Study On Melts Dehydrogenation Processing Of Several Magnesium And Aluminium Alloys

Magnesium alloys are recognized as the new light weight metal structural material. However, there are still many fundamental problems to be solved in large-scale applications of magnesium alloys, in which the effect of hydrogen on properties of magnesium alloys has attracted attention in recent years. In this paper, based on the work of effect of melting technical parameters on hydrogen content and hydrogen evolution behaviour during the solidification process of magnesium alloys, the effects of removing hydrogen methods such as degasser (including C2Cl6 and RJ-2 flux), ultrasonic and floating gas in the melts of the alloys on the hydrogen content in the melt, the density, mechanical properties at room and high temperature and corrosion resistance of the alloys were studied and compared with those of aluminum alloys by the optical microscopy observation, liquid-state hydrogen testing, solid-state hydrogen testing, X-ray diffraction, scanning electron microscopy, energy dispersive spectrum analysis, tensile testing and corrosion testing. These provide the basis for purifying the melts of magnesium alloys and improving their properties.The research results show that melt temperature and holding time have obvious effect on hydrogen content in the magnesium alloys melt. The reduced pressure test indicates that reducing melting temperature and holding time can reduce hydrogen content in the melt and increase the density of Mg-6.04Zn-0.62Y alloy, and the melting temperature has intenser effect than that of holding time. The microporosity amount in different positions of magnesium alloy ingots is different, and decreases from center to edge of the ingot in same section because solidification sequence is different. Compared with the aluminum alloy, the AZ91 alloy has more microporosity than 2024 alloy, but fewer microporosity than 5083 alloy.The effects of degasser on removing hydrogen efficiency for magnesium and aluminum alloys were studied. The results indicate that C2Cl6 has higher degassing efficiency both in magnesium and aluminum alloys, and can refine the grain size obviously in magnesium alloy so that the mechanical properties of the alloys can be improved significantly. The highest degassing efficiency can reach 52.7% and 41.8%, respectively for Mg-6.04Zn-0.62Y and AZ91 alloy. The lowest hydrogen content of AZ91 alloy is 11.3cm3/100g, and the corresponding tensile properties at room temperature are Rm=208MPa, Ro.2=137MPa and A=5.3% respectively. The tensile strength and yield strength increase by 32.5% and 7.9% respectively. The highest degassing efficiency can reach 47.6% and 44.8%, respectively for Al-3Mg binary and 2024 alloy, at which the hydrogen content are 0.163cm3/100g and 0.096cm3/100g respectively. The tensile properties of 2024 alloy at room temperature are Rm=288MPa, Ro.2=198MPa and A=4.7%, respectively, and the tensile strength and yield strength of 2024 alloy increase by 27.4% and 11.2% respectively. If the addition amount is considered, the addition amount for magnesium alloy is more than that for aluminum alloy. RJ-2 flux for magnesium alloy has poor removing hydrogen effect in melt. The best degassing efficiency is 29.4% in AZ91 alloy, but has larger effect on the tensile properties at room temperature, which is possibly related to removement of inclusion. The tensile strength and yield strength increase by 26.8% and 7.9% respectively.The effects of ultrasonic on removing hydrogen efficiency in magnesium and aluminum alloys were investigated. The results indicate that the application of ultrasonic in magnesium and aluminum alloys can remove more hydrogen than C2Cl6 and refine alloy structure obviously. The highest degassing efficiency can reach 53.8%?67.5% and 50.5% for Mg-3.03Ca, Mg-6.04Zn-1.17Ca and AZ91 alloys respectively. The lowest hydrogen content of AZ91 alloy can reach 9.6cm3/100g, and the corresponding tensile properties at room temperature are Rm=194MPa, Ro.2=133MPa and A=4.8% respectively. The tensile strength and yield strength increase by 27.6% and 6.4% respectively. The highest degassing efficiency can reach 51.1% and the hydrogen content reaches 0.085cm3/100g for 2024 alloy correspondingly, and the corresponding tensile properties at room temperature are Rm=288MPa, Ro.2=190MPa and A=4.9% respectively which increase by 27.4% and 6.7% respectively. The addition C2Cl6 before ultrasonic treatment (C2Cl6-ultrasonic combination method) can significantly shorten degassing time and obviously improve degassing efficiency.Removing hydrogen by floating gas (Ar gas) in AZ91 alloy has a better degassing efficiency than that of 5083 and 2024 aluminum alloy, the highest degassing efficiency is 50.7% and the hydrogen content can reach 7.3cm3/100g, and the corresponding tensile properties at room temperature are Rm=200MPa, Ro.2=128MPa and A=5.2% respectively. The tensile strength and yield strength increase by 24.2% and 5.8% respectively. However, the dehydrogenation effect becomes worse in the AZ91 made from alloy scraps which has higher non-metallic inclusion.In the four methods of C2Cl6, RJ-2 flux, ultrasonic treatment and Ar gas dehydrogenation, Ar gas dehydrogenation has the highest dehydrogenation efficiency in AZ91 alloy. However, the lowest hydrogen content is obtained by C2Cl6-ultrasonic combination and ultrasonic methods in 2024 alloy.Reducing hydrogen content of magnesium and aluminum alloys can obviously improve as-cast mechanical properties at room temperature of the alloys. When the hydrogen content in AZ91 alloy is less than about 11cm3/100g, further reducing hydrogen content has little effect on mechanical properties. Tensile properties at room temperature continuously increase with the decrease of hydrogen content for 2024 alloy. When the hydrogen content is less than about 0.12cm3/100g for 2024 alloy, the yield strength not markedly increases with the decrease of hydrogen content.Removing hydrogen treatment for magnesium alloy can markedly reduce average corrosion rate of alloy ingot. However, removing hydrogen treatment has a very small impact on self-corrosion potential (Ecorr) for AZ91 alloy. Experiment results of full immersion corrosion indicate that the average corrosion rate of AZ91 alloy reduces by 39.5%.The densities have obviously effect on the mechanical properties of magnesium and aluminum alloy at high temperature. The effect of density on tensile and yield strength of material decreases with the increase in temperature in AZ91 alloy. However, the effect of temperature on elongation is opposite to that on strength. For 5083 alloy, the effect of densities of 5083 alloy on strength is the similar to that of AZ91 alloy. However, the improvement level of elongation is more obviously caused by high densities and not relative to temperature.