Effects Of Al-Mn-Ti-P-Cu Master Alloy And Mg On Microstructures And Properties Of Al-25Si Alloy

With the propulsion of lightweighting for high power and turbocharged internal-combustion engine, more and more concern about Al-Si piston alloy, which is on the basis of light elements aluminum and silicon, is being earned from researchers. And hypereutectic high-Si aluminum alloy often has as high as 22wt.% to 26wt.% Si-content, compared with lower Si-content Al-Si alloy, the massive over-solution Si in the form of primary and eutectic Si in matrix, can greatly reduce density, thermal expansion coefficient and casting costs of this alloy, improve its volume stability, casting properties etc., even these advantages can have a further play with the increase of Si content. But the more Si content increasing, the thicker Si phase will be, and the more difficult modification will be. At the same time, its propertyies especially strength, ductility, wear resistance etc. will be damaged severely. Compared with other similar alloys, modification and strengthening technology for high silicon aluminum alloy is not mature enough at home and abroad, so its widespread use is limited.In the experiment, the high Si-content hypereutectic Al-25Si alloy was used as the research object. Aiming its obvious advantages and disadvantages, prepared a new type of Al-Mn-Ti-P-Cu master alloy to process it, assisted with some Mg under certain technology and appropriate heat treatment process, to improve its microstructure and properties greatly and give full play to the advantages brought by high Si-content. The mainly achievements are as follows:1) The as-cast microstructure of experimental Al-25Si alloy contained: five-star or bulk primary Si, long-needle eutectic Si and irregular a-Al, all of which were thick and nonuniform. And numerous defects such as cavities and cracks etc. distributed inner and boundaries of primary Si.2) AlP and Al3Ti in experimental Al-Mn-Ti-P-Cu master alloy can be nucleation particles of primary Si and a-Al respectively and refine them because of some similarities in crystallography. Also eutectic Si is refined through the together-growing relationship with a-Al, so this master alloy have excellent refining effects on all primary Si, eutectic Si and a-Al. Adding 4wt.% master alloy into Al-25Si melt at 880℃and keeping 20min, we can get the best modification effect. At the same time, its manufacturing and modifying processes are green. At last, a kind of ideal anti-recession alloy can be obtained.3) After modification, Adding Mg into Al-25Si melt before pouring (770℃), under the precondition that the microstructure won't become thicker, the addition of Mg can be extended to 1.5wt.% which is far more than its usual range 0.4-0.7wt.%, and also small strengthening phase Mg2Si (granular or small bone-like) can be brought into the matrix, not character-like or reticular, so solution and deposition strengthening will play maximum effects. The main reasons are the adhering-growing relationship between Mg2Si and eutectic Si and AlP being the nucleation particles of Mg2Si.4) The best heat treatment process of experimental alloy is 510℃×6h +210℃×10h underT6 condition. After heat treatment, primary Si was changed into small rounded lump with improved nodularity and granulating effect, whose average size reached 13.09μm from about 250μm. Eutectic Si became granular or short rod-like, and nearly all Mg2Si became granular. Meanwhile, a large number of fine precipitated strengthening phases appeared in the matrix. Finally, an ideal microstructure, where the relatively fine and rounded phases above distributed in a-Al matrix uniformly and dispersedly, was gained.5) While its microstructure optimization, nearly all main properties of experimental alloy were improved considerably step by step along with adding master alloy, Mg and being heat-treated. Hardness was improved from 81.6HB to 127HB; Tensile strength under room temperature and 300℃reached 293 MPa and 158MPa respectively, rised by up to 208.4% and 107.9%, and also ductile fracture characteristics on the tensile fracture surface became obvious gradually; With about a half decline of wear loss, the wear mechanism was changed from both abrasive and adhesive to only the former; Heat-resistance and corrosion-resistance also got an obvious more than 60% improvement. The main reasons for the improvements of properties above as follows. Diminution of defects and destructive effects on the matrix and fine-grain strengthening ofα-Al after modification, alloying function of Mn, Cu and lots of Mg, deposition strengthening of Mg2Si, Al2Cu and other precipitated phases after heat treatment in the matrix.