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2022.10.20
The semi-solid slurry of Al-27Si-4.5Cu-0.6Mg alloy was prepared by using pure copper serpentine channel composite Sr modification, and rheo die-casting was carried out. The effect of T6 treatment on the properties and microstructure of rheo die-casting samples with different Sr addition was studied. The results show that the primary Si is fully refined and the eutectic Si is effectively modified by the composite process. After T6 treatment, round granular eutectic Si and primary Si with edges and corners passivated can be obtained. The better the modification effect before heat treatment, the more round and fine the eutectic Si shape after T6 treatment; T6 treatment further improved the tensile strength of semi-solid rheo die-casting Al-27Si-4.5Cu-0.6Mg alloy specimen prepared by composite treatment. The highest tensile strength before and after heat treatment appeared under the condition of 0.02% Sr content.
Hypereutectic Al Si alloys have excellent wear and corrosion resistance, low density, high thermal stability and excellent mechanical properties. It is mainly used to manufacture piston, cylinder liner, pulley, transmission device and other parts of automobile or motorcycle engines. However, when the content of Si is high, coarse polygonal block, lath and five-star shaped irregular primary Si will appear in the hypereutectic Al Si alloy structure, and eutectic Si will also grow into long needle flake. This state seriously splits the alloy matrix, and under stress conditions, it is easy to produce stress concentration at the edge of primary Si, causing microcracks and deteriorating the mechanical properties of the alloy. Therefore, in order to improve the properties of hypereutectic Al Si alloys, it is necessary to fully refine primary Si and effectively modify eutectic Si at the same time.
In order to fully refine the primary Si and effectively modify the eutectic Si, the traditional method is to add an appropriate amount of P or S to the hypereutectic Al Si alloy to refine the primary Si, and add an appropriate amount of Na, Sr, Sb, etc. to modify the eutectic Si. However, when P or S and Na, Sr, Sb are added at the same time, chemical reaction will occur, which not only reduces the refining effect of the primary Si, but also weakens the modifying effect of the eutectic Si. The preparation of semi-solid alloy slurry by pure copper serpentine channel casting is a method proposed in recent years, which can produce semi-solid alloy slurry with good quality. In this project, it is envisaged that the pure copper serpentine channel is used to fully refine the primary Si, and then Sr is added to the alloy in advance to chemically modify the eutectic Si, so that the primary Si can be fully refined and the eutectic silicon can be effectively modified at the same time. Moreover, the semi-solid hypereutectic Al Si alloy slurry prepared by the pure copper serpentine channel can realize rheological die casting, which can effectively reduce the entrainment of the sample. T6 treatment can further improve the mechanical properties of rheo die-casting samples of hypereutectic Al Si alloys. Therefore, in this project, semi-solid Al-27Si-4.5Cu-0.6Mg alloy slurry was prepared by water-cooled pure copper serpentine channel and Sr composite process, and then the rheo die-casting sample was treated with T6 to investigate the changes in tensile strength and microstructure of the rheo die-casting sample, in order to provide reference for its application.
1 Test plan
The test alloy is Al-27Si alloy with high purity. Sr is added in the form of Al-10Sr master alloy.
The Al-27Si-4.5Cu-0.6Mg alloy is heated in an electric resistance furnace, and the melting temperature is controlled at 850 ℃. When the alloy is melted, keep it warm for 2h, then add Al-10Sr master alloy, and then keep it warm for 30min. The amount of Sr added is 0,0.01%, 0.02% and 0.03% respectively. After the heat preservation of the alloy, add Ar for 10 min into the melt for degassing, and then when the melt temperature is reduced to around 830 ℃, remove the slag and prepare to pour it into the serpentine channel. Before pouring, the serpentine channel is filled with cooling water in advance. Pour the 830 ℃ Al-27Si-4.5Cu-0.6Mg alloy liquid into the pure copper serpentine channel, the semi-solid slurry flows into the ceramic ladle preheated to 200 ℃ in advance, and immediately pour it into the pressure chamber of the die-casting machine for rheo die-casting. Preparation of semi-solid Al-27Si-4.5Cu-0.6Mg by pure copper serpentine channel. Take metallographic photos with 4XC inverted optical microscope (OM), and calculate the equivalent circle diameter D of primary Si and eutectic Si.
Where, Ai is the area of primary Si; N is the total number of primary Si particles.
