Increase knowledge | How to improve the quality of 6063 aluminum rod?
1. The choice of alloy composition
1. Selection of alloying element content
The composition of 6063 alloy has a wide range. In addition to the mechanical properties and processing properties, the specific composition must also consider the surface treatment performance, that is, how the profile is surface treated and what kind of surface is to be obtained. For example, if you want to produce abrasive materials, Mg/Si should be smaller, generally in the range of Mg/Si=1-1.3. This is because there is more relatively excess Si, which is conducive to the sandy surface of the profile; if the production is bright For materials, coloring materials and electrophoretic coating materials, Mg/Si is better in the range of 1.5-1.7. This is because there is less excess silicon, the profile has good corrosion resistance, and it is easy to get a bright surface. In addition, the extrusion temperature of aluminum profiles is generally selected at about 480°C. Therefore, the total amount of alloying elements magnesium and silicon should be about 1.0%, because at 500°C, the solid solubility of Mg2Si in aluminum is only 1.05%, which is too high. The content of alloying elements will cause the Mg2Si to not completely dissolve into the matrix during quenching, and there will be more undissolved Mg2Si phases. These Mg2Si have little effect on the strength of the alloy, but will affect the surface treatment performance of the profile, and give the profile oxidation, coloring (or Paint) cause trouble.
2. The influence of impurity elements
①Iron and iron are the main impurity elements in aluminum alloy. In 6063 alloy, the national standard stipulates that it is not more than 0.35. If the first-class industrial aluminum ingot is used in the production, the iron content can generally be controlled below 0.25, but if it is to reduce production Cost, large use of recycled aluminum or other external aluminum, iron is likely to exceed the standard. There are two forms of Fe in aluminum, one is the β phase (Al9Fe2Si2) with acicular (or flake) structure, and the other is the α phase with granular structure (Al12Fe3Si). Different phase structures are used for aluminum alloys. There are different effects. The β phase of the flaky structure is much more destructive than the α phase of the granular structure. The β phase can make the surface of the aluminum profile rough, the mechanical properties and the corrosion resistance worse, and the surface of the oxidized profile becomes blue and shiny. Decrease, no pure color can be obtained after coloring, therefore, the iron content must be controlled.
In order to reduce the harmful effects of iron, the following measures can be taken:
a) All tools used for smelting and casting shall be coated with paint before use to minimize iron dissolution and molten aluminum.
b) Refine the crystal grains to make the iron phase finer and smaller, reducing its harmful effects.
c) Add appropriate amount of strontium to transform β phase into α phase and reduce its harmful effects.
d) Carefully select the waste and miscellaneous materials, and try to reduce the iron wire, nails, iron filings and other sundries entering the aluminum melting furnace to increase the iron content.
②Other impurity elements
Other impurity elements are very few in electrolytic aluminum ingots, far below the national standard, and may exceed the standard when using recycled waste aluminum; in production, not only must each element not exceed the standard, but also the total impurity elements must be controlled. The amount cannot exceed the standard. When the content of a single element does not exceed the standard, but the total amount exceeds the standard, these impurity elements also have a great impact on the quality of the profile. In particular, it needs to be emphasized that practice has proved that when the zinc content reaches 0.05 (not greater than 0.1 in the national standard), white spots appear on the surface of the profile after oxidation, so the zinc content should be controlled below 0.05.
2. Smelting of 6063 aluminum alloy
1. Control the melting temperature
Aluminum alloy smelting is one of the most important process links in the production of high-quality cast rods. If the process is not properly controlled, slag inclusions, pores, coarse grains, feather crystals and other casting defects will be produced in the casting handle. Therefore, strict control must be performed.
It is better to control the melting temperature of 6063 aluminum alloy between 750-760℃, too low will increase the generation of slag inclusion, too high will increase hydrogen absorption, oxidation, nitriding burning loss. Studies have shown that the solubility of hydrogen in molten aluminum rises sharply above 760°C. There are many ways to reduce hydrogen absorption when heat is reduced, such as drying the melting furnace and smelting tools, and preventing the use of flux from being degraded by moisture. But melting temperature is one of the most sensitive factors. Excessive melting temperature not only wastes energy and increases costs, but also causes defects such as pores, coarse grains, and feather crystals.
