The tools used to repair the damage include pneumatic grinder, pneumatic small milling cutter, electric small milling cutter and flat shovel. Before repairing, the defective parts should be found by corrosion, and the repaired parts should be smooth transition, and the width should be 5 ~ 10 times of the depth. Aluminum alloy pressure processing products are divided into seven categories: rust prevention (LF), hardness (LY), forging (LD), superhard (LC), cladding (LB), special (LT) and brazing (LQ). The commonly used aluminum alloy materials are annealed (M-annealed), quenched (Y) and hot rolled (R).
Characteristics of die casting
Pressure casting (die casting for short) is a casting method in which molten alloy is injected into a pressure chamber at a high speed to fill a steel mold cavity, and the alloy liquid solidifies under pressure to form a casting. Die casting is different from other casting methods at high pressure and high speed.
Molten metal (1) fills the cavity under pressure and crystallizes and solidifies under higher pressure. Common pressure is15-100 MPa.
(2) The cavity is filled with molten metal at a very high speed, usually 10-50m/s, and some of them can even exceed 80m/s (the linear speed of introducing the cavity through the inner gate-the speed of the inner gate), so it can be filled with molten metal in about 0.0 1-0.2s (depending on the size of the casting). Die-casting machine, die-casting alloy and die-casting mold are the three elements of die-casting production, and they are indispensable. The so-called die casting process is the organic comprehensive application of these three elements, so as to produce qualified castings with good appearance and internal quality, and the size meets the requirements of drawings or agreements, even high-quality castings stably, rhythmically and efficiently.
Die casting fluidity
Fluidity refers to the ability of alloy liquid to fill the mold. Fluidity determines whether the alloy can cast complex castings. Among aluminum alloys, * * * crystal alloy has the best fluidity.
There are many factors that affect the fluidity, mainly the composition, temperature and solid particles of pollutants such as metal oxides and metal compounds in the alloy liquid, but the external fundamental factors are pouring temperature and pouring pressure (commonly known as pouring head).
In actual production, when the alloy has been determined, in addition to strengthening the smelting process (refining and slag removal), it is necessary to improve the casting process (sand mold permeability, metal mold exhaust and temperature) to increase the pouring temperature without affecting the casting quality, so as to ensure the fluidity of the alloy.
aluminum
Aluminum and aluminum alloys processed into certain shapes are collectively called aluminum materials, including plates, belts, foils, tubes, rods, wires and profiles. Aluminum alloy material, high strength and light weight. The main welding processes are TIG welding, metal inertia gas, FSW and resistance spot welding.
Protective measures for aluminum alloy welding
1. Before welding, remove the oxide on the groove and peripheral part of the workpiece and the surface of the welding wire by chemical+mechanical method, and the order is chemical cleaning first, then mechanical polishing;
2. Qualified shielding gas should be used for protection during welding;
3. Flux is used in gas welding, and the oxide film on the surface of molten pool is constantly broken by welding wire during welding.
Welding difficulty
(1) is easily oxidized. In air, aluminum is easily oxidized to form a dense alumina film (thickness about 0. 1-0.2 micron) with a high melting point (about 2050℃), far exceeding the melting points of aluminum and aluminum alloys (about 600℃). The density of alumina is 3.95-4. 10g/cm3, which is about 1.4 times that of aluminum. The surface of alumina film is easy to absorb moisture, which hinders the fusion of basic metals during welding, and is easy to form defects such as porosity, slag inclusion and incomplete fusion, which leads to the decline of weld performance.
(2) It is easy to produce pores. Hydrogen is the main cause of porosity in aluminum and aluminum alloy welding. Because liquid aluminum can dissolve a lot of hydrogen, solid aluminum hardly dissolves hydrogen. When the temperature of the molten pool cools and solidifies rapidly, hydrogen cannot escape, and it is easy to gather in the weld to form pores. It is difficult to completely avoid hydrogen holes. There are many sources of hydrogen, such as hydrogen in arc welding atmosphere, and the oxide film on the surface of aluminum plate and welding wire absorbs moisture in the air. Practice has proved that even if the purity of argon reaches above 99.99% according to GB/T4842 standard, a large number of dense pores will appear when the moisture content reaches 20ppm, and when the relative humidity of air exceeds 80%, if no measures such as heating are taken, the weld will obviously appear pores. At the same time, slow welding with low current, increasing the cooling time of weld seam and stirring the molten pool with welding wire arc are helpful for gas to be discharged from the molten pool.
