Study On The Microstructural And Elevated Temperature Properties Of Cast Al-Si-Cu-Ni-Mg Alloys

As one of the light metallic structural materials,cast Al alloys have many advantages,such as low density,high specific strength and stiffness,good machining capability,superb castability and easy to recycle,etc.,which is in line with the current social demand for lightweight material and green environmental protection.Recently,the development of the industry and automotive engines efficiency bring forward higher request for the cast Al alloys and greater demands for those with the elevated-temperature properties.Therefore,it is necessary to exploit the new heat resistant cast aluminium alloys used for engine cylinder blocks and pistons above250?.Al-12Si-4Cu-2Ni-0.8Mg?M174??wt.%?alloy was prepared in this study.Effects of alloy elements and heat treatment on the microstructure and properties were systematically investigated by computer data collection system,inductively coupled plasma analyzer?ICP?,optical microscopy?OM?,image analysis software,X-ray diffractometer?XRD?,differential scanning calorimetry?DSC?,dynamic mechanical analyzer?DMA?,dilatometer?DIL?,scanning electron microscopy?SEM?with energy dispersive X-ray analysis?EDX?and transmission electron microscopy?TEM?etc..Through experimental such as hardness,room and elevated temperature tensile test,thermophysical properties,compression creep resistance and friction and wear properties,gravity-cast M174?M174-xSr?x=0.006,0.02,0.03,0.04??M174-?0,0.1?Ti-?0,0.02?Sr?M174-xGd?x=0.1,0.2,0.5,1.0,3.0??M174-xY?x=0.1,0.2,1.0??M174-xNd?x=0.2,0.4,1.0,3.0??M174-xSm?x=1.0,3.0?and M174-xCe?x=1.0,3.0??wt.%?alloys were investigated and the optimal alloy was designed——M174-0.2Gd alloy.The effect of heat treatment on the microstructure and mechanical properties was studied and the optimal heat treatment parameters was achieved.The time-varying reduce mechanism,the creep deformation mechanism and friction mechanism of Al-Si-Cu-Ni-Mg-Gd alloy were studied in detail.The purpose of the present work is to provide theoretical and practical results for the development of high-performance cast heat resistance aluminium alloys.The main conclusions can be summarized as follows:1.The as-cast M174 alloy contains?-Al,primary Si,eutectic Si,?-Al₃CuNi??-Al₇Cu₄Ni?Q-Al₅Cu₂Mg₈Si₆?Al₁₁?MnFeNiCu?₄Si and?-Al₂Cu.The intermetallics formed in the process of alloy solidification are,in order,primary Si,?-Al,eutectic Si,?-Al₃CuNi,?-Al₇Cu₄Ni,Q-Al₅Cu₂Mg₈Si₆ and?-Al₂Cu.2.Effects of Sr content?0,0.006,0.02,0.03,0.04?on the microstructure and tensile properties at room temperature of as-cast M174 alloy were investigated.After Sr additions,the sizes of both the entectic Si and Al₁₁?MnFeNiCu?₄Si are decreased and the elongation to failure at room temperature of the alloy is increased.As the content of Sr in the alloy is increased,the volume fraction of Al₁₁?MnFeNiCu?₄Si is decreased,and the secondary dendrite arm spacing is decreased gradually and then increased.When the Sr content reachs 0.02 wt.%,the secondary dendrite arm spacing is about 13?m and the alloy exhibits the optimal tensile properties at room temperature.3.Effects of Al-10Sr or/and Al-5Ti-B master alloys on the microstructure and ambient and elevated temperature properties of as-cast M174 were studied.The B that reacts with Sr to form SrB₆ can act as the nucleating substrates for?-Al.It is found that adding Al-5Ti-1B into alloy can improve tensile properties at both ambient and elevated temperature.However,owing to the modification of eutectic Si,the tensile properties of the alloys is enhanced at room temperature and then slightly degraded at elevated temperature.4.Effects of heat treatment?solution treatment and ageing treatment?on the microstructure and mechanical properties of M174-xGd?x=0,0.1,0.2??wt.%?alloys were investigated.The optimum solid-solution condition of alloy is 500?×2h+540?×3h.After solution treatment,the morphologies of primary and eutectic Si gradually change into a spherical shape,and both the?-Al₂Cu and Q-Al₅Cu₂Mg₈Si₆ phases are dissolved in the matrix.The optimum ageing treatment of alloy is 175?