Thermodynamic characterization of solidification and defects that occur in Mg-alloy AM60

J. Min. Metall. Sect. B-Metall. 53 (2) B (2017) 107 - 114 Jo u rn al o f Min in g an d Met allu rgy, S ect ion B: Met a llu rg y TherModynaMIc characTerIzaTIon of SolIdIfIcaTIon and defecTS ThaT occur In Mg-alloy aM60 M. Vončina *, M. Petrič, P. Mrvar, J. Medved University of Ljubljana, Faculty of Natural Science and Engineering, Department of Materials and Metallurgy, Ljubljana, Slovenia Abstract (Received 09 June 2016; accepted 06 February 2017) The AM60 alloy was thermodynamically examined using chemical analysis, thermodynamic calculation made by ThermoCalc program, “in situ” thermal analysis and differential scanning calorimetry (DSC), whereas the microstructure constituents were confirmed using optical and scanning electron microscopy (SEM). At the eutectic temperature of 437 °C the equilibrium solubility of Al in Mg is 12.6 wt. % Al. On the boundaries of the primary Mg grains the intermetallic compound of Al12Mg17 is precipitating according to the solvus line of the Mg-Al phase diagram. Solidification of the AM60 alloy has been investigated using “in situ” simple thermal analysis. The investigation of solidification has been taking place by evaluation of the cooling curves in connection with metallographic examinations, differential scanning calorimetry and thermodynamic calculations. All defects, nonmetallic inclusions and intermetallic compounds that occur in investigated AM60 alloy were identified. Keywords: AM60 Mg-alloy; Thermodynamic calculation; Inclusions; Differential scanning calorimetry (DSC). 1. Introduction Aluminium and magnesium are two most important lightweight metals used in automotive applications. The increasing need to lower the fuel economy has created a huge interest in the development of lightweight automotive structures, for aircraft parts, car parts, etc. Magnesium alloy products are 34 % lighter than aluminium and 76 % lighter than steels and show high light-to-weight ratio (UTS=36 KSI / 243 MPa). They show good corrosion resistance and good mechanical properties. The disadvantages of Mg processing occur due to technological melting processes, casting and working of the magnesium alloys, whereas all Mg alloy components-painted or unpainted-are fully recyclable. Therefore, fully understanding of solidification and cooling are needed for the optimization of the technological processes. Basic magnesium alloys MgAl (marked by ASTM as AMxy) contain from 2 to 9 wt. % Al [1-3]. The majority of magnesium elements is produced by means of sand or die casting. Despite Mg-based alloys are claimed to exhibit good casting properties, they are dramatically difficult to cast properly [4]. Beside the casting technology one of the important problems, which are connected with recycling and also re-melting, is the presence of nonmetallic inclusions as well as non-homogeneous and unbalanced chemical Corresponding author: * DOI:10.2298/JMMB160609009V composition in ingots of magnesium alloys. The latter has influence on a portion of inter-metallic compounds such as Mg17Al12 and Al4Mn. It has been found, that different input materials with chemical composition within valid standards have consequences on different technological, thermal, physical and mechanical properties. As demonstrated some authors [5] the microstructure of Mg-alloys for high pressure die casting process (HPDC) is a strong function of Al concentration such that alloys with 4–6 wt. % Al typically result in a well-defined skin region, related to the formation of the so-called externally solidified crystals (ESCs). Microstructure is also affected by local cooling rates that depend on the detailed structure of the casting and in particular local wall thickness. Shrinkage pores and gas pores are both observed in AM60 [6-8]. The most conventional way of melt purification is adding fluxes to the melt during melting, which contains chloride salts [9, 10] and/or fluoride salts [11]. It is true that some commercial fluxes can effectively remove nonmetallic inclusions from the melt of magnesium alloys, but the effects of these flux additions on decreasing the impurity elements of magnesium melt are not very satisfactory [12]. What is more, using fluxes may result in loss of alloying elements from the melt and secondary pollution through bringing in some nonmetallic impurity elements like F and Cl. M. Vončina et al. / JMM 53 (2) B (2017) 107 - 114 108 2. experimental For purpose of thermodynamic characterization of solidification and determination of defects, nonmetallic inclusions and intermetallic compounds, that occur in investigated AM60 alloy, following investigation methods were done on different AM60 alloys (from various manufacturers) in the high pressure die cast and gravity cast (blocks) state: chemical analysis, thermodynamic calculation, which was made by ThermoCalc program, “in situ” thermal analysis, differential scanning calorimetry (DSC), optical and scanning electron microscopy (SEM). The “in situ” thermal analysis were made in the laboratory. The measuring process started with casting of the investigated molten metal into the sand measuring cell. Cooling curves were plotted and the characteristic solidification temperatures were marked. Furthermore, the chemical analysis was made and the specimens for DSC and optical and SEM were prepared out of castings from “in situ” thermal analysis. DSC was made on Jupiter 449c Instrument (NETZSCH) in order to determine the influence of defects and various inclusions on the solidification characteristics. To analyze the microstructure components optic microscope OLYMPUS BX61 equipped with video camera DP70 and analySIS 5.0 program was used and to identify the defects, nonmetallic inclusions and inter-metallic compounds that occur in investigated AM60 alloy, SEM JEOL 5610 with EDS and electron microanalyzer JEOL SUPERPROBE 733 with two WDS spectrometers was used. 3. results and disscusion 3.1 Thermodynamic calculation Thermodynamic calculations (Fig.1 and 2) were made by the ThermoCalc software TCW5 using SSOL database according to the chemical composition given in Table 1. Furthermore, all the equilibrium phases and their temperature range of stability were calculated for all four investigated samples from AM60 alloy (Fig.1). Fig.1 shows that the first phase to appear is the primary crystals of aMg from 616 to 639 °C and start growing. During the cooling the solubility of the alloying elements in aMg changes, which is shown with the shading. The intermetallic compounds Al8Mn5 and Mg2Si also precipitates between liquidus and solidus temperature. The intermetallic compound Al13Fe4 forms from 515 to 548 °C, respectively, but the portion of it is very low. The intermetallic compound AlMnSi-β forms from 504 to 530 °C, respectively. At approximately 285 °C the intermetallic compound AlMn forms at the equilibrium conditions. It can be concluded that regarding the small changes in the chemical composition of AM60 alloy, although all a (...truncated)