is 3m / min;

[0067] Step 7 pre-stretching: the stretching amount is 0.7%;

[0068] Step 8 two-stage solid solution: the first stage temperature is 385℃, the holding time is 3h, and the heating-up time is 3h; the second stage temperature is 545℃, the holding time is 1h, and the heating-up time is 30min; after the solid solution treatment, quenching is performed in the cooling water (water temperature is 25℃) in the case that the transfer time is not more than 25s.

[0069] Step 9 two-stage aging: immediately after the solid solution treatment, aging treatment is performed, the pre-aging temperature is 120℃, the holding time is 2h, and the heating-up time is controlled to be 0.75h; the final aging temperature is 165℃, the holding time is 5h, and the heating-up time is 15min; after the aging treatment is completed, air cooling is performed to room temperature.

[0070] Comparative Example 1-1

[0071] On the basis of Example 1, the difference is that in Step 6, the extrusion mode is positive extrusion.

[0072] Table 1

[0073] No. Coarse grain ring thickness Note Example 1 1 mm High surface roughness due to high surface layer grain size grade, needs to be turned to remove skin Comparative Example 1-1 10 mm Low surface roughness due to low surface layer grain size grade

[0074] It can be seen that the reverse extrusion process can refine the surface layer grains of the rod, effectively reduce the thickness of the coarse grain ring, although a larger machining allowance needs to be reserved due to the lower surface quality, but the thickness of the coarse grain ring is significantly reduced.

[0075] Example 2-1 to Example 2-8

[0076] Examples 2-1 to 2-5 are based on Example 1, changing the process parameters of two-stage homogenization and comparing, Comparative Examples 2-6 to 2-8 are based on Example 1, the process parameters are not within the scope of the application, and 5 sampling points are taken equidistantly from the center to the surface along the diameter direction, the samples are cut and prepared, and the microstructure is characterized by using a scanning electron microscope SEM, the dispersed phase size, distribution and the dissolution of the grain boundary strengthening phase are observed. See Table 2 for details.

[0077] Table 2

[0078]

[0079] From the above table, it can be seen that Example 2-3 is the optimal two-stage homogenization process: the first stage temperature is 395℃, and the holding time is 6h; the second stage temperature is 570℃, and the holding time is 25h; after the holding is completed, rapid cooling is performed, and the cooling rate is 500℃ / h; the increase of the first stage holding temperature is conducive to accelerating the homogenization of alloying elements and the precipitation of the dispersion phase, and inhibits the growth of recrystallization in the extrusion process; the increase of the second stage holding temperature is conducive to the full re-dissolution of the intergranular strengthening phase, so that the ingot maintains high plasticity in the extrusion process, reduces the extrusion resistance and reduces the tendency of coarse grain ring formation. From Example 2-4 and Example 2-5, it can be seen that the continuous increase of the holding temperature and the holding time has little effect on the performance improvement, and the second phase has a tendency to grow, the production efficiency is reduced and the energy consumption is increased. Comparative Example 2-6 is a common single-stage homogenization process, compared with Example 1, the single-stage homogenization process has limited effect on the re-dissolution of the intergranular strengthening phase, which is reflected in the size of the intergranular strengthening phase being too large, which is due to the lack of the first stage low temperature holding treatment, the first stage treatment can improve the morphology of the intergranular strengthening phase, promote the spheroidization of the intergranular strengthening phase, and make it more easily dissolved into the aluminum alloy matrix in the second stage high temperature holding treatment; in addition, the introduction of the first stage treatment can also effectively prevent the overburning of the low melting point phase, after the first stage treatment, the dispersion phase is uniformly distributed in the matrix, and it is not easy to melt at high temperature. Comparative Example 2-7 and Comparative Example 2-8 are respectively insufficient first stage treatment and insufficient second stage treatment relative to Example 1, in Comparative Example 2-7, the size of the dispersion phase increases obviously and is unevenly distributed, and the re-dissolution effect of the intergranular strengthening phase is poor, which leads to the weakening of the pinning effect of the dispersion phase on the grain boundary, the increase of the coarse grain tendency, and the presence of more residual intergranular strengthening phase, which reduces the plasticity of the ingot and increases the deformation stress in the extrusion process, and the recrystallization and coarsening tendency increases; in Comparative Example 2-8, the alloy composition is not completely homogenized, which leads to a large difference in recrystallization temperature in different parts, and the intergranular strengthening phase is coarse, which aggravates the formation of coarse grains. From Comparative Example 2-6 to Comparative Example 2-8, it can be seen that, compared with the two-stage homogenization process within the process parameter range of the present application, the common single-stage homogenization process and the two-stage homogenization process with process parameters not within the range of the present application have poor inhibition effect on the coarse grain ring.