Model YYC180B horizontal cold chamber die-casting machine is used for rheologic die-casting. The injection specific pressure is 90 MPa, the injection head speed is 0.5 m/s, and the pressure holding time is 9 s. At the same time, four standard tensile specimens and material columns are prepared.
T6 treatment process: solution temperature is 490 ℃, holding time is 6h, water quenching; The aging temperature is 180 ℃ and the aging time is 6 h.
2 Test results and analysis
Effect of T6 treatment on microstructure of rheo die-casting sample
The microstructure of semi-solid Al-27Si-4.5Cu-0.6Mg alloy before and after T6 treatment prepared by pure copper serpentine channel composite casting. Among them, white particles are primary α- Al phase: primary Si in gray block, eutectic Si in gray needle flake and Al2Cu in black dendrite. When Sr is not added, the eutectic Si in the alloy structure is long needle like and mainly distributed in the interdendritic of the matrix. The average length of long needle lamellar eutectic Si can reach 7.85 μ m. This seriously splits the alloy matrix. In addition, due to the chilling effect, there is also a small amount of modified fibrous eutectic Si in the structure. The coarse dendritic Al2Cu is overlapped with the long needle like eutectic Si. When the content of Sr is 0.01%, the acicular and flaky eutectic Si is obviously refined, and some eutectic Si is modified into fibrous, and the average length of the remaining long acicular and flaky eutectic Si is reduced to 4.64 μ m. And with the addition of Sr α- Al dendrite length decreases. When the content of Sr is 0.02%, the long needle like eutectic Si is further modified, and the average length of eutectic Si is reduced to 2.85 μ m. The distribution of eutectic Si is more uniform. When the content of Sr is 0.03%, the long needle like eutectic Si basically disappears, and the length of eutectic Si is only 1.07 μ M or so. Therefore, when the addition amount of Sr is 0.03%, eutectic Si can be effectively modified from long needle like to fibrous. α- Al dendrites are transformed into rosettes, and it is found that some eutectic Si is connected with primary Si particles. Eutectic Si exists as fibers around primary Si, but Al2Cu still exists as dendrites.
Rheo die casting microstructure of semi-solid Al-27% Si-4.5Cu-0.6Mg alloy before and after T6 treatment with different Sr content
（a）w（Sr）=0； (b) After T6 treatment; w（Sr）=0； （c）w（Sr）=0.01%； (d) After T6 treatment, w (Sr)=0.01%; （e）w（Sr）=0.02%； (f) After T6 treatment, w (Sr)=0.02%; （g）w（Sr）=0.03%； (h) After T6 treatment, w (Sr)=0.03%
After T6 treatment, the eutectic Si in the rheo die-casting sample of Al-27Si-4.5Cu-0.6Mg alloy changes to granular, but the size of eutectic Si is related to whether it is modified, and the dendritic Al2Cu basically disappears. The equivalent circle diameter of eutectic Si with different Sr content after T6 treatment is different. After Sr modification, the equivalent circle diameter of eutectic Si decreases. When Sr is not added, the equivalent circle diameter of eutectic Si is 5.2 μ m； When the content of Sr is 0.01%, the equivalent circle diameter of eutectic Si is greatly reduced to 3.7 μ m； When the Sr content is 0.02%, the equivalent circle diameter of eutectic Si is further reduced to 3.2 μ m； When the Sr content is 0.03%, the equivalent circle diameter of the final eutectic Si is 2.9 μ m。
Solidification structure of Al-27Si-4.5Cu-0.6Mg alloy without Sr. It can be seen that the primary silicon in the structure of Al-27Si-4.5Cu-0.6Mg alloy is massive without the treatment of pure copper serpentine channel, which seriously splits the alloy matrix and weakens the mechanical properties of the alloy. The primary silicon particles in Al-27% Si-4.5Cu-0.6Mg alloy were obviously refined after the copper serpentine channel treatment. T6 treatment has little effect on the primary silicon in the microstructure of semi-solid Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting samples. Before T6 treatment, primary Si exists in the form of polygonal blocks; After T6 treatment, the primary Si has a certain degree of passivation, but the size has not changed.
