2020-11-05
1. Segregation of chemical components in ingots The phenomenon of uneven distribution of chemical components in ingots is called segregation. In the wrought aluminum alloy, segregation mainly includes intragranular segregation and reverse segregation.
1.1 Intracrystalline segregation
The phenomenon of uneven chemical composition in grains in the microstructure is called intragranular segregation.
The microstructure characteristic of intragranular segregation is that the corrugated intragranular structure resembles a tree ring after etched. The microhardness in the grains is different, the microhardness near the grain boundary is high, and the microhardness in the center of the grain is low.
The presence of intragranular segregation makes the chemical composition inside the crystal grains and the structure of the ingot extremely uneven, and seriously deteriorates the performance of the ingot, mainly:
1) The inhomogeneity of the chemical composition and the unbalanced excess phase caused by the intracrystalline segregation of the solid solution reduce the stability of the alloy against electrochemical corrosion. 2) The appearance of non-equilibrium eutectic or low-melting composition reduces the starting melting temperature of the alloy, which makes the ingot easy to produce local overburn during the subsequent thermal deformation or quenching heating process. 3) Intragranular segregation not only causes the appearance of non-equilibrium phases and increases the number of second phases, but these low-melting phases form a hard and brittle dendritic network around the crystal branches, which sharply reduces the plasticity and processing properties of the ingot. 4) The inhomogeneity of chemical composition caused by intragranular segregation is inherited into the semi-finished product, resulting in the formation of coarse grains in the processed material after annealing.
Intracrystalline segregation is caused by unbalanced crystallization. Therefore, in the actual production of aluminum alloy continuous casting, intragranular segregation is inevitable. An effective method to eliminate intragranular segregation is to homogenize the ingot for a long time.
In continuous casting, the method to reduce intragranular segregation is:
First, increase the cooling rate and adopt modification treatment to refine the grains and intra-granular structure, and reduce the range of intra-granular segregation.
Second, use the completely opposite method to reduce the cooling rate and perform deep-cavity casting similar to ingot casting to reduce the degree of supersaturation of elements such as iron and manganese to reduce the degree of segregation.
Third, select some additives that can appropriately change the crystalline properties of the alloy. For example, adding an appropriate amount of iron to the 3A21 alloy reduces the concentration of manganese in the solid bath, thereby reducing the uneven distribution of manganese within the grains. In actual production, in the presence of impurity iron, adding titanium is beneficial to reduce the segregation in 3A21 alloy solid solution grains, because the directions of titanium segregation and manganese segregation are exactly opposite, and the center of the dendritic crystal contains high titanium, thereby reducing casting The difference between the solid solution concentration in the center and the edge of the crystal grain.
1.2 Inverse segregation
The phenomenon that the solute concentration at the edge of the ingot is higher than the solute concentration in the center of the ingot is called reverse segregation. The structural characteristics of reverse segregation are not easy to distinguish from the microstructure, and can only be confirmed from the analysis of chemical components.
Reverse segregation in aluminum alloy ingots is an important reason for the great differences in mechanical and physical properties of ingots and their pressure-processed products. The chemical composition of the area with serious degree of reverse segregation even exceeds the specified range of the standard, causing the mechanical properties to exceed the standard and be scrapped.
Reverse segregation is a mixing phenomenon in the solidification process of aluminum alloy continuous ingots, which cannot be completely avoided, nor can it be eliminated by high temperature homogenization. However, according to the formation law of inverse segregation and influencing factors, the segregation of elements can be controlled within the allowable range. The method is:
1) Improve the cooling strength of the ingot. 2) Choose an appropriate casting speed so that the inclination angle of the transition zone to the exposed liquid surface is not too large, generally not more than 60°. 3) Increase the casting temperature appropriately. 4) Use a suitable casting funnel to evenly divert and make it flow to the edge of the ingot. 2. Gas and non-metallic inclusions in ingots 2.1 Non-metallic inclusions (slag inclusions)
The slag mixed into the ingot or other non-metallic impurities falling into the ingot are called non-metallic inclusions. The fracture features are black strips or flakes, and the microstructure features are mostly black linear, massive, and flocculent disordered tissues, with obvious color difference from the matrix. Non-metallic inclusions are an important cause of delamination and many surface defects in aluminum processed products. In the heat treatment and heating process, the presence of non-metallic inclusions can promote the formation of secondary porosity and bubbles. In terms of mechanical properties, non-metallic inclusions are places where stress is concentrated, which reduces the strength limit and elongation of the alloy. In particular, the reduction in transverse elongation and dynamic mechanical properties (impact toughness, fatigue strength and fracture toughness) is more serious. In addition, non-metallic inclusions will reduce the stress corrosion resistance of the alloy.
