| Zn-Al alloy has excellent self-lubricating ability,excellent casting performance,low melting point,good fluidity and processing performance,etc.It is widely used as structural parts and wear-resistant parts in many fields such as aerospace,transportation and industrial production.On the other hand,Mg2Si has low density(1.99×103 Kg/m3),high elastic modulus(120 GPa),high melting point(1085℃),high hardness(4500 MN/m2),and low thermal expansion coefficient(7.5×10-6 K-1)and other advantages,it is an ideal reinforcement particle in the field of particle-reinforced metal matrix composite materials.The use of in-situ crystallization technology to generate Mg2Si in the ZA40 alloy matrix is an important development direction of zinc-based composites.However,the primary Mg2Si particles produced by the traditional casting process are coarse and irregular in shape,and the eutectic Mg2Si phase is distributed in flakes,which will destroy the continuity of the matrix and weaken the mechanical properties of the alloy.In order to improve the morphology,size and distribution of the Mg2Si phase,researchers have used a variety of methods to improve the Mg2Si phase,such as:directional solidification,rapid solidification,extrusion treatment,centrifugal casting,modification and heat treatment.Among these treatment methods,modification and heat treatment are undoubtedly the easiest,most efficient,and most cost-effective methods.Previous researchers have found that a single addition of modifier elements can refine the primary Mg2Si particles in the alloy.Further heat treatment can passivate the sharp edges and corners of the modified Mg2Si particles and improve the morphology of the eutectic Mg2Si phase.However,there are few studies on compound modification.Generally speaking,the effect of compound modification is better than that of single modification.Under the situation that the single modifiers element is fully developed,it is urgent that finding more compound modifiers for obtain better Mg2Si phase size and morphology to improve the composite material Performance.Therefore,this paper studies the effects of Ce、Sb and Ce、Bi compound modification and heat treatment on the microstructure and mechanical properties of ZA40-12 wt.%Mg2Si composites,and explores modification mechanism.The purpose of experiment is to modify ZA40-12 wt.%Mg2Si phase in Mg2Si composite materials and improve the performance of composite materials.The results of the study are summarized as follows:(1)Adding 0.2 wt.%Ce,0.5 wt.%Bi or 2 wt.%Sb to ZA40-12 wt.%Mg2Si composite material can all make the primary Mg2Si phase change from coarse large particles to regular polygonal particles.The size has also been reduced from 65 μm to about 18 μm,21 μm and 20 μm,respectively.At the same time,the existence range of large eutectic Mg2Si has been reduced,and the hardness of the material has been increased from the original 168.1 HB to 184.6 HB,181.5 HB and 177.4 HB respectively.(2)After the compound modification of ZA40-12 wt.%Mg2Si composite with 0.2 wt.%Bi and 0.1 wt.%Ce,the size of primary Mg2Si particles are decreased from 65 μm to about 18μm,the Brinell hardness increased to 186.8 HB,and the ultimate tensile strength and elongation are increased to 411 MPa and 0.78%,respectively.The mechanism of compound modification is that Ce and Bi elements are enriched on the growth surface of the primary Mg2Si particles,which hinder the growth of the particles and finally make them grow into a regular octahedral structure.(3)After the compound modification of ZA40-12 wt.%Mg2Si composite with 0.2 wt.%Ce and 1 wt.%Sb,the size of primary Mg2Si particles is decreased to about 15 μm,the Brinell hardness increased to 188.4 HB,the ultimate tensile strength and elongation have been increased from 396 MPa and 0.55%to 418 MPa and 0.93%,respectively.The dry friction coefficient of the material decreased from 0.277 to 0.192.The size of eutectic Mg2Si phase has been greatly reduced.The mechanism of Ce-Sb compound modification is that the formed CeSb phase can be used as the heterogeneous nucleation core of primary Mg2Si particles,which improves the nucleation rate of Mg2Si particles and optimizes the shape and size of the particles.(4)The heat treatment test shows that the solution treatment at 400℃×3h can improve the sharp edges and sharp corners of the primary Mg2Si particles after Ce-Sb compound modification,and eutectic Mg2Si phase change from flocculent to dots;in solution treatment of 320℃,the size of primary Mg2Si particles was reduced,but the morphology will not be improved;the solution treatment under the conditions of 420℃×2h and 440℃×1h,2h will cause the material to overburn,and destroy the material structure and performance. |