×5h.5.It is found that the rare element of Gd can influence the microstructure of as-cast,cast-T4 and cast-T6 M174 alloy:1)In as-cast condition,the Al₃CuGd phase forms and the morphologu of Gd-rich phases gradually develops into needle-like.With the increase of the Gd content??0.5 wt.%?,the sizes of Al₁₁?MnFeNiCu?₄Si and?-Al₃CuNi phases decreased gradually.2)In cast-T4 condition,with the increase of the content of Gd in the alloy??0.2 wt.%?,skeleton-like?-Al₃CuNi gradually develops into two parts:one was?-Al₃CuNi which has the framework shape;the other was broken intermetallics which include Al₃CuNi?5¹0?m?and Gd-rich??5?m?phases.3)In cast-T6 condition,Gd can inhibit the precipitation of?-Al₂Cu in the process of ageing treatment.There are mainly two types of precipitated phases in the microstructure of peak ageing M174-0.2Gd alloy:one was Q-Al₅Cu₂Mg₈Si₆?diameter?500nm?which have spherical shape in the interior of the grains;the other was Al₃CuGd?a=b=4.157?,c=10.651?,size?200nm?distributed in the grain boundary.6.The research was focused on the tensile properties of as-cast,cast-T4 and cast-T6 M174-xGd?x=0,0.1,0.2??wt.%?alloy at room and elevated temperature.It is found that the tensile properties of alloy at ambient and elevated temperature is increased at the first stage and then decreased.It turns out that adding 0.1 wt.%Gd into alloys can improve tensile properties when the temperature is below 200?while adding 0.2 wt.%Gd into alloys can improve ultimate tensile properties when the temperature exceeds 300?.7.Effects of Gd content on the physical properties of as-cast and cast-T6 M174alloys at ambient and elevated temperature were investigated.At the same heat treatment condition,as the content of Gd in the alloy is increased,the coefficient of thermal expansion is decreased,and the elastic modulus is increased.8.The time-varying reduce mechanism of cast-T6 M174-0.2Gd after thermal exposure was investigated systematically by TEM.The secondary precipitates and coalescence of?-Al₂Cu and Q-Al₅Cu₂Mg₈Si₆ phases were the main reason resulting in the gradual reduction in the tensile properties at thermal exposure experiment.When the thermal exposure temperature was 250?,the?-Al₂Cu phase precipitates and then grows up to 100²50nm or even 500nm in the local area.Furthermore,the distance between the adjoining?-Al₂Cu is increased from 20¹00nm to?200nm,and the spot shape Q-Al₅Cu₂Mg₈Si₆ precipitates?1² nm?forms parallel to<100>?_-Al,when the thermal exposure time increases from 0h to 200h.9.The research was focused on the compression creep properties of cast-T6M174-0.2Gd?wt.%?alloy at high temperature.The Al₃CuGd phases pin up the grain boundaries and impede their movement.At low stress???50MPa?and low temperature?T?250??,the creep stress exponent values range from 1.3 to 2.5,and the average activation energy of 32.0⁷4.6kJ/mol implies that the diffusion or grain boundary slip is the creep mechanism.At high stress???80MPa?and high temperature?T?250??,the stress exponent n is from 3.7 to 5.6 and the activation energy of deformation Q=18.0⁷9.0kJ/mol indicates that the grain boundary slip and dislocation climb are the creep mechanism.The cast-T6 M174-0.2Gd alloy can be used below 300?and under 80 MPa.10.Effects of Gd content on the wear properties of as-cast and cast-T6 M174alloys at ambient and elevated temperature were investigated.The volume abrasion rate and dynamic wear coefficient of alloys decline with the increase of the Gd content.The main wear mechanism of M174 alloy are abrasire wear and delamination wear,and the main wear mechanism of M174-0.2Gd alloy is delamination wear under high temperature or high load.11.Under the oil lubrication,the volume abrasion rate was much lower than that under the dry friction due to the oil film between the counterfaces.Moreover,the dynamic wear coefficient in oil lubricated state is only about 1/5 of that in dry sliding.The intermetallics that are broken and dropped off may be responsible for the main worn surface morphology in oil lubricated state of alloys.