The study about as cast microstructure and solidification process of Al-Mg-Si alloys
Introduction High-strength and high formability aluminum alloys are the highly demanded alloy in automobile industry, in this industry lightweight are popular alloys. The 6000 series Al-Mg-Si alloys (heat treatable) with good strength, corrosion resistance, and formability are the high applicable alloy in automobile industry. The elements of alloy have significant effect on the mechanical properties and microstructure. Silicon and magnesium are the elements of the 6000 series alloys to form Mg2Si.the formation of manganese-bearing dispersions is because of adding Manganese to alloys which will postpone the re crystallization. The impurity in alloys forms harmful intermetalic phases. In this research the effect these element on mechanical properties and microstructures will be explained. The process of solidification for Al-Mg is simple even if the magnesium has high amount in alloy the process has single α-al phase which formed after slow solidification, But in Al-mg-si (mg<wt%2) and si(<%1wt) the structure of as cast is complex, so SI addition has huge effect on solidification process. in this research based the solidification process of Al-Mg-Si will explained
The as-cast microstructure
Fig. 1 shows the as-cast microstructures of alloy II, It consisted of three phases: α-al, Mg2Si and Al15Si2(FeMn)3. They were identified by a combination of morphology, EPMA (Electron probe micro-analyzer)analysis (a) The as-cast microstructure of alloy II. (b) The eutectic compounds Mg2Si and Al15Si2(FeMn)3. (c) The ternary eutectic structure (backscattered imaging).
The as-cast microstructure The α-al phase formed the matrix of the material. Mg2Si had two shapes: one possessed a lamellar characteristic, while the other was small blocky. The Al15Si2(FeMn)3 phase also had two shapes: one was coarse blocky, the other was small blocky. The comparatively coarse lamellar Mg2Si and coarse blocky or Chinese script Al15Si2(FeMn)3 were the products of binary eutectic reactions L α-al+mg2si and L α-al+al15si2(femn)3, the small blocky Mg2Si and Al15Si2(FeMn)3 were the products of ternary eutectic L α-al+mg2si+al15si2 (FeMn)3. They formed at different solidifcation stages and were distributed at the grain boundary and interdendritic regions
The as-cast microstructure The compositions of the phases formed at different solidification stages were analysed by EPMA and are summarized in Table below. About Mg2Si, the compositions of the constituents formed from the binary eutectic reaction (the lamellar) and the ternary eutectic reaction (the smaller block) are almost the same. But their stoichiometry deviated from the nominal com-position of Mg2Si. For Al15Si2(FeMn)3, the small blocky shape resulting from the ternary eutectic reaction possessed slightly lower silicon and manganese contents than the coarse shape. Their stoichiometry deviated from the nominal composition of the Al15Si2(FeMn)3. These stoichiometry deviations resulted from the non-equilibrium eutectic reaction
Fig. 2 shows the as-cast microstructures of the alloys I and III. In general, they consisted of α-al matrix plus eutectic compounds. In alloy I, the eutectic structure Mg2Si was much less than alloys II and III, it only existed in a few interdendritic regions. Moreover, the ternary eutectic structure is hard to find, while in alloy III, the fraction and size of Mg2Si increased remarkably. From imaging analysis results, alloy II had 3.01% eutectic structure, alloy III had 4.76%. Obviously, they were influenced by both magnesium content and silicon content in the alloy. On the other hand, another intermetallic compound, coarse blocky Al15Si2(FeMn)3 was almost the same in all three alloys, both in size and in volume fraction The as-cast microstructures of (a) alloys I and (b) III.
Tensile testing during solidification For evaluation of mechanical properties in mushy zone of alloys (like strength) to methods are existing : 1. Performing the test after the specimen reached to solidus temperature,(heating up form ambient solidus temperature before the test) 2. Performing the mechanical property test after melting the sample and the cooled it down the called it re melt type
Tensile testing during solidification In this experiment solidification structure was gained. as we could see Tensile strength increase slowly by decreasing temperature from 893K to 833K, but for temperature less than 833K tensile strength increase significantly. the elongation increase slowly for the temperature less than 833K,so is obvious that tensile strength and elongation behaviour are different, this because the liquid existence along dendrites, which will effect on strength and ductility
Relation of tensile strength, breaking elongation, solid fraction and temperature
Mechanical properties; ZDT and ZST Figure above shows the relationship of tensile strength, elongation, solid fraction and temperature. At the 893 K, tensile strength is zero ZST is(definition): The minimum temperature above which strength was not Perceptible,based on definition ZST of this alloy is 893K so the temperature 893K is ZST of this alloyand solid fraction at this point is 0.69 In the same manner (Zero Ductility Temperature) was defined as the minimum temperature above which alloys showed no ductility. ZDT and ZST are important point in solidification az we could see in Fig 4 there is tmperture rang between ZDT and ZST because of no ductility and low strength thealloy is snsetive for hot tearing in this range.
Observation of macro structure at breaking section of tensile testing sample: a); T = 888K (f s =0:74), b); T = 863K( f s = 0:85), c); T = 833K (f s = 0:91) and T = 783K (f s = 1:0).
Effects of cooling rate Fig blow is presenting the relation between cooling rate and area fraction of eutectics,you observe that there is a positive relation between area fraction and cooling rate it means by increasing the cooling rate area of fraction will be increased too, the relationship could be describe based on following equation: f= 2.04 + 2.43 log (T)
Relationship between area fraction of eutectics and cooling rate
Effects of cooling rate In this equation f and T are respectively presenting the area fraction of eutecticsand cooling rate. even though the alloy composition is less than the solubility limit of A1-Mg-Si system, solid solution segregation will cause the formation eutectics by increasing the cooling rate, segregation will be more effecting on the solute atom so that base on above discussion the eutectic area of fraction will be increased by increasing the cooling rate, the higher the cooling rate will cause a better microstructure. of the alloy is much less than the solubility limit
Effects of cooling rate So that it means the fraction of ternary eutectics has the highest amount at the cooling rate of 430 C/min. So based on result from microstructure observations and image analysis,,it is obvious that cooling rate is effecting the process of solidification, for the low cooling rate the sequence of solidification process is : L α-a1 + L 1 α-a1 + (α-a1 + Mg2Si ) + L 2 α-a1 + (α-a1 + Mg2Si) +(α-a1 + Mg2Si + Si) When the cooling rates is high, the sequence od the process is: L α-a1+ L 1 α-a1 + (α-a1 + Si) + L α-a1 + (α-a1 +Si) + (α-a1 + Mg2Si + Si)
Conclusions 1. Solidification process of Al-Mg-Si alloys is complicated for the studied alloy the process was: L α-al+l1 α-al+al15si2 (FeMn)3+L2 α-al+al15si2(femn)3+(α-al+ Mg2Si )+L3Pa-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(a- Al+Mg2Si+Al15Si2(FeMn)3), and a wide solidification temperature of 75 C. 2. The amount of magnesium present in alloys has huge effete on the as cast microstructure, when the amount of magnesium is high more Mg2Si will be present,and in this type of alloy both binary eutectic structure, and ternary eutectic structure will be formed. 3. The temperature for ZST was 893k and soli fraction of 0.69,and for ZDT happened at the temperature of 883K and amount of solid fraction was0.77. 4.For studied alloys for effect of cooling rate,the results of image analysis show that relative fraction of ternary eutectics could have the highest amount at the cooling rate of 430 C/min