[0080] Examples 3-1 to 3-8

[0081] Examples 3-1 to 3-5 are based on Example 2-3, the two-stage solid solution process parameters are changed and compared, and Comparative Examples 3-6 to 3-8 are based on Example 2-3, the process parameters are not within the range of the present application, the middle section is cut, and three sampling positions are taken from the center to the surface along the diameter direction, cylindrical tensile specimens are prepared, and their tensile strength, yield strength and elongation are measured respectively. See Table 3.

[0082] Table 3

[0083]

[0084] It can be seen from Example 3-1 to Example 3-5 that the comprehensive mechanical properties of the bar in Example 3-3 are the best; Comparative Example 3-6 is a common single-stage solid solution treatment outside the scope of the application, and compared with the double-stage solid solution treatment, the mechanical properties are poor due to the lower solid solution degree of the single-stage solid solution and the tendency of recrystallization coarsening and overburning; it can be seen from Comparative Example 3-7 that if the first-stage solid solution temperature is lower or the holding time is shorter, the strain energy is not fully released, leading to grain coarsening and reducing the strength of the bar; it can be seen from Comparative Example 3-8 that if the second-stage solid solution temperature is lower or the holding time is shorter, the Mg2Si strengthening phase is not fully dissolved, leading to less β' phase precipitated from the matrix after aging, poor aging strengthening effect, reducing the strength of the bar and making the plasticity higher; in addition, if the holding time of the first-stage or second-stage solid solution is too long, the matrix grain may be coarsened; if the holding temperature of the second stage is too high, the bar may be overburned, and the strength and plasticity of the bar are both reduced.

[0085] Example 4-1 to Example 4-4

[0086] Example 4-1 to Example 4-2 are adjusted on the basis of Example 3-3, and the difference is that the process parameters of the double-stage aging are changed and compared; Example 4-3 is a treatment method in the application that the solid solution treatment cannot be immediately followed by aging treatment; Comparative Example 4-4 is a common single-stage aging treatment outside the scope of the application, the middle section of the sample is cut, and 3 sampling positions are taken from the center to the surface along the diameter direction to prepare cylindrical tensile samples, and the tensile strength, yield strength and elongation are measured respectively. See Table 4.

[0087] Table 4

[0088]

[0089] From the above table, it can be seen that the mechanical properties of the bar of Example 3-3 are the best; if the holding temperature of the first and second aging is too low or the holding time is too short, the matrix structure of the bar is still in a supersaturated state, the precipitation of metastable phase is insufficient, the resistance to dislocation movement is weak, the strength is reduced and the plasticity is improved, i.e. the "under aging" phenomenon; if the holding temperature of the first aging is too high or the holding time is too long, the GP zone and atomic clusters are converted to metastable phase in advance, and these metastable phases formed in advance will provide heterogeneous nucleation cores for the formation of stable phase in the second aging, thereby leading to the formation of coarse β phase, weakening the aging effect, reducing the strength of the bar and improving the plasticity; and if the holding temperature of the second aging is too high or the holding time is too long, coarse β phase particles will also be formed, the resistance to dislocation movement is weakened, the strength of the bar is reduced and the plasticity is improved, i.e. the "over aging" phenomenon; in addition, it can be seen from Example 4-3 that for the bar which cannot be aged immediately after solid solution treatment, the bar is placed for 50 h and then subjected to single-stage aging, and a relatively high strength can also be obtained, because in this placement time, the GP zone and atomic clusters begin to form in the matrix, which is equivalent to replacing the first aging with natural aging. Comparative Example 4-4 is a single-stage aging treatment system outside the scope of the present application, and compared with Example 4-1 and Example 4-2, the strength of the bar is improved and the plasticity is reduced, because after solid solution, the bar is directly subjected to high-temperature holding, the nucleation rate is relatively fast, the GP zone and atomic clusters are not uniformly and dispersedly formed in the matrix in advance, leading to uneven distribution of nucleation sites, the fine and dispersed precipitates are concentrated in the grain interior, the resistance to dislocation movement is large, which is manifested as high strength and low plasticity; in addition, the comprehensive mechanical properties of Comparative Example 4-4 are worse than those of the appropriate double-stage aging system (Example 3-3), which may be due to the fact that the precipitates concentrated in the grain grow under high-temperature holding to form coarse and continuous precipitates, which deteriorate the mechanical properties, and compared with the double-stage aging system, the single-stage aging is more sensitive to process parameters, leading to poor controllability of the properties.

[0090] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application but not to limit the technical solutions, and those of ordinary skill in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, which should be covered in the scope of the claims of the present application.