(3) The weld is easy to deform and crack. The linear expansion coefficient and crystal shrinkage rate of aluminum are about two times larger than that of steel, which is easy to produce larger welding deformation internal stress and will promote the generation of hot cracks for structures with greater stiffness.
(4) The thermal conductivity of aluminum is high (0.538 calories/cm.s.cm for pure aluminum). It is about four times that of steel, so welding aluminum and aluminum alloys consumes more heat than welding steel.
(5) Evaporation and burning loss of alloying elements. Aluminum alloy contains low-boiling elements (such as magnesium, zinc, manganese, etc. ), it is easy to evaporate and burn under the action of high temperature arc, thus changing the chemical composition of weld metal and reducing the performance of weld.
(6) Low high temperature strength and plasticity. At high temperature, the strength and plasticity of aluminum are very low, which destroys the formation of weld metal and sometimes easily leads to the collapse and penetration of weld metal.
(7) does not change color. When aluminum and aluminum alloys change from solid to liquid, there is no obvious color change, so it is difficult for operators to master the heating temperature. Melting and pouring of aluminum alloy are the main links in casting production. Strictly controlling the whole process of melting and casting plays an important role in preventing casting defects such as pinhole, inclusion, insufficient pouring, crack, porosity and shrinkage porosity. Because aluminum melt has a strong tendency to absorb hydrogen, strong oxidation ability and easy to dissolve iron, simple and cautious preventive measures must be taken during melting and casting to obtain high-quality castings.
1, aluminum alloy batching preparation and quality control
In order to smelt high-quality aluminum melt, it is necessary to select qualified raw materials first. The raw materials must be scientifically managed and properly handled, otherwise the quality of the alloy will be seriously affected. Production practice has proved that raw materials (including metal materials and auxiliary materials) will be scrapped in batches if they are not strictly controlled.
(a) raw materials must have qualified chemical composition and organization, and the specific requirements are as follows:
Besides analyzing the main components and impurity content, the eutectic structure and fracture surface of the incoming alloy ingot were also examined. Practice has proved that it is difficult to obtain dense castings by using aluminum liquid containing serious shrinkage cavities, pinholes and bubbles, and even the whole furnace and batch of castings will be scrapped.
When someone is studying the influence of Al-Si alloy ingot on pinhole of aluminum alloy, it is found that there is no pinhole when using molten pure casting sand test block. When the Al-Si alloy ingot with unqualified low microstructure is added, the pinhole of the test block is serious and the grain is coarse. The reason is the heredity of materials. Al-Si alloy and heritability increase with the increase of silicon content, and the heritability is significant when the silicon content reaches 7%. Continue to increase the silicon content to * * * crystal composition, the heritability decreased slightly. In order to solve the casting defects caused by burden heredity, aluminum ingots, master alloys and other high metallurgical quality burden must be selected. The specific criteria are as follows:
(1) There should be no pinholes and blowholes on the fracture surface.
The pinhole should be within Grade III, and the local area (no more than 25% of the inspected area) should not exceed Grade III. Those exceeding the third grade must be remelted to reduce pinhole. The remelting refining method is the same as general aluminum alloy smelting, and the casting temperature shall not exceed 660℃. For those aluminum ingots and alloy ingots with large original grains, lower ingot mold temperature should be adopted first to make them solidify quickly and refine the grains.
2. Charge treatment
Blast blasting should be carried out before the furnace charge is used to remove rust, grease and other dirt on the surface. The aluminum alloy ingot and metal mold return charge with clean surface can be taken out without sand blowing treatment, but the iron filter and insert mixed in the charge should be removed, and all the charge should be preheated before entering the furnace to remove the moisture attached to the surface, so that the smelting time can be shortened by more than 3 hours.
3, cost management and storage
The reasonable preservation and management of burden is of great significance to ensure the quality of alloy. The burden should be stored in a dry warehouse with little temperature change.
2. Preparation of Crucibles and Melting Tools
(1) Crucibles Iron crucibles are often used for casting aluminum alloys, and can also be welded with cast steel and steel plates.
