How to Improve The Quality of 6063 aluminum billet

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How to Improve the Quality of 6063 aluminum billet

　
How to Improve the Quality of 6063 aluminum billet

1. Main character of Al-Mg-Si series aluminum alloy of 6063

According to the chemical composition of 6063 alloy at the standard of GB/T5237-93, Silicon is 0.2%-0.6%, Magnesium is 0.45%-0.9% and the maximum Iron is 0.35%, while other element (such as Cu, Mn, Zr, Cr) are less than 0.1%, where there still large space for chosen.

6063 alloy is belongs to Al-Mg-Si series aluminium alloy, which is heat-treatable and intensified. In the ternary system of Al-Mg-Si series, there are only two binary compounds- Mg2Si and Mg2Al3. And it is divided by pseudo binary section of α(Al)-Mg2Si into two ternary systems, ?(Al)-Mg2Si-(Si) and ? (Al)-Mg2Si-Mg2Al3.

The main strengthening phase in Al-Mg-Si Series Alloy is Mg2Si, whose quantity changes while quenching. The more the Mg2Si melting in matrix is, the higher the strength of alloy after aging is, or the less, the lower. When the eutectic temperature on pseudo binary phase diagram of ? (Al)-Mg2Si is 595?, the largest solubility of Mg2Si is 1.85%, while 500? is 1.05%, from which we can know that temperature affects the solubility of Mg2Si: The strength after aging depends on the temperature of quenching. Some aluminium profile cannot reach the standard of hardness while their chemical components are qualified. Because the temperature of aluminum billet is not enough (inner of billet is cold while outer of billet is hot) when extrusion.

In Al-Mg-Si series Alloy, the weight ratio of Mg2Si between Magnesium and Silicon is 1.73. If Mg in alloy is more than Si, the solubility of Mg2Si in the alloy will reduce; if Si in alloy is more than Mg, the solubility of Mg2Si in the alloy will not change, from which we can know that, to get higher strength of alloy, the ratio should be Mg?Si?1?73.

2. Selection of 6063 aluminum alloy composition

(1) Content of aluminium alloy elements

The fact that there is a wide scope of 6063 aluminum alloy composition. But when selecting the specific composition, we should consider mechanical performance, processing performance and surface treatment performance. For example, when the ratio between Mg and Si is 1 to 1.3, products can be easily manufactured with abrazine surface (frosted), because relative surplus Si will be easy to get the sand shape surface; if produce the aluminum profile for gloss surface, coloration surface and the electrophoresis coating surface, then the ratio 1.5 to 1.7 will be better, because the less Si, the corrosion resistance of aluminum profile will be better, it can be easy to get the gloss surface of aluminum profile.

On the other hand, the extrusion temperature of aluminum billet is around 480? and the total amount of Mg and Si should be about 1.0%. Because when the aluminum billet is 500?, the solubility of Mg2Si is 1.05%, high content of alloy elements will not make Mg2Si fully fused into the matrix, the dis-fused Mg2Si can no effect the strength of alloy while have a negative effect on and the performance of surface treatment, bring a lot of trouble for anodizing, coloring and electrophoresis.

(2) Influence from other margin elements

1) Iron

Iron is the main margin element in the 6063 aluminum alloy, the content of it is not allowed to over 0.35 according to China standard. The content of iron can be controlled under 0.25 when first-grade industrial aluminium ingots are put into production. But for reduce the production cost, using a lot of recycle aluminum or aluminum scrap, the iron will be exceeded easily. There are two existing forms of Fe in aluminium: ? diagram (Al9Fe2Si2) with acicular structure; ? diagram (Al12Fe3Si) with granulous structure. Different structures have different effects on aluminum alloy. The destructiveness of ? diagram is greater than ? diagram, causing roughness of aluminum profile surface, worst mechanical performance and bad resistance to corrosion. After anodizing, the aluminum profile surface is pastiness, the glossiness decline; even after coloration, the aluminum profile surface can’t get the real color. Therefore, the content of iron should be controlled.

Solutions to decrease adverse effects from iron:

? All of tools used in the process of melting and casting should be coated for less iron melting into the aluminum liquid.

? Refine grains to make iron thinner and smaller.

? Add appropriate strontium in the melting liquid, so that ? phase turns into ? phase.