2. Choose good flux and proper refining process
Flux is an important auxiliary material used in aluminum alloy smelting. At present, the main components of fluxes sold on the market are chloride and fluoride. Among them, chloride has strong water absorption and is easy to be damp. Therefore, the raw materials used in the production of flux must be dried. Thoroughly remove water, seal the packaging, prevent damage during transportation and storage, and pay attention to the production date. If the storage date is too long, moisture absorption will also occur. In the smelting of 6063 aluminum alloy, the slag remover, If the flux such as refining agent and covering agent absorbs moisture, it will cause the aluminum liquid to produce different degrees of hydrogen absorption.
It is also very important to choose a good refining agent and to choose a suitable refining process. At present, most of the refining of 6063 aluminum alloy uses powder spray refining. This refining method can make the refining agent and the aluminum liquid fully contact, so that the refining agent can play the best. efficacy. Although this feature is obvious, attention must be paid to the refining process, otherwise the desired effect will not be obtained. The pressure of nitrogen used in powder injection refining is better to meet the requirements of blowing powder. If the nitrogen used in refining is not high-purity Chlorine (99.99%N2), the more nitrogen is blown into the molten aluminum, the more the moisture in the fluorine gas will cause the molten aluminum to oxidize and absorb more hydrogen. In addition, the fluorine gas pressure is high, and the rolling waves generated by the molten aluminum are large, which increases the possibility of oxidation and slag inclusion. If high-purity nitrogen is used in the refining, the refining pressure is high, the bubbles generated are large, the buoyancy of the large bubbles in the aluminum liquid is large, the bubbles rise rapidly, the residence time in the aluminum liquid is short, and the hydrogen removal effect is not good and wasteful. Nitrogen increases costs. Therefore, nitrogen should be used less, refining agent should be used more, more refining agent has only advantages, no harm. The key point of the powder spray refining process is to use as little gas as possible and spray as much refining agent into the aluminum liquid as possible.
3. Grain refinement
Grain refinement is one of the most important processes in aluminum alloy melting and casting, and it is also one of the most effective measures to solve casting defects such as porosity, coarse grains, bright crystals, feather crystals, and cracks. In alloy casting, it is non-equilibrium crystallization, and most of the impurity elements (including alloying elements) are mostly concentrated in the grain boundaries. The smaller the grains, the larger the area of the grain boundaries. The impurity elements (or alloying elements) The higher the uniformity. For impurity elements, high uniformity can reduce its harmful effects, and even turn the harmful effects of a small amount of impurity elements into benefits; for alloying elements, high uniformity can exert greater alloying capabilities of alloying elements. To achieve the purpose of making full use of resources.
In the production of 6063 aluminum alloy, for abrasive materials, the uniform distribution of alloying elements and impurity elements is particularly important because the profile must be corroded to produce a uniform sand surface. The finer the crystal grains, the more uniform the distribution of alloying elements (impurity elements), and the more uniform the sand surface obtained after corrosion.
3. Casting of 6063 aluminum alloy
1. Choose a reasonable casting temperature
Reasonable casting temperature is also an important factor in the production of high-quality aluminum rods. Too low temperature can easily produce casting defects such as slag inclusion and pinholes. If the temperature is too high, casting defects such as coarse grains and feather crystals are likely to occur.
The 6063 aluminum alloy liquid after the grain refining treatment can be appropriately increased, and the casting temperature can generally be controlled between 720-740°C. This is because: ①The aluminum liquid becomes sticky and easy to solidify after the grain refining treatment crystallization. ②Aluminum rod has a liquid-solid two-phase transition zone in the crystallization front of the casting. Higher casting temperature has a narrow transition zone. The narrow transition zone is conducive to the escape of the gas discharged from the crystallization front. Of course, the temperature should not be too high or too high. The casting temperature will shorten the effective time of the grain refiner and make the grains relatively larger.