Claims

1. A method for preparing large aluminum alloy bars that suppress the formation of coarse grain rings, characterized in that, Specifically, the steps include the following: Step 1: Prepare the raw materials for the aluminum alloy according to the element composition of 6082 aluminum alloy; among them, add Sc element to 6082 alloy, and the amount of Sc added is 0.08%-0.15% by weight percentage. Step 2: Heat the raw materials prepared in Step 1 until they are completely melted, then add a refining agent for refining, and then filter the molten aluminum liquid; Step 3: Prepare ingots from the molten aluminum described in Step 2 using a semi-continuous casting method; wherein the diameter of the ingots is >300mm; Step 4: Perform a two-stage homogenization treatment on the ingot obtained in Step 3; wherein, the temperature of the first-stage homogenization treatment is 350℃-420℃, the holding time is 4h-8h, and the heating time is controlled at 4h-6h; the temperature of the second-stage homogenization treatment is 550℃-570℃, the holding time is 20h-30h, and the heating time is controlled at 0.5h-1.5h. Step 5: Machining the ingot obtained in Step 4 into a billet to be extruded, and extruding the billet into aluminum alloy rods using a reverse extrusion method; Step 6: Perform pre-stretching treatment on the aluminum alloy bar obtained in Step 5; wherein, the stretching amount is 0.5%-1% based on the length of the bar. Step 7: Perform a two-stage solution treatment on the aluminum alloy rods obtained in Step 6; wherein, the first stage solution treatment temperature is 350℃-480℃, the holding time is 2h-4h, and the heating time is controlled at 2.5h-3.5h; the second stage solution treatment temperature is 530℃-560℃, the holding time is 0.5h-2h, and the heating time is controlled at 15min-45min. Step 8: Within 3 hours after completing Step 7, perform a two-stage aging treatment on the aluminum alloy bar; or, after completing Step 7, allow the bar to rest naturally for at least 48 hours before performing a single-stage aging treatment. The two-stage aging process is as follows: the first stage aging temperature is 110℃-130℃, the holding time is 1h-3h, and the heating time is controlled within 0.5h-1h; the second stage aging temperature is 160℃-170℃, the holding time is 4h-6h, and the heating time is controlled within 15min; the single-stage aging process is as follows: the aging temperature is 170℃-185℃, and the holding time is 8h-10h. Step 9: Stretch, straighten and saw the bar material processed in Step 8 to obtain the bar material.

2. The preparation method according to claim 1, characterized in that, The aluminum alloy contains the following elements by weight percentage: Si: 0.9-1.1%; Fe≤0.3%; Cu: 0.05-0.1%; Mn: 0.4-0.7%; Mg: 0.6-1.2%; Cr: 0.1-0.25%; Zn≤0.2%; Ti: 0.02-0.05%; Zr: 0.08-0.15%; Sc: 0.08-0.15%; balance Al; and unavoidable impurities, totaling no more than 0.15%.

3. The preparation method according to claim 2, characterized in that, The mass ratio of Sc to Zr is 1:

1.

4. The preparation method according to claim 1, characterized in that, In step 2: Step 2-1: First, put the aluminum ingot and Si material into the melting furnace, heat and melt them, and control the melt temperature at 740℃-760℃; after they are completely melted, add the intermediate alloy containing Cu, Mn, Cr and Zr; finally, add the intermediate alloy containing Mg until it is completely melted. Step 2-2: Reduce the temperature of the melt prepared in Step 2-1 to 720-740℃ and hold it at that temperature. Refine the melt 2-3 times, with each refining time controlled between 30-40 minutes. After refining, remove the slag from the melt and let it stand for 30-40 minutes. After standing, introduce Al-Sc alloy into the melt. Degas the melt using a gas blowing method. After degassing, filter the molten aluminum using a two-stage filtration method.

5. The preparation method according to claim 1, characterized in that, In step 3, the aluminum liquid described in step 2 is prepared into ingots using a hot-top casting crystallizer, and the casting temperature is 680℃-720℃.

6. The preparation method according to claim 1, characterized in that, In step 4, after the two-stage homogenization treatment, the ingot is cooled with cooling water at a rate of 300℃ / h-500℃ / h.

7. The preparation method according to claim 1, characterized in that, In step 5, during the reverse extrusion process, the billet is subjected to gradient heating. The preheating temperature of the billet head is 480℃-495℃, and the preheating temperature of the billet tail is 495℃-510℃; the preheating temperature of the die is 450-470℃; the preheating temperature of the extrusion cylinder is 430-450℃; and the extrusion speed is 2-3.5m / min.

8. The preparation method according to claim 1, characterized in that, In step 7, after each solution treatment, the bar is placed in cooling water for quenching, and the transfer time of the bar in cooling water after solution treatment does not exceed 25 seconds.

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