? Be careful to select the waste sundries like the iron wire, iron nail and iron scrap to avoid putting into the melting furnace in order to lower the content of iron.

2) Other margin element

In primary aluminum ingots, the content of other margin elements are lower than China standard, while which may be exceeded in using recycled aluminum from scrap. During the process of manufacture, we have to control not only the content of single elements, but also other margin elements. Once the content of margin element is over the standard, it may have negative effects on the quality of aluminum products. Most importantly, it has been found that white spots appear on the aluminum profile surface after anodizing when the content of Zinc reaches 0.05 (China standard is less than 0.1). Therefore, the content of Zinc has to control to less than 0.05.

3. 6063 aluminum alloy melting and refining

1) Control the melting temperature

Aluminum alloy melting is one of the most important parts of process of craft to get superior aluminum billet, which will get slag inclusions, pores, thick grain and feather crystal caused by improper control process. So start to aluminum alloy melting, have to control strictly very step.

The best temperature of aluminum alloy melting is 750? ~760?, lower than which will increases the aluminum slag inclusion, while over which will cause hydrogen absorption, oxidation and nitridation and other melting loss. It is found that the solubility of hydrogen may rise rapidly at above 760?. Fortunately, there are many ways to reduce the absorption of hydrogen, such as drying the aluminum melting furnaces and aluminum melting tools and avoiding fluxing agent from humidity or metamorphism. However, the key factor is melting temperature. If it is too high or too low, it may increase extra production cost as well as imperfections such as pore, thick grain and feather crystal.

2) Choose good quality fluxing agent and proper refining process

Fluxing agents are important auxiliary materials in aluminum alloy melting, which main compositions are chloride and fluoride. In the process of manufacture, fluxing agents should be dried and gotten rid of moisture, in particular the chloride with strong moisture absorption. What’s more, attention should be paid to fluxes during the process of packaging, transportation and storage in case of damaged. Furthermore, notice the date in produced of flux, do not keep fluxes too long time in case of humidity. Once fluxes (like Deslagging flux, refining flux and covering flux) are wet, hydrogen absorption may happen in the smelting of 6063 aluminum alloy.

Proper refining craft are as important as good quality fluxes. Nowadays, powder injection refining is the most common craft used in the melting of 6063 aluminum alloy, which takes good use of refining flux with fully contact with aluminium liquid. However, pressure of Nitrogen should be the minimum value in order to receive the best effect. If the nitrogen is not (99?99?N2), moisture from nitrogen will cause oxidation and hydrogen absorption. In addition, large waves caused by turnup and high-pressure nitrogen increase the possibility of oxidic slag inclusions. Even though high-purity Nitrogen is used in refining process, large bubbles caused by high pressure will float up rapidly so that the time of Nitrogen staying at aluminum liquid is very short, then it is not good for remove the hydrogen from aluminum liquid, resulting in a waste of nitrogen and extra production cost. Therefore, use more refining flux but less nitrogen, meanwhile, the key point of powder injection refining process is that using the least nitrogen to spray into the most refining flux.

3) Grain refinement

Grain refinement is one of the most important craft of melting and casting aluminum alloy, but also is one of the effective solutions to solve defects of aluminum billet, such as pore, thick grain, bright crystal, feather crystal and crack. While casting aluminum alloy, non-equilibriums are everywhere, with majorities of margin element gathering on the crystal boundary, including alloy elements. The smaller the grain is, the larger the area of crystal boundary is, and the higher degree of uniformity of margin element is. For margin element, high degree of uniformity can reduce their negative effects, even turn some of them into positive effects; for alloy elements, high uniformity can help them reach their alloying potential.

In production of 6063 aluminum alloy, uniformity of alloy elements and margin element is necessary for abrazine material of aluminum profile to get dull finish surface through corrosion. Therefore, grains should be as small as positive so that the distribution of alloy elements and margin element is uniform, which is helpful for uniformity of abrasive surface caused by corrosion.

4. Casting 6063 aluminium alloy billet

1) Select suitable casting temperature

Proper casting temperature is the key factor to manufacture superior aluminium billets. Too low will cause slag inclusions and pinholes while too high will cause thick grains and feather grains.

