Growth Mechanism And Mechanical Properties Of The Second Phase In Al-Si And Al-Fe Casting Alloys

Al-Si alloys are widely used in automobile bodies and electronic product casings due to their high specific strength,good casting properties,high corrosion resistance and wear resistance.The common Fe,Mn,Mg,and Cu elements in Al-Si alloys mostly exist in the form of intermetallic compounds,and their types,sizes,and morphology have important effects on the mechanical properties of Al-Si alloys.The establishment of the mechanical model between the mechanical properties and microstructure of aluminum alloys can greatly improve the efficiency of alloy design.The mechanical properties of intermetallics are the key input parameters of the mechanical model.In this work,the composition of the alloy is calculated and designed based on the phase diagram,and then the second-phase ultra-large particles are prepared by slow directional solidification and solution growth method,which makes the mechanical properties detection simple and accurate.In addition,the growth transition mechanism of Fe-rich phase with high elastic modulus has also been intensively studied in this work.Based on the above studies,the development of new heat-resistant aluminum alloys was carried out.The main conclusions obtained are summarized as follows:（1）Ultra-largeα-Al Fe Si,β-Al Fe Si,Al₃Fe Si₂,Mg₂Si,Al₆（Mn,Fe）and Al₂Cu particles up to 3 mm in size were obtained.These single particles visible to the naked eye greatly facilitated the detection of the mechanical properties of the second phase.And improve the detection accuracy.Among them,the experimental data of the mechanical properties ofβ-Al Fe Si,Al₃Fe Si₂,and Al₆（Mn,Fe）were obtained for the first time in this work.The method for preparing large particles provided in this work also has important reference significance for the study of other properties of single phase.（2）It is found that the growth ofβ-Al Fe Si in the aluminum alloy is the layer-by-layer growth of the（0 0 1）crystal plane,and the growth rate along the[1 0 0]and[0 1 0]directions is much faster than the[0 0 1]direction.It was found that flakyβ-Al Fe Si tends to exist in giant fan-shaped clusters.This phenomenon is due to the weak atomic bonding force between the（0 0 1）_{β-Al Fe Si}planes,and under the action of thermal stress during the solidification process,cracking occurs between different sheets of（0 0 1）_{β-Al Fe Si},resulting in newβ-Al Fe Si The continuous splitting makes the fully grownβ-Al Fe Si form huge fan-shaped clusters.In addition,it is determined thatβ-Al Fe Si will grow along the direction of heat flow,and the preferred orientation of growth along the direction of heat flow is[1 0 0]/[0 1 0]crystal orientation.（3）Increasing the Fe/Si ratio can promote the transformation ofβ-Al Fe Si toα-Al Fe Si,and increasing the cooling rate can promote the transformation of primaryα-Al Fe Si and Al₁₃Fe₄phases to eutecticα-Al Fe Si.Under the composition of Al-5 wt.%Fe-3 wt.%Si,the Fe-rich phase in the alloy can be completely transformed into a fine eutecticα-Al Fe Si phase by water cooling.Compared with Al-Si alloy,Al-Fe-Si alloy with this structure has extremely high thermal stability of structure,and its raw material richness is extremely high,which is an attractive new heat-resistant aluminum alloy.（4）The variation law of the microstructure and mechanical properties of Al-6 wt.%（Fe,Mn）alloy with Fe/Mn ratio was explored.When the Fe/Mn ratio is 2:1,the alloy has the smallest microstructure and the highest hardness.In Mn-containing alloys,with the increase of Mn content,the defects such as stacking faults in the alloy increase,and the strength of the alloy matrix decreases.The cross-section of Al₆（Fe,Mn）fibers has gradually improved coherence with the aluminum matrix,and the distortion caused by the interface is reduced,which increases the easy movement space of dislocations.Therefore,the overall mechanical properties are low.（5）Substituting Fe with 1 wt.%Mn in Al-3 wt.%Fe alloy can completely suppress the coarse Al₁₃Fe₄.However,the addition of Mn resulted in serious coarsening of Al₆（Fe,Mn）fibers at high temperature,while the addition of a small amount of Zr could inhibit the thermal exposure coarsening of Al₆（Fe,Mn）fibers.The main mechanism is that Zr forms a layer of enrichment on the fiber surface,which inhibits the diffusion of Mn to the aluminum matrix.However,the addition of Zr leads to the formation of coarse Al₆（Fe,Mn）particles.Using the high cooling rate method of die casting,Al-2Fe-1Mn-0.3Zr alloy with completely refined microstructure can be obtained.Compared with Al-3Fe,its yield strength and tensile strength are increased by 132%and74%respectively.,reaching 143.2 MPa and 218.3 MPa,respectively.