Study On Microstructure Control Of Al/Mg₂Si Materials Under Alloying And Physical Fields

Al/Mg₂Si composites have many advantages,such as simple preparation method,high toughness,high specific strength and so on,which have great potential market and wide application.Fine and uniform distribution of primary Mg₂Si phase is beneficial to the improvement of mechanical properties of the materials,while segregation distribution of primary Mg₂Si phase is advantageous to preparation of functionally graded materials.In this paper,Al-20Mg₂Si alloy was taken as the research object,primary Mg₂Si phase was refined by alloying and ultrasonic treatment,Distribution of primary Mg₂Si phase was controlled by pulsed magnetic field to prepare gradient materials.The solidification microstructure and mechanical properties of the alloy were further improved by composite treatment of alloying and external field.Effects of P and La alloying and pulsed magnetic field on the solidification microstructure and mechanical properties of the alloy were studied.On the basis of this,effects of composite treatment of pulsed magnetic field and alloying,composite treatment of ultrasonic and alloying on morphology,size and distribution of primary Mg₂Si phase in the alloy,and mechanical properties of the alloy were studied.The main conclusions are as follows.1)The solidification microstructure and mechanical properties of Al-20Mg₂Si alloy have been improved under the condition of P and La and its complex modification.In the range of 0⁰.75%,with the increase of P or La content,size of the primary Mg₂Si phase in the alloy decreases at first and then increases,and the turning point is 0.5%.Under the condition of complex modification,the size of primary Mg₂Si phase is the smallest,which is 63.7%smaller than that of unmodified sample.Change of mechanical properties of the alloy is basically consistent with the size change of the primary Mg₂Si phase.The mechanical properties of the alloy are optimized under the condition of composite modification.The tensile strength and elongation of the alloy are increased by 40.5%and 153%than those of the unmodified sample,respectively.2)Under the action of high voltage pulsed magnetic field,the primary Mg₂Si phase in the alloy exhibits coarsening,segregation and gradient distribution.Under most experimental conditions,the volume fraction of primary Mg₂Si phase increases gradually from the center to the edge of the sample,and the gradient material with continuous change of primary Mg₂Si phase can be prepared.Only when the pouring temperature is 760℃,the volume fraction of primary Mg₂Si phase in the edge region of the sample decreases.When the pulse voltage is 100V,the pulse frequency is 10Hz and the mold temperature is 400℃,there is no primary Mg₂Si phase in the core of the sample,and the primary Mg₂Si phase is distributed mainly in the edge of the sample.The surface reinforced in-situ gradient composites can be prepared.In the range of 0³00V,with the increase of pulse voltage,the equivalent circle diameter of the primary Mg₂Si phase increases at first and then decreases,and the turning point is200V.In the range of 1¹0Hz,the change of pulse frequency has no obvious effect on the equivalent circle diameter of the primary Mg₂Si phase.When the pouring temperature is in the range of 670⁷60℃or the mold temperature is in the range of200⁶00℃,with the increase of pouring temperature or the decrease of mold temperature,the equivalent circle diameter of primary Mg₂Si phase decreases.The hardness of the sample also shows gradient change.The variation of hardness at different positions of the same alloy is consistent basically with the change of volume fraction of primary Mg₂Si phase.3)Low voltage pulsed magnetic field can control the average distribution width of primary Mg₂Si phase and Al/Mg₂Si surface reinforced composites can be prepared.When the pulse voltage is in the range of 0¹75V,the pulse frequency is in the range of 1¹0Hz or the mold temperature is in the range of 200⁶00℃,the equivalent circle diameter of the primary Mg₂Si phase increases with the increase of the pulse voltage,pulse frequency or die temperature.In the range of 670⁷60℃,with the increase of pouring temperature,the equivalent circle diameter of primary Mg₂Si phase decreases gradually.With the increase of pulse voltage or pouring temperature,the average distribution width of primary Mg₂Si phase decreases at first and then increases,and the turning point is 105V and 730℃,respectively.The average distribution width of primary Mg₂Si phase decreases with the increase of pulse frequency or die temperature.The hardness of different regions of the alloy is not consistent,the hardness at the edge of the alloy is higher than that in the central region,and the hardness of the alloy in different regions is mainly determined by the volume fraction of the primary Mg₂Si phase in the region.4)With composite treatment of pulsed magnetic field and alloying,the addition of P and La can refine the primary Mg₂Si phase in the alloy,but at the same time inhibits movement of the primary Mg₂Si phase,which make the volume fraction of the primary Mg₂Si phase present continuous gradient distribution in the radial direction of the sample.From the center to the edge of the sample,there is no obvious segregation distribution.With compound treatment of high voltage pulsed magnetic field and alloying,in the range of 0³00V,with the increase of pulse voltage,the equivalent circle diameter of primary Mg₂Si phase increases at first and then decreases,and the turning point is 200V.In the range of 1¹0Hz,the change of pulse frequency has no obvious effect on the equivalent circle diameter of primary Mg₂Si phase.In the range of 200⁶00℃,with the increase of mold temperature,the equivalent circle diameter of primary Mg₂Si phase increases gradually.Under the condition of composite treatment of low voltage pulsed magnetic field and alloying,when pulse voltage is in the range of 70¹75V or pulse frequency is in the range of1¹0Hz,with the increase of pulse voltage or pulse frequency,the equivalent circle diameter of primary Mg₂Si phase increases.In the range of 200⁶00,with the increases of mold temperature,the equivalent circle diameter of the primary Mg₂Si phase increases.The hardness of the alloy increase gradually from the center to the edge in the radial direction of the sample,and the variation of hardness and wear resistance at different positions isconsistent basically with the change of volume fraction of primary Mg₂Si phase.5)The solidification microstructure and mechanical properties of the alloy are improved significantly with combined treatment of ultrasonic and alloying,and the effect is better than single ultrasonic treatment or alloying treatment.Compared with the untreated sample,the size of the primary Mg₂Si phase decreases 68%,the tensile strength increases 62.3%,and the elongation increases 47%.With the increase of ultrasonic power,the size of primary Mg₂Si phase decreases gradually and the mechanical properties of the alloy improve gradually.With the increase of ultrasonic treatment time,the size of primary Mg₂Si phase decreases at first and then increases,and the turning point is 730℃,but the mechanical properties of the alloy increase gradually.With the increase of treating temperature,the size of primary Mg₂Si phase decreases at first and then increases,and the turning point is 730℃.The change law of mechanical properties corresponds to change of primary Mg₂Si phase.