After grains refinement, casting temperature of aluminium liquid can be raised between 720? and 740?. Reasons for that are followed: First, aluminum liquid becomes sticky after grains refinement and it is easy to crystallize. Second, before crystallizing, there is a transitional zone. High billet casting temperature will have narrow transitional zone, which is beneficial to release gas. However, casting temperature can’t too high because over-heat will shorten the effective time of grains refining flux and makes grains become too large.

2) If possible, preheat fully, dry the launder and shunt plate and other casting system in case of hydrogen absorption caused by combination between moisture and aluminium liquid.

3) In the aluminum billet casting process, try to avoid the aluminum liquid rolling and foaming. Do not stir the aluminum liquid in lanuder or shunt plate in order to let the aluminum liquid steadily flow into crystallizer to crystallize under protection of oxidation film of aluminum liquid surface. Because aluminum liquid rolling or stirring aluminum liquid will be easy to damage the oxidation film of aluminum liquid and it will create a new oxidation film and mix into the aluminum liquid. Study shows that oxidation film with strong ability of absorption and 2% moisture, when oxidation film mixs into aluminum liquid, it will lead to hydrogen absorption and slag inclusion caused by the reaction between moisture and aluminium liquid

4) .Filter is the most effective solution to eliminate non-metal slag inclusions. Multilayer glass wool cloths or ceramic plank are usually used in filter during casting aluminum billet of 6063 alloy. To ensure the process of filter can be carried out normally, skimming on aluminum liquid surface should be removed before filter, because the skimming will be easy to block the filter screen holes, so that the filter can’t work better. To solve that problem, set a skimming block in the launder to remove the skimming before filter.

5. Homogenization of 6063 aluminium alloy billet

1) Problems caused by non-equilibrium crystallized

There are two major problems in aluminium alloy billet: one is casting stress that exists among grains; the other is imbalance of chemical composition caused by non- equilibrium crystallized. Owing to these two problems, the aluminum billet will be difficult to extrude at the aluminum profile extrusion process. Meanwhile, the mechanical performance of aluminum profile and the surface treatment performance will decrease. Hence, aluminum billet should be homogenized before extrusion to eliminate casting stress and imbalance of chemical compositions.

2) Homogenization treatment

Homogenization treatment is a kind of heat treatment to eliminate casting stress and imbalance of chemical components by keeping warm in the heat (lower than burnt temperature). The burnt temperature of Al-Mg-Si series alloy is 595?. However, the real one is lower than 595? because of margin element in 6063 aluminum alloy, which is a polynary system. Therefore, temperature of homogenizing 6063 aluminum alloy can be set between 530? and 550?, which is high enough to increase productivity.

3) Influence from size of grains to homogenization treatment

Homogenization treatment is a long process that alloy elements spread from edge of crystal to the centre in the heat for the great binding force among atoms in solid. That means the smaller the grains are, the shorter homogenizing time.

4) Solutions to save energy for homogenization treatment

Nowadays, few factories adopt homogenization treatment for its high cost and large demand of energy. Fortunately, there are several solutions to lower the cost:

1) Grain refinement. Small size of grain can shorten the homogenizing time, the smaller the grains are, the shorter homogenizing time.

2) Adopt multi-log aluminum billet heating furnace with hot shear and separately control temperature of homogenization treatment and extrusion. So we can leave out an extra furnace for homogenizing process. It can make good use of the heat energy after homogenization to avoid heating up again when extrusion. Meanwhile, long holding time for heating aluminum billet, same temperature inside or outside will be helpful to extrusion and following heat treatment. That is why most of China aluminum profile extrusion factories adopts multi-log aluminum billet heating furnace with hot shear, because the aluminum billet didn’t process homogenization treatment. But for large ton aluminum extrusion press above 1800T, it is equipped with single log billet heating furnace with hot shear to get faster heating up time. But single log billet heating furnace can’t hold the temperature, so the aluminum billet can’t reach the function of homogenization treatment. So when use single billet heating furnace, need to consider about the billet homogenizing process.

To sum up, in order to produce superior 6063 aluminium alloy billets, first at all, choose suitable alloy compositions according to different characteristics of aluminum profiles. Secondly strictly control alloy melting and billet casting temperature, carefully controls refinement of grain and aluminum liquid filter in case of fracture and involvement of oxidation films. At last, homogenization treatment can improve aluminum billet quality, then superior 6063 aluminium alloy billets provides high-quality aluminum extrusion products with a reliable material basis.
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