| It is well known that microporosity always presents in magnesium alloy castings, so that the quality and mechanical properties decrease sharply. The metallurgical quality of molten magnesium alloy, including gas and inclusion contents, affects directly on the quality of its castings. Moreover, for lack of measurement instruments of hydrogen content on shop floor during melting and refining processes, the quality of the melt is not well controlled. Furthermore, the recycling of magnesium scrap becomes a very serious problem, as it usually increases hydrogen content in Mg melt. Therefore, the objective of this paper is to develop a new instrument for hydrogen measurement in magnesium melt to study hydrogen content in different conditions during melting process, and the relation of hydrogen content and microporosity together with degassing technology. By means of these, the purity of molten magnesium and the quality of castings could be improved a lot.At first, a mathematical model suited for measurement of hydrogen content has been modified based on materials thermodynamic analysis and induction. Then a new technique for determination of hydrogen content in molten magnesium has been developed successfully based on this model with PLC controlling circuits. Its principle is based on the First Bubble method. The apparatus can quickly determine hydrogen content in 1~3min, and testing process and results are stable, and the precision level is±0.1cm3/100gMg. The reliability of the instrument is very good by the anti-jamming design of hardware and software. This technique can be widely applied in foundry shop and research work, because the testing method is simple, rapid and precise. By the way, the technique firstly provides a means of quantitative determination of hydrogen content in molten magnesium for controlling process of production in factory in our country.The systematical constitution of the apparatus is established, and it includes the selection of hardware, processing and controlling of data collected, function designing and programming of software, etc. The collected data are processed through valid checking, linear processing, digitized filtration and numerical setting, therefore truth-value and accuracy of data are insured and response speed becomes fast by setting sampling period. Furthermore, it is fully considerable for convenience of on-site application, utility of function and simpleness of operation.By utilizing this apparatus, the factors, such as melting temperature, holding time in the molten condition, protection modes, stewing time, air humidity, degassing process, that affecting on the hydrogen content in molten AZ91 alloy have been investigated. The results show that that hydrogen content in magnesium melt increases with the increase of temperature. Gas protection during melting is more efficient than flux protection, and there has less hydrogen content in the melt. the hydrogen content in Mg melt at 700℃is 13.9cm3/100g under flux protection, but 10.3cm3/100g under gas protection. With the increase of holding time hydrogen content in magnesium melt under flux protection increases slowly, but nearly no difference under gas protection. Degassing with Ar gas (decreased to 6cm3/100g~7cm3/100g) is more effective than C2Cl6 degassing. In addition, hydrogen content sharply decreases at first with the increase of holding time after degassing, then only a little decrease after 30 min. The mathematical models of hydrogen content and temperature in magnesium melt are obtained via linear regression analysis. It is very important with these results to instruct actual production.This paper has also investigated the relations between microporosity and hydrogen content, samples'thickness or degassing processes in magnesium alloy. The results show that under sand casting, the amount of microporosity becomes more, as hydrogen content in magnesium melts under flux protection is higher than that under gas protection. Microporosity increases with the increase of samples'chickness at the same hydrogen content. Degassing with Ar gas under flux protection is so effective that microporosity is much less, and coalesced pores disappear and only small pin-pores exist when sample thickness is up to 30mm. At the same time, Degassing with C2Cl6 under flux protection is also effective. In addition, with respect to metallic-mold casting, microporosity ratio is decreased with the decrease of the hydrogen content. By comparing of sample density under different cast conditions, the density of sand samples decreased with the increase of sample thickness or hydrogen content or microporosity ratio, but that of metallic-mold samples is almost the same whether the melting condition is under flux protection or under gas protection or C2Cl6 flux degassing under flux protection or Ar gas degassing under flux protection. Results of mechanical property tests indicate that tensile strength of the sample degassed with Ar gas is 21% higher and elongation is 50% higher as well.Purging effects of Ar fluxing were investigated on hydrogen content in magnesium alloy melt, and the influence of three factors of Ar flow rate, degassing time and temperature of magnesium melt on purging effect were also studied. The optimal parameters are obtained by experiment under conduction of orthogonal test design that Ar flow rate is 1 L/min～1.5L/min, degassing time 20min～25min and temperature of magnesium melt 725℃～750℃. By nonlinear regression analysis, the mathematic model of degassing rate in magnesium melt is established. In addition, the degassing mechanism of molten magnesium alloy was analyzed according to thermodynamic theory, and thermodynamic equations of Ar gas volume entering into the melt had been established. Furthermore, degassing dynamics was also analyzed.The refining process of Mg melt when magnesium scraps are used had been studied. By comparing of the variations of hydrogen contents before and after degassing, the purging of Ar fluxing is very effective. Moreover, the variation of microporosity before and after degassing was studied. Microporosity after degassing sharply decreases under sand-cast or iron cast but the porosity is still more than that only Mg alloy ingot and no scraps are used. Compared to as-cast sample with no degassing, tensile strength of the sample degassed with Ar gas is 19% higher and elongation is 63% higher as well.
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