Study On SiC The Coating For Melting Al-Li Alloys

This work is a part of the key project of the National Natural Science foundation of China-Fundamental research on processing of the purity,fine grain and homogeneous ingots of Al-Li alloys(U1608252,to develop new coatings for melting Al-Li alloys for meeting the need of development of Al-Li alloys.SiC was selected as aggregate.Na2B4O7 and phosphate were selected as adhesive.Low carbon steel,stainless steel,graphite and high alumina brick were basic materials.The composition,sintering method and ability of anti-thermal fatigue,corrosion behavior in Al-(1.5-2.5)wt%Li alloys were worked out.The main results are as followings;1.Fe-Al compounds formed at the interface of Al-(1.5-2.5)wt.%Li alloys and low carbon steel or stainless steel when they were hot dipped in the liquid alloys.Li promoted corrosion of low carbon steel and stainless steel.For stainless steel,for 0.5h,thickness of compound without Li is 5 ?m,increase to 28?m with 2.5wt.%Li.For 1h,thickness of compound without Li is 8 ?m,increase to 35?m with 2.5wt.%Li.For steel,for 0.5h,thickness of compound without Li is 3 ?m,increase to 120?m with 2.5wt.%Li.For 1h,thickness of compound without Li is 5 ?m,increase to 165 ?m with 2.5wt.%Li.Thickness of compound layer and needlelike compounds containing Fe ang Ni increases with increase in Li content and holding time.2.No corrosion takes place when the graphite and high alumina brick hot dipped in liquid pure Al.Corrosion occurs when they hot dipped in liquid Al-Li alloys.Corrosion intensified with increase in Li content and holding time.Some particles falled off from coatings in severe conditions.The ability anti-corrosion of high alumina bricks is better than that of graphite.3.If Na2B4O7 was selected as adhesive and SiC as aggregate coating is easy tofolled off from steel and stainless steel when cooled after sintering.Coating layers were bonded tightly with graphite and high alumina brick coated by Na2B4O7/SiC,which has high anti-thermal fatigue.However,Corrosion taked place when they were hot dipped in liquid Al and Al-Li alloys because of the low melting temperature of Na2B4O7 which means that the ability of anti-corrosion of adhesive is the key to process the coating with high quality.4.If the phosphate was chosen as adhesive and SiC as aggregate good coating can be processed when they were coated on stainless steel.The firm bounding between the steel and coating and high ability of anti-thermal fatigue can be obtained.No corrosion occurred and the interfaces of steel/coating/alloy were intact and plain,which means that this coating is perfect.5.The dilatation coefficient is the key parameter for bounding of coating and matrix.One of principles to design the coating is that dilatation coefficient of coating approximates that of matrix.