Al-27Si-4.5Cu-0.6Mg alloy is treated by pure copper serpentine channel, and its primary Si is fully refined by the chilling effect of the inner wall of the channel. This physical refining effect has no effect on the chemical modification of eutectic Si by Sr. Therefore, the composite Sr treatment of pure copper serpentine channel can fully refine the primary Si and effectively modify eutectic Si at the same time. Sr is a typical “adsorbed” metamorphic element. The Sr atom is adsorbed on the growth surface of eutectic Si, poisoning the growth position of Si, and inducing more twin growth, which provides more growth directions for eutectic Si , so that the growth characteristics of eutectic Si change from anisotropy to isotropy, so the growth state of eutectic Si changes, and finally changes from long needle sheet to fiber, resulting in effective modification. With the addition of Sr, α- The Al dendrite finally turns into a rose shape due to α- The disappearance of Al dendrites increases the contact opportunity between primary Si and the final solidified liquid phase, and because primary Si and eutectic Si have essentially the same crystal structure, more eutectic Si can be attached to primary Si for nucleation and growth. This indicates that the modification of Sr on eutectic Si is to α- The results of Al phase interaction.
Effect of T6 treatment on tensile strength of rheo die-casting specimen
T6 treatment has a great influence on the microstructure of Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting samples. Cu, Mg, and other solute atoms diffuse into α- Al matrix forms a supersaturated solid solution and disperse and precipitate the strengthening phase during aging, which is conducive to the improvement of mechanical properties of the alloy. During T6 treatment, eutectic Si necks and granulates, and becomes rounder granular eutectic Si, which enhances its bonding force with the matrix, reduces the possibility of stress concentration and crack initiation under stress, and is conducive to the stability or improvement of its tensile strength. When T6 is treated, the edges and corners of the primary Si are passivated, which also improves the bonding between the primary Si and the matrix, and is conducive to the stability or improvement of its tensile strength.
The tensile strength of semi-solid Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting specimens prepared for copper serpentine channel composite Sr treatment. It can be seen that after T6 treatment, the tensile strength of the alloy with different Sr content has been further improved. When the Sr content is 0.02%, the tensile strength of the alloy reaches 254.7 MPa. When Sr is not added, the tensile strength before T6 treatment is 161.5 MPa, and the tensile strength after T6 treatment is 205.3 MPa. When Sr content is 0.01%, the tensile strength before T6 treatment is 175.1 MPa, and the tensile strength after T6 treatment is 227.8 MPa. When Sr content increases to 0.03%, the tensile strength before T6 treatment is 200.2 MPa, and the tensile strength after T6 treatment is 225.3 MPa.
The tensile fracture morphology of semi-solid Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting samples with Sr content of 0 and 0.03% before and after T6 treatment. The fracture surface of tensile specimen is composed of brittle section and dimple. The flat primary Si presents brittle section, and dimple shaped α- Al phase fracture surface, which indicates that the fracture of the alloy is a mixed fracture mode of coexistence of toughness and brittleness. The addition of Sr modifier and T6 treatment increased the dimples in the fracture surface, but did not improve the brittle fracture plane of primary silicon.
With the addition of Sr, the acicular lamellar eutectic Si in semi-solid Al-27Si-4.5Cu-0.6Mg alloy is modified into fibrous. After T6 treatment, the eutectic Si is further rounded, the edges and corners of primary Si are also passivated, and Al2Cu is also dispersed in the matrix. All these improve the adhesion between eutectic Si and primary Si and the matrix, which further improves the tensile strength of rheological die cast Al-27Si-4.5Cu-0.6Mg alloy samples.
3 Conclusion
(1) The semi-solid Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting samples were prepared by the water-cooled pure copper serpentine channel casting process and the long-term modifier Sr composite refining process, which can fully refine the primary Si and effectively modify the eutectic Si.
(2) After T6 treatment, the semi-solid Al-27Si-4.5Cu-0.6Mg alloy rheo die-casting sample can obtain granular eutectic Si and corner passivated primary Si, and the better the modification effect before T6 treatment, the more round and fine the morphology of eutectic Si obtained after T6 treatment. When the addition of Sr is 0.03%, the minimum equivalent circle diameter is 2.9 μ m。
(3) T6 treatment can further improve the tensile strength of semi-solid rheo die-casting sample of Al-27Si-4.5Cu-0.6Mg alloy. For example, when the Sr content is 0.02%, the tensile strength of the alloy before and after T6 treatment is the highest, which is 228.8 MPa and 254.7 MPa respectively.