Research On Low Hot-tearing And High Fluidity Aluminum Alloy With(La+Yb) Mixed Rare Earth And Die Casting

The morphology and size of?-Al and eutectic silicon phase in Al-Si alloy affect the performance of the alloy.Furthermore,the iron phase in the aluminum alloy will also affect the mechanical properties of the alloy.In addition,the fluidity and hot tearing resistance of the alloy are two very important parameters in the casting performance,which is particularly important for the molding of complex thin-walled parts.In this dissertation,rare earth aluminum alloys were studied.The mixed rare earth?La+Yb?was added into the cast aluminum alloy ADC12 and A356.2 alloy matrix.The modification effect of rare earth on the alloy and the fluidity and thermal cracking properties of the alloy were studied.A rare earth aluminum alloy with low thermal cracking and high fluidity was designed.At the same time,the thermodynamic calculation and nucleation kinetics analysis of the aluminum-rare earth binary and ternary alloys involved in this paper were performed.Finally,the rare earth aluminum alloys were die-casted and the microstructure and mechanical properties of the alloys under die casting conditions were studied.The results are as follows.The master alloy was prepared by high-energy ultrasonic method.And the master alloy Al-5La-5Yb was analyzed by Optical microscopy?OM?,scanning electron microscopy?SEM?,energy dispersive X-ray spectroscopy?EDS?and X-ray diffraction?XRD?to determine its elemental composition and phase composition.Results showed that there are Al,La,Yb elements and the Al-rare earth?RE?compounds Al₁₁La₃,Al₃Yb in the mast alloy.RE-aluminum alloy?ADC12+xRE??x=0⁰.9wt.%?and?A356.2+xRE?were prepared and the microstructure of the alloy was observed and analyzed.It was found that the addition of rare earth played an important role in refinement of?-Al.The addition of rare earths has a modification effect on eutectic silicon.when the amount of mixed rare earth is 0.6 wt.%,the modification effect is best.When rare earth is added in excessive amounts,rare earth compounds are formed in the alloy.The rare earth compounds in the alloy have many types of morphologies,which are characterized by small granular,long needles,and granular shapes.At the rare earth segregation sites,Larger needle-like and bulky rare earth compounds will appear.A comparative study of the fluidity of rare-earth aluminum alloy?ADC12+xRE?and?A356.2+xRE?and related factors was concluded.The conclusion is as follows:The fluidity of the alloy is related to the content of added rare earth.When the rare-earth addition is 0.6 wt.%,the?-Al grain refinement effect is best and the flowability is the best.The flow length of the?A356.2+xRE?alloy increased from 500 mm to 555 mm?11%?compared to the matrix alloy?700°C casting?.Meanwhile,the fluidity of the alloy increases as the pouring temperature increases.When an excessive amount of mixed rare earth is added,the fluidity of the alloy also begins to decline.The addition of rare earths refines the grains of the alloy and increases the fluidity.The ADC12 alloy has better fluidity than the A356.2 alloy.The fluidity of the alloy is related to the morphology and size of the?-Al alloy grains,the morphology and size of the iron phase in the alloy,and the surface tension of the alloy.The addition of rare earths changes the morphology and size of the alloy grains and the morphology and size of the iron phase.As a result,the solidification mode of the alloy changes,the viscosity of the alloy decreases,the surface tension decreases,and the fluidity of the alloy increases.The change of morphology and size of?-aluminum crystallites due to the addition of rare earths is the main reason for the increase of fluidity of the alloys.`The thermal cracking resistance and microstructure of the alloys with different content of rare earth elements?La+Yb?were studied.The conclusions are as follows:The eutectic silicon morphology of the alloys is improved by adding appropriate amount of niobium to the alloys.The?-Al grain size is refined and the thermal cracking resistance is improved.When the addition amount of rare earth is 0.6%,the modification and refinement of the ADC12 alloy are best,and the alloy has the best hot cracking resistance.It's hot-tearing force is 1400N,which is 11%less than the matrix?1570N?.The liquidus and eutectic temperatures of ADC12 alloys with different rare-earth yttrium were analyzed by temperature solidification curve and DTA.As an aluminum-silicon alloy,ADC12 has lower hot cracking tendency due to its smaller solidification range,resulting in less thermal contraction during solidification,good fluidity,and good feeding properties.Furthermore,its hot cracking tendency can be further improved by adding a suitable amount of rare earth element.Using Miedema method and TOOP method,the formation of enthalpy,excess entropy,and Gibbs free energy of some rare earth-aluminum binary systems and ternary alloys involved in this experiment were calculated and analyzed.Through lattice matching calculation and TEM transmission electron microscopy tests,it was found that the Yb element in the mixed rare earth can generate Al₃Yb compound in the aluminum alloy.This compound can be used as the core of?-Al heterogeneous nucleation in the solidification process of the alloy and promote the alloy crystal grain refinement.However,the Al₁₁La₃ compound produced by the rare earth La in the aluminum alloy cannot be regarded as the core of the?-Al heterogeneous nucleus,so it may be distributed at the grain boundary or form a compound with other elements.The impact of mixed rare earth?La+Yb?on the microstructure of ADC12aluminum alloy under die-casting conditions and the influence of mechanical properties were investigated using a die-casting process.The microstructure of ADC12 in the state of die-casting was observed and the die-casting of the alloy was found.Compared with gravity casting,the?-Al grain size is greatly reduced,the?-Al morphology is relatively larger size rose-like,dendritic crystals and smaller spherical crystals;the eutectic silicon phase morphology in the alloy has also changed compared with gravity casting,it shows a small size of rods and fibers under die casting conditions.The size is also reduced accordingly;unlike gravity casting,the iron phase in the alloy is mainly represented by the larger size of the bulk?-AlFeMnSi and the smaller size of the spherical?-AlFeMnSi under different casting temperatures.The grain size is different.When the pouring temperature is high,a large amount of latent heat of the crystal is released,the solidification time becomes longer and the grain size is larger.When the pouring temperature is too low,the primary?-Al is generated in the early stage of solidification.Under the condition of die casting,the mechanical properties of the alloy were improved after the addition of rare earth.The tensile strength increased from 280 MPa to 313 MPa;the elongation increased from 4%to 4.5%.From the fracture morphology analysis,the strength was lower before the addition of rare earth.The alloy is mainly cleavage fracture.After the addition of rare earth,the grain size becomes smaller due to the metamorphism of rare earth.At the same time,a large number of dimples appear in the fracture,indicating that the fracture mechanism is dominated by ductile fracture.