Basic knowledge of the casting aluminum ingots (3)
4. The main elements of aluminum alloy casting performance
Silicon (Si) is the main component for improving flowability, and the best fluidity is obtained from eutectic to hypereutectic, but the precipitated silicon (Si) tends to form hard spots and deteriorate the machinability, Do not let it exceed the eutectic point.In addition, silicon (Si) can improve the tensile strength, hardness, machinability and high temperature strength, while reducing the elongation of the content of 10.5% -12.0% of the ingot is easy to shrink but does not affect quality.
In aluminum alloy, copper (Cu) is dissolved in solid solution to improve the mechanical properties and improve the machinability, however, the corrosion resistance is reduced and hot cracking is likely to occur, as is copper (Cu) as an impurity.
Aluminum-magnesium alloy has the best corrosion resistance, therefore, ADC5 and ADC6 are corrosion-resistant alloys and have a large solidification range, so they have hot brittleness and are prone to cracking in castings and are difficult to cast. As an impurity, magnesium (Mg) -Si-Cu This material, Mg2Si will make the casting brittle, so the general standard of less than 0.3%.
Impurities of iron (Fe) will generate FeAl3 needle crystal, because the die-casting is rapid cooling, so the precipitated crystal is very thin, can not be said to be harmful ingredients. Less than 0.7% of the content is not easy demoulding phenomenon, so iron (Fe) 0.8 ~ 1.0% instead of good die-casting. Contains a lot of iron (Fe), will generate metal compounds, the formation of hard spots. And iron (Fe) content of over 1.2%, reducing the fluidity of the alloy, the quality of the casting damage, shorten the life of metal components in die-casting equipment.
Like copper (Cu), there is a tendency to increase the tensile strength and the hardness, which greatly affects the corrosion resistance. To improve high-temperature strength and heat resistance, nickel (Ni) is sometimes added, but it has a reduced effect on corrosion resistance and thermal conductivity.
Can improve the copper (Cu), silicon (Si) alloy high temperature strength. If more than a certain limit, easy to generate Al-Si-Fe-Mn quaternary compounds, easy to form hard points and reduce thermal conductivity. Manganese (Mn) can prevent the recrystallization process of aluminum alloy, increase the recrystallization temperature and remarkably refine the recrystallized grains. The refinement of recrystallized grains mainly hinders the recrystallization grain growth through the diffusion of MnAl6 compound particles. Another function of MnAl6 is to dissolve the impurity iron (Fe), forming (Fe, Mn) Al6 to reduce the harmful effects of iron. Manganese (Mn) is an important element of aluminum alloy, Al-Mn binary alloy can be added alone, and more with other alloying elements added, so most of the aluminum alloy contains manganese (Mn).
If the impurity zinc (Zn), high temperature brittleness, but with Hg (Hg) to form a strengthened HgZn2 obvious strength of the alloy. JIS is specified within 1.0%, but foreign standards to 3%, of course, here is not about the alloy composition of zinc (Zn), but in the role of impurity zinc (Zn), it has cracks in the casting Propensity.
Chromium (Cr) forms intermetallic compounds such as (CrFe) Al7 and (CrMn) Al12 in aluminum, hinders the nucleation and growth of recrystallization, has a certain strengthening effect on the alloy, improves the toughness of alloy and reduces the stress corrosion Cracking sensitivity. But will increase the sensitivity of quenching.
Only a small amount of the alloy can improve the mechanical properties, but the conductivity decreases. Peritectic reaction of Al-Ti alloy, the critical content of titanium (Ti) is about 0.15%, if there is boron can be reduced.
Phosphorus added to Al forms only one metal compound, AlP, and forms AlP up to 800 degrees Celsius below 800 degrees Celsius without dissolving. Will not break down, the crystal lattice lattice constant a 5.42A minimum distance of 2.35A atoms in the alloy when the precipitation of Si into a different kind of nuclear role and become the Si primary crystal nucleus Si refinement, the improvement of its role after several times The experimental comparison of Na and P Si8-14% of Na is more effective for more than 15% to 20%, then the effect of P is superior. For refinement such as S (sulfur), PbS (lead sulfide), are thin The role of the universal is not universal.