2. When possible, fully preheat and dry the casting system such as the launder and the splitter plate to prevent the reaction of moisture and the aluminum liquid from causing hydrogen absorption.
3. During casting, avoid turbulent flow and rolling of the molten aluminum as much as possible. Do not easily stir the molten aluminum in the launder and shunt plate with tools, so that the molten aluminum can flow into the crystallizer smoothly under the protection of the surface oxide film. It is because the tool agitates the molten aluminum and the flow of the molten aluminum will rupture the oxide film on the surface of the molten aluminum, causing new oxidation, and at the same time, the oxide film is drawn into the molten aluminum. Studies have shown that the oxide film has a very strong adsorption capacity, it contains 2% of water, when the oxide film is involved in the aluminum liquid, the water in the oxide film reacts with the aluminum liquid, causing hydrogen absorption and slag inclusion.
4. Filter the molten aluminum. Filtration is the most effective method to remove non-metallic slag in the molten aluminum. In the casting of 6063 aluminum alloy, it is generally filtered with multi-layer glass cloth or ceramic filter plate, no matter what kind of filtering is adopted. Method, in order to ensure that the molten aluminum can be filtered normally, the surface scum should be removed before the filtration of the molten aluminum, because the surface scum can easily block the filter mesh of the filter material, and the filtration can not be performed normally. The easiest way to remove the surface scum of the molten aluminum The method is to set a slag baffle in the launder to remove the scum before filtering the aluminum liquid.
4. Homogenization treatment of 6063 aluminum alloy
There are two problems in the internal structure of the aluminum alloy rod produced by casting: ① There is casting stress between the grains; ② The imbalance of chemical composition in the grain caused by non-equilibrium crystallization. Due to the existence of these two problems, extrusion becomes difficult, and at the same time, the extruded product has a decline in mechanical properties and surface treatment properties. Therefore, the aluminum rod must be homogenized before extrusion to eliminate the casting stress and the imbalance of chemical composition in the crystal grain.
The homogenization treatment is a heat treatment in which the aluminum rod is kept at a high temperature (below the over-firing temperature) to eliminate the casting stress and the imbalance of the chemical composition in the crystal grain. The over-firing temperature of Al-Mg-Si series alloy should be 595℃, but due to the presence of impurity elements, the actual 6063 aluminum alloy is not a ternary system, but a multi-element system. Therefore, the actual over-firing temperature is higher than 595℃. Lower, the homogenization temperature of 6063 aluminum alloy can be selected between 530-550 ℃, the temperature is high, can shorten the holding time, save energy, and improve the productivity of the furnace.
Energy-saving measures for homogenization treatment: Homogenization treatment needs to be kept at high temperature for a long time, which requires a large energy source and high processing costs. Therefore, most of the current aluminum rods are not homogenized. The most important reason is that the homogenization process requires high cost. The main measures to reduce the cost of homogenization treatment are:
① Refine the grain
Refining the grains can effectively shorten the holding time, and the finer the grains, the better.
②The lengthened aluminum rod heating furnace is controlled in sections according to the homogenization and extrusion temperature to meet the requirements of different processes. The main benefits of this process are:
a) Do not increase the homogenization treatment furnace.
b) Make full use of the heat energy after homogenization of the aluminum holder to avoid reheating the aluminum rod during extrusion.
c) The aluminum holder has a long heating and holding time, and the temperature inside and outside is uniform, which is conducive to extrusion and subsequent heat treatment.
In summary, to produce high-quality 6063 aluminum alloy cast rods, firstly select reasonable ingredients according to the profile produced, and secondly strictly control the melting temperature and casting temperature, and do a good job in grain refinement, alloy liquid refining, filtration, etc. Technological measures, careful operation, to avoid rupture and involvement of the oxide film. Finally, the aluminum rods are homogenized, so that high-quality aluminum rods can be produced, which provides a reliable material basis for the production of high-quality profiles.