Non-metallic inclusions (slag inclusions) can usually be found when inspecting the transverse low-magnification test piece of the ingot. YS67-1993 stipulates that for all alloy products, there should be no more than two slag inclusions on the ingot test piece, and the single area is less than 0.5 mm2, otherwise it will be rejected. Non-metallic inclusions (slag inclusions) are the most frequently occurring waste products in aluminum alloy ingots. According to statistics, ingots scrapped due to slag inclusion account for 10% to 25% of the total amount of waste calculated by quality. Slag inclusions are usually caused by slag, lining fragments, and larger oxides that fall into the ingot along with the liquid metal during the casting process. The method to prevent slag inclusion is: 1) Carefully refine, ensure the standing time, and filter with glass cloth in the flow plate or funnel; 2) Shorten the transfer distance as much as possible, establish good transfer conditions, and close the flow channel, flow plate, All the exposed drop in the funnel can prevent the metal level from fluctuating. It is best to use the same level of casting when possible; 3) Thoroughly bake the transfer tool, appropriately increase the casting temperature, carefully slag in the casting process, and carefully after the casting. Clean the stove, flow plate and other tools; 4) Use clean charge.
2.2 Oxide film
According to the definition of YS/T417.1-1999, the oxide film is the non-metallic inclusions and unremoved gas (mainly hydrogen) that are mainly formed by alumina in the ingot. Therefore, the oxide film is a unique non-metallic inclusion type defect in the ingot, but it is different from the slag inclusion. It is generally not found when the low-magnification test piece and fracture structure of the ingot are inspected. It can be found only by using a special process sample (upsetting the ingot sample).
The oxide film found on the fracture of the ingot process sample can be roughly divided into three types: one is light yellow, brown or dark brown flake oxide film; the other is the surface morphology similar to the flake oxide film but present Bright white "sequins"; the other type is round or oval "small bright spots" with silver-white luster.
There is no unified understanding of the nature of oxide film. However, many researchers believe that the three types of oxide films in the ingot are caused by the contamination of the melt with moisture, hydrogen and oxygen. Among them, the flaky oxide film is a layered layer formed by solid non-metallic inclusions or air bubbles with an oxidized surface, the sequins are layered by uncompressed loose and air bubbles, and small bright spots are formed by supersaturated solid solution. Secondary hydrogen bubbles precipitated in the. 2.3 Porosity and shrinkage
When the melt crystallizes, due to insufficient liquid metal replenishment between the matrix dendrites or due to the presence of unremoved gas (mainly hydrogen), the micropores formed in the dendrites after crystallization are called loose. The micropores formed by insufficient feeding are called shrinkage porosity; the porosity formed by gas is called gas porosity. The loose macroscopic tissue is characterized by black pinholes, and the fractured tissue is characterized by rough tissues and not dense. When the looseness is severe, there are small white bright spots on the fracture. The microstructure is characterized by angular black holes. The more severe the looseness, the greater the number of black holes and the larger their size.
Porosity reduces the density and compactness of the ingot, especially the impact toughness and transverse elongation of high-strength aluminum alloys. In addition, the loose ingots will cause cracks during hot rolling and forging, and the semi-finished products will have bubbles during spheroidizing annealing. Therefore, as an important structural defect of ingots, porosity must be prevented. Strict control should be carried out on the porosity that has occurred in the ingot. When the degree exceeds the standard allowed, the ingot will be rejected. The degree of porosity in the aluminum alloy ingot can generally be determined by measuring the density of the sample and comparing it with the theoretical density of the alloy.