Whether it is a new crucible or an old crucible that has not been used for a long time, sand should be blown before use and heated to 700-800 degrees for 2-4 hours to burn off the moisture and combustible substances attached to the inner wall of the crucible. When the temperature is lower than 300 degrees, carefully clean the inner wall of the crucible, and when the temperature is not lower than 200 degrees, spray paint.
Before use, the crucible should be preheated to dark red (500-600 degrees) and kept for more than 2 hours. Before melting outside the new crucible, it is best to melt a batch of the same brand of recycled materials.
(2) Preparation of melting tools
Bell jar, pressure ladle, mixing ladle and ladle
Ingot mold, etc. Before use, it should be preheated, coated with 150-200 degree protective coating, and thoroughly dried at 200-400 degree for more than 2 hours. After use, the oxides and fluorides attached to the surface should be thoroughly removed (preferably sand blasting).
3. Control of melting temperature
Too low melting temperature is not conducive to the dissolution of alloying elements and the discharge of gases and inclusions, which will increase the tendency of segregation, cold insulation and underfilling. In addition, due to insufficient heat in the riser, the casting cannot be properly fed. It is pointed out that the melting temperature of all aluminum alloys should reach at least 705 degrees and be stirred. Excessive melting temperature not only wastes energy, but also is more serious because the higher the temperature is, the more hydrogen is absorbed, the coarser the grain is, the more serious the oxidation of aluminum is, and the more serious the burning loss of some alloy elements is, which leads to the decline of the mechanical properties of the alloy, the deterioration of the casting performance and processability, the weakening of the modification treatment effect and the reduction of the air tightness of the casting.
The production practice has proved that the alloy liquid can be quickly heated to a higher temperature and stirred reasonably, which can promote the dissolution of all alloy elements (especially refractory metal elements), and then reduce to the pouring temperature after removing scum. In this way, the degree of segregation is minimum, and less hydrogen is melted, which is beneficial to obtain alloys with uniform density and high mechanical properties. Because it is difficult to judge the temperature of aluminum melt with naked eyes, no matter what type of melting furnace is used, a thermometer should be used to control the temperature. Temperature measuring instruments should be inspected and maintained regularly. Thermowells should be cleaned regularly with a metal brush and coated with a protective coating to ensure the accuracy of temperature measurement results and the service life of the guide.
4. Control of smelting time
In order to reduce the oxidation, gas absorption and iron dissolution of aluminum melt, the residence time of aluminum melt in the furnace should be shortened as much as possible and the aluminum melt should be melted quickly. From the beginning of melting to the completion of pouring, sand casting shall not exceed 4 hours, metal casting shall not exceed 6 hours and die casting shall not exceed 8 hours.
In order to speed up the smelting process, the middle piece of recycled material with low melting point and Al-Si master alloy should be added to form a molten pool at the bottom of the crucible as soon as possible, and then the larger piece of recycled material and pure aluminum ingot should be added to make them slowly immerse in the gradually expanding molten pool and melt quickly. After the main part of the burden is melted, a small amount of high melting point master alloy is added, and the melting is accelerated by heating and stirring. Finally, the temperature is lowered and the oxidizable alloy elements are pressed in to reduce the loss.
5. Transfer and pouring of melt
Although the density of solid alumina is similar to that of aluminum melt, it takes a long time to sink to the bottom of crucible after entering the aluminum melt. However, the alumina film formed by oxidation of aluminum melt is dense only on the side in contact with aluminum melt, and loose on the side in contact with air, with a large number of small holes with a diameter of 60 60-100A, which has a large surface area and strong adsorption, and is easy to adsorb on water vapor, but easy to float. Therefore, the specific gravity difference between this oxide film and aluminum melt is small, and the floating and sinking speed is very slow when mixed with the melt, so it is difficult to remove it from the melt, and the pores formed in the casting are too inclusion. Therefore, the key to transfer aluminum melt is to minimize the stirring of molten metal and the contact between melt and air.
When transferring the melt with an inclined crucible, in order to avoid mixing the melt with air, the ladle should be placed as close as possible to the furnace mouth, so that the melt can flow down the side wall of the ladle, and it will not directly impact the bottom of the ladle, causing stirring and splashing.
