| In this paper, the synthetical principle of Al-P-Si master alloys and their modification treatment technique have been investigated to achieve stable modification effect and avoid environmental pollutions in ingot metallurgy Al-Si piston alloys, using optical microscopy (OM) and electron probe micro-analyzer (EPMA), etc. This will offer more effective modification routes and technical support for the piston industry.A high efficient Al-P-Si master alloy has been developed by " a two-step process", in which Cu-Si-P master alloys are firstly prepared as an intermedia containing P and then added into Al melts, so that a Al-P-Si master alloy with P content of 3.5% can be produced under industrial condition. There are large numbers of micron dimension AlP compounds, which act as heterogeneous nuclei of primary Si, evenly distributed in matrix alloy.The excellent modification efficiency on Al-Si piston alloys could be obtained through the addition of Al-P-Si master alloys. It is considered that there are two modification mechanisms for eutectic Al-Si alloys treated by Al-P-Si master alloy, one is heterogeneous nucleation and refining of primary Si phase by AlP particles, the other is P atoms dissolved in Si phases and modify the structure and morphology of Si phases. The modification for the hypereutectic Al-Si alloy depends on heterogeneous nucleation mechanism induced by AlP particle.It has been found that Ca in the Al-Si alloy melts can make phosphorus modification inefficiency and even Na modification efficiency appears, the abnormal phenomena is due to the formation of Ca_xP_ySi_x compounds which can not act as the heterogeneous nucleating sites of primary Si grains during solidification.The efficiency of P modification could be recovered through refinement treatment by C2C16 because C2C16 can remove most of the Ca element. It is difficult to present phosphorus modification effect when the Ca content in the ZL109 alloy exceeds 100 ppm.With thedecrease of Ca content from 100 ppm to 12 ppm, the average primary silicon particles size is reduced to 16.2 um from 31.7 urn. When the content of Ca in the melt is below 50 ppm, trace residual P with 57 ppm content can lead to excellent modification effect.During the smelting process, the addition sequence of Mg would influence the P modification effect. The experimental results show that when 1.2wt.% Mg is added to the near-eutectic Al-Si alloy melt modified by phosphorus, the P modification efficiency is weakened, even similarly producing sodium modification characteristic due to the formation of compounds MgxSiyPz which is more stable than A1P phases in the melt. In the case of adding 1.2wt.% Mg into the melts before P modification treatment, Mg will combine with some elements to form compounds, such as Mg2Si and AUMg5Cu4Si4, which do not combine with P and have much less adverse effect on P modification effect.The room temperature ultimate tensile strength (UTS) of ZL109 alloy increased about 50 MPa after addition of 0.15wt.% Al-5Ti-lB master alloy into the melt. The results showed that the eutectic colony can be refined. On the other hand, Ti element or TiB2 particles react with alloying agent in the Al-Si piston alloy and form the strengthening phases distributed in the matrix alloy, which lead to the increase of UTS. When the quantity added to the melt is more than 0.15wt.%, the primary Si phases coarsen distinctly, and the range of the UTS that could be promoted decreases.Under commercial applications conditions, modification of Al-Si alloys by Al-P-Si master alloy shows various advantages, such as the high modification efficiency, high modification stability and avoiding of pollution. Furthermore, the use of Al-P-Si can reduce the loss of aluminium and simplify operative technolgy.At present, the Al-P-Si master alloys have been widely applied in more than 40 piston manufacturing enterprises countrywide, such as Shandong Binzhou Bohai Piston Co.,Ltd. etc. It has produced the obvious economic and social benefit. |
PDF Full Text Request