However, in production, for the sake of convenience, it is usually based on the four-level standard picture (the standard is formulated according to the size and number of loose holes per unit area of ??the low-power test piece, see YS67-1993). of. For general-purpose aluminum alloy ingots, the diameter is greater than 290 mm, the level 3 porosity is allowed, and the diameter is less than 290 mm, the level 2 porosity is allowed; but the ingots for important products are not allowed to have porosity. The porosity in continuous aluminum alloy ingots is always distributed in the equiaxed crystal zone with a wider transition zone and is always filled with gas. Under normal conditions, the largest loose area of ??the slab ingot occurs within 30 mm of the wide surface skin layer. The largest loose zone of the ingot occurs in the center of the ingot and develops to the edge as the height of the mold increases. The supply flow conditions have a great influence on the distribution of porosity, and may disrupt the above-mentioned rules and cause maximum porosity in some areas.
According to the formation process of porosity and influencing factors, the method to prevent porosity in ingot is:
1) Reduce the gas content in the melt. The main points should be as follows:
(1) The furnace must be thoroughly baked after large or medium repair; (2) Refining agent, casting tools, etc. must be preheated and baked thoroughly; (3) Refining must be thorough; (4) To prevent the melt from staying in the furnace for too long .
2) Reduce the size of the transition zone in the ingot. The main measures are:
The porosity in continuous aluminum alloy ingots is always distributed in the equiaxed crystal zone with a wider transition zone and is always filled with gas. Under normal conditions, the largest loose area of ??the slab ingot occurs within 30 mm of the wide surface skin layer. The largest loose zone of the ingot occurs in the center of the ingot and develops to the edge as the height of the mold increases. The supply flow conditions have a great influence on the distribution of porosity, and may disrupt the above-mentioned rules and cause maximum porosity in some areas.
According to the formation process of porosity and influencing factors, the method to prevent porosity in ingot is:
1) Reduce the gas content in the melt. The main points should be as follows:
(1) The furnace must be thoroughly baked after large or medium repairs; (2) Refining agents, casting tools, etc. must be preheated and baked thoroughly; (3) Refining must be thorough; (4) To prevent the melt from staying in the furnace for too long .
2) Reduce the size of the transition zone in the ingot. 2.5 White spots
White dot refers to a gray-white dotted film without light selectivity in the fracture structure of aluminum alloy ingots. White spots are usually distributed on the bottom of the ingot, the gate and the edge of the cross section. The nature and formation mechanism of white spots are still unclear. Preliminary research shows that white dots have the following characteristics:
1) It is easy to appear in aluminum alloys with high magnesium content; 2) It is alkaline; 3) It is brittle. When the white spot is severe, it can significantly reduce the plasticity of the ingot; 4) Usually distributed at the bottom, gate and edge of the ingot; 5) The white spot mainly depends on the degree of gas saturation of the melt and the ingot The cooling conditions. The higher the gas content of the melt, the lower the cooling strength of the ingot, the greater the tendency to form white spots. To prevent white spots, one is to reduce the gas content in the melt, and the other is to increase the crystallization rate of the ingot. 3. Cracks in aluminum ingots There are four common crack forms in aluminum alloy flat ingots, namely: side cracks, bottom cracks, gate cracks, and surface cracks. The common forms of cracks in aluminum alloy round ingots are: central cracks, surface cracks, ring cracks and transverse cracks. Common cracks in aluminum alloy hollow round ingots include radial cracks in the inner hole, annular cracks and transverse cracks. In addition, in aluminum alloy ingots, there is also a form of cracks called crystal layer splitting. The distribution of the oxide film in the ingot is uneven, and there is no strict regularity, but according to a large number of industrial production statistics, there are roughly the following tendencies:
1) Different alloys have different types of oxide films in ingots. Generally, flaky oxide films are mostly found in alloys such as 2A12, 7A04, 2A11, 5A06, and 2A14, sequins are more commonly found in alloys such as 6A02, 2A50, 2B50, 2A14, 2A02, and small bright spots are more common in alloys such as 2A70 and 2A80.