Adopting correct and reasonable pouring method is one of the important conditions to obtain high quality castings. In production practice, it is very effective to pay attention to the following matters to prevent and reduce casting defects.
(1) Before pouring, carefully check whether the melting temperature, ladle capacity, dryness of surface coating and preparation of other tools meet the requirements. Place the metal sprue cup on the sand mold within 3-5 minutes before pouring. At this time, the temperature of the ladle is not higher than 150 degrees. If it is placed too early or the temperature is too high, a large amount of gas will be trapped in the gate, which may cause explosion during pouring.
(2) It is not allowed to pour under the condition of "draught", and the melt is strongly oxidized and burned, resulting in defects such as oxide inclusions in the casting.
(3) When taking out the melt from the crucible, gently remove the oxide scale or flux layer on the surface of the melt with the bottom of the ladle, slowly immerse the ladle into the melt, scoop out the melt with the wide mouth of the ladle, and then lift the ladle smoothly.
(4) Don't put your hands flat when lifting the ladle, keep your feet steady and don't lift the ladle too high. The metal level in the ladle must remain stable and cannot move.
(5) Before pouring, the slag of the ladle should be scraped clean, so as to avoid bringing slag and oxide belt into the mold during pouring.
(6) During the pouring process, the melt flow will remain stable and cannot be interrupted, and it cannot go straight to the bottom hole of the mouth. The gate should be filled from beginning to end, the liquid level should not be turned, and the pouring speed should be controlled properly. Usually, the pouring starts slowly to make the melt filling stable, and then the pouring speed is slightly faster, and the pouring speed remains basically unchanged.
(7) In the pouring process, the distance between the nozzle and the gate of the ladle should be as close as possible, with a limit of no more than 50 mm, so as to avoid excessive oxidation of molten metal.
(8) For the blocked injection port, the blockage should not solidify prematurely. After the melt fills the sprue, it should be put aside slowly and obliquely to avoid vortex when the melt is injected into the sprue.
(9) The melt less than 60mm from the bottom of the crucible is not suitable for casting.
Aluminum alloy casting (ZL)
According to the main elements other than aluminum, silicon, copper, magnesium and zinc are divided into four categories, and the codes are 100, 200, 300 and 400 respectively.
In order to obtain high-quality precision castings of various shapes and specifications, aluminum alloys for casting generally have the following characteristics.
(1) has good fluidity and is used to fill the narrow part of the tank.
(2) The melting point is lower than that of ordinary metals, but it can meet the requirements of most cases.
(3) Good thermal conductivity, the heat of molten aluminum can be quickly transferred to the mold, and the casting cycle is short.
(4) Harmful gases such as hydrogen in the melt can be effectively controlled by treatment.
(5) When aluminum alloy is cast, there is no tendency of hot brittle cracking and tearing.
(6) Good chemical stability and strong corrosion resistance.
(7) it is not easy to produce surface defects, and the surface smoothness and luster of the casting are good, so it is easy to carry out surface treatment.
(8) The machinability of cast aluminum alloy is good. It can be produced by die casting, hard mold casting, green sand casting, dry sand casting and investment plaster casting. It can also be molded by vacuum casting, low-pressure and high-pressure casting, extrusion casting, semi-solid casting, centrifugal casting and other methods to produce castings with different uses, specifications and properties.
Cast aluminum alloy is widely used in automobiles, such as engine cylinder head, intake manifold, piston, wheel hub, steering booster housing and so on. In the process of aluminum alloy production, casting defects such as shrinkage cavity, sand hole, air hole and slag inclusion are easy to occur. How to repair defects such as air holes in aluminum alloy castings? If electric welding, argon arc welding and other equipment are used for repair, it is easy to produce side effects such as thermal deformation due to large heat release, which can not meet the requirements of repair welding.
The cold welding repair machine uses the principle of high frequency electric spark instantaneous discharge and non-thermal surfacing to repair casting defects. Because the heat affected zone of cold welding is small, it will not cause annealing deformation, cracks, hard spots and hardening of the base metal. Moreover, the welding strength is high, the auxiliary materials and the matrix are solidified after melting at the same time, and the combination is firm, which can be processed by grinding, milling, filing and the like. , and dense does not fall off. Cold welding repair machine is an ideal method to repair tiny defects such as air holes and sand holes in aluminum alloy.