Studies On Fracture Behavior Of Al-xSi-0.7Mg Alloy Sheets

Al-Si alloys were commonly used in aerospace industry,medical instruments and automobile industry due to their low density,low thermal expansion,high corrosion resistance and high abrasion resistance.With the increase of Si content,the size of Si particles became larger,the brittle fracture of the primary and/or eutectic Si particles made Al-Si alloys difficulty to be used as structural materials.In this paper,DC casting without chemical modification was employed to generate Al-Si-Mg ingots with fine structure,proper thermoplastic processing and heat treatments were performed to produce wrought Al-Si-Mg alloys with good plasticity and medium strength.As new kinds of Si particulate reinforced aluminum matrix composites,produced by in-situ generation and matrix can be aging treated,it had become one of the research focuses of high-performance structural materials,and its fractural behavior has caught great attention.However,attentions on fracture behavior of Al-Si-Mg alloy mainly focused on cast alloy and Al-12.7Si-0.7Mg wrought alloy sheet,the effect of Si content and Mg2Si precipitate on microstructure,mechanical propertities and fracture behavior of Al-xSi-0.7Mg alloy sheet were rarely reported.In order to enhance the massive application of the structural material,it is necessary to systematically study the effect of Si content and heat treatment on the fractural behavior of Al-Si-Mg wrought alloys.This paper systematically studied the microstructure,mechanical properties and the fracture behavior of Al-xSi-0.7Mg(x=1.44%,2.48%,4.92%,6.61%,8.81%,12.40%and 17.20%)alloy sheets after natural aging and peak aging(170?×8 h)treatments by means of metallographic analysis,scanning electron microscope(SEM)and transmission electron microscope(TEM)analysis,hardness measurement and tensile test.The effect of aging treatment(natural aging,artificial aging 170?×2 h,170?×8 h and 170?×18 h)on fracture behavior of Al-6.61Si-0.7Mg alloy sheets was also studied.The main results were shown as follows:(1)Al-(1.44?17.20)Si-0.7Mg alloy sheets contained three kinds of second-phase particles with various size:dark gray Si phase in size of 1?3 ?m,the light gray AlSiMgFe phase and the niddle-like white AlFeSi phase.Bulk primary silicon with size of 10 ?m could be found in Al-17.20Si-0.7Mg alloy sheet.With the increase of Si content,the mean diameter of the eutectic Si particles became larger,the spacing became smaller and the size kept stable.When the aging time elongated from 0 h to 18 h,the quatitive,diameter,size and spacing of the Si paticles had no significant change.(2)Si particles had apparent effects on grain refining of the Al-(1.44?17.20)Si-0.7Mg alloy sheets.As the Si content increased from 1.44%to 17.20%,the average grain size decreased from 46 ?m to 8.6 ?m.After solution and quenching treatments,the size and shape of the recrystallized grains had no obvious change with the aging time elongated from 0 h to 18 h.(3)No precipitates could be found in Al-6.61Si-0.7Mg alloy sheets after natural aging;minute quantities of precipitates could be obversed on dislocations in Al matrix after artificial aging 170?×2 h;a large amount of needle-like Mg2Si precipitates distributed along certain directions could be seen in grains after artificial aging 170?×(8 h?18 h).The size of the precipitate became larger as the aging time elongated.(4)With the increase of Si content,the macrohardness and matrix microhardness of the natural aged Al-(1.44?17.20)Si-0.7Mg alloy sheets increased gradually;the yield strength increased gradually,the ultimate tensile syrength were peaked at the autectic point and declined subquently;the elongation had a step by step decline.The macrohardness and matrix microhardness of the peak aged Al-(1.44?17.20)Si-0.7Mg alloy sheets increased obviously,the yield strength,the ultimated strength and elongation increased firstly and decreased subsequently.The Al-12.40Si-0.7Mg alloy sheet had a maximum ultimate strength and the Al-4.81Si-0.7Mg had a maximum elongation.(5)The fractural morphology of the tensile specimens of the natural aged Al-(1.44?17.20)Si-0.7Mg alloy sheets showed that the area,size and depth of equiaxed dimples were increased,the quatitives of debonded and/or fractured eutectic Si paricles increased gradually,the propogation of the cracks were diverted from the main manner of inter-paticles crack to the manner of trans-particles crack with the increase of Si content.The fractural morphology of the Al-17.20Si-0.7Mg alloy sheet showed typical brittle fracture and large amounts of fractured primary Si paricles could be seen.As the Si content increased,the length of secondary cracks was decreased,the spacing was increased and the recrystallized grain boundaries had evident hampering effects on the propogations of the secondary cracks.(6)With the increase of Si content,the fractural morphology of the tensile specimens of the peak aged Al-(1.44?17.20)Si-0.7Mg alloy sheets showed that the intergranular fracture gradually disappeared,the area,size and depth of equiaxed dimples were increased and some secondary dimples could be observed in certain area,the quatitives of debonded and/or fractured eutectic Si paricles increased gradually.The propogation of the cracks were diverted from the main manner of inter-paticles crack to the manner of trans-particles crack with the increase of Si content.The fractural morphology of the Al-17.20Si-0.7Mg alloy sheet showed typical brittle fracture and large amounts of fractured primary Si paricles could be seen.As the Si content increased,the length of secondary cracks was decreased,the spacing was increased and the recrystallized grain boundaries had evident hampering effect on the propogations of the secondary cracks.(7)The fractural morphology of the tensile specimens of the Al-6.61Si-0.7Mg alloy sheets after different heat treatments showed that the size,depth and distribution of the equiaxed dimples had no obvious differences,the propogation of the cracks were determined by the interface bonding strength between the Al matrix and eutectic Si particles.When the alloy sheets were after natural aging and artificaial aging 170?×2 h,most of the secondary cracks distributed in the Al matrix,only a tiny amounts of them propogated in the manner.of inter-paticles crack and/or trans-particles crack.Most of the eutectic Si particles in the dimples were debonded,the rest were fractured.With the artificial aging time elongated to 18 h,the length of the secondary cracks decreased evidently,the spacing increased obviously,the propogation of the cracks were diverted from the main manner of inter-paticles crack to the manner of trans-particles crack,the fractured eutectic Si particles in the dimples were increased.