2) The tendency of ingots to be polluted by oxide film increases with the lengthening of the melt transfer distance, the decrease of ingot specifications, the increase of temperature and the increase of absolute humidity of the atmosphere.
3) The tendency of the ingot to be polluted by the oxide film is often larger in the first and last casting times and the head and tail of the ingot.
4) The appearance of the oxide film in the ingot increases with the increase of the unidirectional deformation ratio.
The method to prevent and reduce the production of oxide film in aluminum alloy ingots is as follows: First, in every process from the storage and preparation of raw materials to smelting and casting, start with prevention, protection and refining. Corresponding measures, joint governance, to ensure the purity of the metal. Secondly, the work of preventing and reducing the oxide film should not only focus on removing the solid non-metallic inclusions in the melt, but also on greatly reducing the hydrogen content in the melt. Among the above measures, the most critical should be the following four points:
1) Adopt the most effective combined refining method of slag removal and degassing; 2) Adopt perfect pilot furnace and transfer injection technology to implement casting at the same level; 3) Implement high temperature casting; 4) Prevent secondary pollution.
3.1 Side cracks of flat ingot
1) Characteristics of side cracks in flat ingots
(1) It is a cold crack; (2) It usually occurs in hard alloys (such as 7A04, 2A12, etc.); (3) In direct water-cooled semi-continuous casting, it usually occurs after the ingot length reaches 1.5 to 2 m; 4) The crack at the beginning is often mixed with slag, layering, cracking, or crystalline microcracks; (5) The angle between the crack plane and the horizontal plane depends on the casting speed and the ingot width to thickness ratio. The lower the casting speed and the wider the ingot, the smaller the included angle.
During continuous casting, the small face of the slab ingot is cooled on three sides, while the center of the large face is cooled on both sides. The temperature gradient and cooling rate of the small face along the axis of the ingot greatly exceed the temperature gradient of the center of the large face along the axis of the ingot. The cooling rate causes the ingot facet to generate tensile stress acting in the height direction. At the beginning, it may be due to the stress concentration caused by the cold barrier or non-metallic inclusions, which caused the formation of very shallow original cracks in the small area. With the gradual cooling of the ingot, the sensitivity of the metal to the notch is greatly improved. The residual stress in the ingot is locally concentrated at the original crack. When it exceeds the level allowed by the strength of the metal, it causes the sudden development of side cracks. .
2) Ways to prevent side cracks
(1) Reduce the tensile stress in the small area of ??the ingot
①Using a small face notched mold to cool the metal in the small face area in advance during casting. At this time, the metal in the central area of ??the wide face of the ingot that is at the same level as the small face area only forms a hard shell near the mold wall , Will not cause big resistance to the shrinkage of the small area, thus reducing the tensile stress of the small area. At the same time, as a result of the advance cooling of the small area, the quality of the metal resistant to tensile stress is increased.
②In the case of incision on the small face of the mold, appropriately increase the water pressure of the small face to enhance the effect of the incision.
③ Properly increase the casting speed to move the maximum tensile stress point in the ingot to the center of the wide face. At this time, the temperature of the metal in the center of the wide face of the ingot is higher, and the plasticity is better, and the stress generated is easily deformed by the metal. Eliminated. At the same time, the amount of metal that resists tensile stress increases, so the tendency of the ingot to side crack decreases.
④Choose a suitable ingot cross section. The tensile stress in the facet area increases with the increase of the width-to-thickness ratio when the thickness of the ingot is constant, and increases with the increase of the thickness when the width-to-thickness ratio is constant. Therefore, for alloys with high cold cracking tendency, a smaller ingot width to thickness ratio should be selected.
(2) Special attention should be paid to prevent small noodles from layering, small noodles cracking and small noodles from slag.
(3) Control the chemical composition strictly in accordance with internal standards to improve the alloy's ability to resist hot and cold cracks.
3.2 Cracks at the bottom of the ingot
1) The crack at the bottom of the flat ingot has the following characteristics:
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