Study On High Casting Speed Direct Chill Casting Of A390 Alloy Thin Walled Hollow Billet

Due to good performance,such as corrosion resistance,abrasion resistance,low thermal expansion coefficient and other characteristics for A390 aluminum alloy,it is widely used in the automotive industry,especially in the piston industry.However,the main hard phase of the primary Si in the cast structure is too large and the distribution is extremely uneven,which seriously damages its comprehensive performance.Based on the domestic and overseas achievements of improving the uniformity and refining of primary Si in hypereutectic Al-Si alloy by various modifiers,the effect of single modifier and complex modifier on primary Si in A390 alloy was studied by water-cooled metal mould which has a equivalent solidification rate to the direct chill casting process,in turn,the optimal modifier was achieved for DC casting of A390 alloy.Combined with numerical simulation and casting experiment,the effect of casting speed on the process of A390 alloy thin-wall hollow billet DC casting and the size and distribution of primary Si was studied.The conclusions obtained are as follows:(1)Among different kinds of single modifier for A390 alloy,the smallest primary Si average size(20.59μm)was achieved with the addition of Al-P modifier,however,no refinement effect on the eutectic Si.With addition of La-Ce rare earth,although eutectic Si was significantly modified as the morphology changed from large lamellar to small rounded,whereas,the primary Si were roughened and with holes inside(70.85μm)and even larger than that in unmodified alloy(41.07pm).(2)Complex modifiers such as P-La-Ce,P-Sr,P-Y and P-Y-B have modification effect on primary Si but show different weaken extent on the refinement effect of P on primary Si.The refinement effect of P-La-Ce is almost equivalent to the single P modifier,in addition,eutectic Si were also modified with decreased length-width ratio.The addition sequence of P and Sr in P-Sr binary modifier results in different effects on Si particles,when Sr was added 10 min ahead of P,the eutectic Si particles were not refined.On contrary,when P was added 10min ahead of Sr,the eutectic Si were significantly modified but a portion of primary Si particles got coarsen.P-Y complex modifier weakened the refinement effect of P to some extent.The change of addition amount of AlTiB in P-B-Y complex modifier results in different size of primary Si,when the addition amount of AlTiB was 0.03 wt%,the minimum primary Si size(23.52μm)was obtained.(3)The results of numerical simulation showed that:During DC casting of A390 Alloy thin-wall hollow billet with a size of Φ106mm/Φ60mm,as the casting speed increased from 180mm/min to 270mm/min,the sump depth increased gradually.When the casting speed exceeds 210 mm/min,the inner surface of the thin-wall tube was partially remelted.(4)The results of DC casting experiment showed that:Horizontal crack emerged at the inner surface with the casting speed of 240 mm/min and 270 mm/min during DC casting process,and this is attributed to the thin solidification shell at the inner wall teared.At the same time,the distribution of primary Si got extremely worse.As a result,210mm/min was considered as the optimum casting speed for A390 alloy thin-wall hollow billet DC casting.With P-La-Ce modified and casting speed of 210mm/min were chosen for DC casting of thin-wall hollow billet,fine primary Si(23.01-23.26μm)distributed evenly(area fraction of 8.68 to 9.57%)in the hollow billet.