The eutectic temperature of tin and Al is 228.3 ℃, and Al is completely insoluble to Sn. If the content is small, the eutectic composition is in the eutectic state between the crystal grains. If the temperature is above 228.3 ℃, , The thermal cracking occurs. In the Al-Cu-based alloy, if Sn exceeds 0.1, the natural aging is retarded, and Sn is not more than 0.3 in the corrosion-resistant alloy (Al-Si-based, Al-Mg-based) Will affect the corrosion resistance, but the special phenomenon is to require a high degree of machinability, that is, must be Sn, Pb, Bi and other non-crystallizable reaction elements to promote good cutting results.
Lead and aluminum are also crystallized into non-metallic compounds by partial crystallites. Although the same amount of Pb as Pb does not melt into the solid solution, it precipitates between crystal grains at 326.8 ° C and is still a liquid-mature brittleness The reason is that when the Pb is above 1.52%, the temperature rises to the molten state, and the two phases are precipitated in the crystal grains when they are finally coagulated. Since the electrode potential is -0.55V, the corrosion resistance is serious damage. Special purpose only with Sn, Zn, Cd made of low melting point alloy, the welding of aluminum alloy welding rod.
The reaction of calcium (metal calcium) and water to produce H2 its strong chemical reaction It can be seen in the Al dissolution soup is more active than the oxidation of Mg, Al-Ca alloy corrosion resistance is poor, the casting surface will be thick In the Al-Si alloy, Si can be partially surrounded by Fe and Fe, while the other part of Si is solidified in Al, resulting in a decrease in electrical conductivity. For example, adding a little Ca to form a CaSi2 metal compound can change the SiCa solid solution The nature of the conductivity will be greatly improved, CaSi2 metal compounds will form alloy casting surface wrinkles effect in the general alloy because of the impact of casting surface so the presence of Ca, with the exception of AC8B use of other alloys were dismissed as Ca Casting, liquidity have a negative impact (AC3A, AC2A). Such as the harmful degree of ADC-12, that is more significant in more than 0.05, 0.05 is not any serious problem, and CaSi2 easily lead to the existence of Ca is more harm than good. Al-Si-Ca system into a ternary eutectic precipitation, the eutectic point of 576.5 ℃ CaSi2 which is a square black crystal.
Sodium and aluminum binary system for the partial crystal reaction 659 ℃ Al solubility of 0.003, the precipitation of Na in the crystal grain to 97.5 ℃, then dissolved into the liquid is the reason for the hot brittleness, the improvement of the silicon alloy ( Micronization) has been widely adopted by the industry, because Si is less than 5% of the crystal will not be roughened to 12% of the maximum, so to make such alloy elongation of more than 5%, no light processing can not be achieved in the past There is a method of using NaF to increase the temperature of the soup to 800 ° C to promote the grain refinement. However, there are many problems that arise when the aluminum soup exceeds 800 ° C. Although the purpose of the refining can be achieved, another aspect of the alloy Problem, so not ideal. Until Na was added to the alloy to be refined to solve these problems, but if the use of excess will occur over the phenomenon of improvement (OVER-MODIFICATION) and the alloy embrittlement, usually the amount of 0.05-0.1% is appropriate, The improvement effect is to add Na to form AlNaSi4 metal compound with a eutectic concentration of Si11.7%, Na0.017% so that when the crystal is directly prevented from diffusing Si, dendrites can not form after condensation, then refinement occurs organization. After the testification of the effectiveness of Na by this refinement mechanism, it has been proposed that rare earth metals have the same effect as above. Such as NaF, for example, the chemical formula is as follows: Al +3 NAF-> AlF3 +3 Na, so only pinch the role of the decomposition point can be flexibly applied.
Aluminum casting temperature, die-casting mold temperature and die-casting paint
(1) casting temperature: refers to the aluminum liquid poured into the pressure chamber to fill the cavity when the average temperature. Aluminum alloy casting temperature of the alloy pouring in the table below:
(2) Die-casting mold temperature: die-casting mold is both a heat exchanger and a regenerator, preheating before production to avoid quench of liquid metal, reducing the fatigue stress of die-casting mold, so in the die-casting process to maintain a certain temperature. Die-casting aluminum die-casting mold working temperature table below:
(3) Die Casting Coatings:
(a) The role of paint: to prevent sticky mold, reducing the thermal conductivity of die-casting mold to improve the forming of liquid aluminum to avoid direct liquid aluminum die-casting erosion, improve the die-casting mold filling conditions, the die-casting mold play a role in lubrication.
(b) Common paint for aluminum die casting: 5% zinc oxide + 1.2% water glass + hydrocolloid graphite (3% ~ 5%) Polyethylene + kerosene