Aluminum bronze is a copper alloy with aluminum as the main alloy element. There are two kinds of aluminum bronze, one is binary aluminum bronze, the other is multi-aluminum bronze, the alloys containing less than 7.4% AL have single-phase a solid solution structure at all temperatures. Extremely Malleable and easy to work with. In the two-phase structure, the β → a phase transformation process in the alloy can not be completed completely under the cooling condition of production, and some β solid solution is still retained, subsequent eutectoid decomposition occurs into (a + Γ2) eutectoid, in which Γ2 is a hard brittle, which increases the hardness, strength and decreases the plasticity. When the alloy with 9.4% ~ 15.6% AL content is slowly cooled to 565 °c, the β → a + y 2 transformation will occur and the eutectoid structure will be formed. The microstructure of (a + Γ2) in aluminum bronze is similar to that of pearlite in annealed steel. However, the EUTECTOID decomposition of β phase is relatively slow, and when it is cooled rapidly, it will not decompose in time, resulting in metastable structure. When al-bronze containing 8.5% ~ 11% AL is slowly cooled, β phase decomposes into eutectoid (A + 2) and forms continuous, chain-like coarse Γ2 particles, which leads to embrittlement of the alloy. This phenomenon is called “Self-annealing”, this phenomenon should be avoided in production.

The properties of aluminum bronze are related to aluminum content. It has high mechanical properties, corrosion resistance, wear resistance, cold resistance, no spark during impact, good fluidity and little segregation tendency. Its disadvantage is that it is easy to form columnar grain during solidification, easy to be polluted by alumina film, difficult to be welded, and not stable enough in superheated steam. The addition of alloying elements such as FE, Ni and MN to the binary aluminum bronze can further improve the properties of the alloy and expand its application range. Aluminum in aluminum bronze is a strong reductant, it is easy to be oxidized in the melting process, forming A203 with high melting point, which will form continuous AL202 film on the surface of the melt, it can protect the melt from further oxidation without any covering agent, and if it is not handled properly, it will be difficult to remove and become the source of slag inclusion defects in castings. The elements such as phosphorus, arsenic, Bismuth, zinc and lead, which are disadvantageous to the improvement of melting and casting properties, should be restricted.

Fe and MN are high melting point elements. They should be added into the furnace in the form of Cu-Fe (20% ー30% Fe) , Cu-Mn (25% ー35% MN) , Cu-Al (50% AI) , Cu-Fe-Al, Cu-Fe-Mn, Al-Fe and so on, in order to avoid a long time to high temperature smelting caused by a large number of inhalation, oxidation burning loss and energy waste. 25% ~ 75% of the alloy scrap is usually used in smelting aluminum bronze. Detritus containing oil, emulsion and moisture should be dried or remelted before being put into the furnace. The melting temperature of each alloy element in aluminum bronze is iron, manganese, nickel, copper and aluminum from high to low. Due to the strong exothermic effect of aluminum and copper, some copper material (commonly known as cooling material) can be left in advance, and then added into the furnace at the later stage of smelting. In this way, the melt will not overheat due to the exothermic effect associated with the addition of aluminum, and energy can be saved. Iron does not melt easily in copper, and manganese facilitates this process. Therefore, manganese should be added to the melt before iron. NIO and Nio. CU2O and other inclusions, should pay attention to avoid the oxidation of the melt, if necessary, can be in copper, nickel melting first deoxidation. It is suitable to smelt aluminum bronze in medium and power frequency coreless induction furnace. It is not suitable to melt in the mains frequency cored induction furnace. The basic reason is that the slag composed of A203 or A203 and other oxides is easily stuck on the wall of the channel, so that the effective cross section of the channel is continuously reduced until the whole section of the channel is blocked by the slag.

The melting atmosphere of induction furnace is easy to control, and the melting speed is fast, which is helpful to reduce or avoid the danger of hydrogen absorption and formation of Al2O3 which is difficult to be discharged from the melt. Although very small Al2O3 may have the effect of refining crystallization, the greater harm is that Al2o3 may become the root of lamellar fracture defect of processed products. It is necessary to add flux for treatment, flux has better wetting ability to A203, can effectively carry out slag removal and thus reduce slag quantity. In fact, in medium and power frequency coreless induction furnace melting, as long as the charge is good, depending on the surface of the molten pool formed naturally AL203 film, can also prevent the melt from further oxidation and slagging. When aluminum bronze is smelted in a gas furnace, the molten metal is often treated with nitrogen or even gas before pouring to remove the hydrogen absorbed by the molten metal during smelting. The amount of nitrogen injected depends on the mass of the melt. For example, the amount of nitrogen injected is 20L/100kg melt. The melting temperature of aluminum bronze generally does not exceed 1200 °C. 

Source: Copper Alloy Casting

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