Study on properties of cast heat resistant aluminum alloy

In view of the complex service environment of automobile engine cylinder and piston, cast heat-resistant aluminum alloy not only has excellent high temperature mechanical properties, but also has high creep resistance, heat exposure resistance, fatigue resistance and certain dimensional stability.

1. High temperature mechanical properties

At present, the research on the mechanical properties of heat-resistant aluminum alloy at home and abroad mainly focuses on the improvement of high temperature strength at 350 ℃. Figure 1 summarizes the recent research results of domestic and foreign scholars on the tensile properties of cast aluminum alloy at 350 ℃. Xiao Daihong et al produced al-5.3cu-0.8mg-0.3ag-xce alloy by vacuum melting and casting, then homogenized at 500 ℃ and hot extruded at 400 ℃, finally quenched at 525 ℃ and aged at 185 ℃. The test results show that the alloy with 0.2 wt.% CE has the best mechanical properties, and the tensile strength reaches 170 MPa at 350 ℃. On the basis of al-12si-4cu-mg alloy, Meng et al. Introduced Fe and Ni elements into al-12si-4cu-mg alloy by designing different Fe / Ni ratios for strengthening. It was found that appropriate Fe / Ni ratio could form t-al9feni phase with excellent thermal stability, which presented flaky filling δ- The network gap of al3cuni phase. At the same time, when Fe / Ni is 0.23, the tensile strength of the alloy can reach 106 MPa at 350 ℃, which is 23.3% higher than that of the base alloy. Feng et al. Combined with JMatPro thermodynamic calculation, designed a system composed of a single ε- The microstructure of al-12si-0.9cu-0.8mg-xni alloy strengthened by al3ni phase is fine α- Al, Si and Al ε- Al3Ni。 Research shows that with ε- With the increase of al3ni content, the tensile strength of the sample increases monotonously from 94 MPa to 116 MPa at 350 ℃ ε- Al3ni is easier to debond and become the source of crack due to its coarsening size. Li et al strengthened al-12si-3.5cu-2mn alloy by adding CR element. It was found that CR would dissolve into the primary Mn rich phase to form al15 (Mn, Cr) 3si2. With the increase of Cr content, the morphology of the phase changed from rod like to dendrite like and finally to star like particles. The results show that when the content of Cr is 1.0wt%, the most Cr is dissolved in the Mn rich phase, and the tensile strength of the alloy reaches the highest value at room temperature and high temperature, which are 279 MPa and 106 MPa respectively. The 211zl. 1 aluminum alloy developed by Guizhou Huake aluminum material engineering technology research Co., Ltd. is strengthened by Cu, Mn, Ti, Zr, CD, re and other elements. The test shows that the tensile strength of the alloy after T6 heat treatment can reach 490 MPa at room temperature and not less than 130 MPa at 350 ℃.

It can be seen from Figure 1 that the three-dimensional structure and morphology of the strengthening phase or reinforcement are closely related to the high-temperature strength of the material, and the alloy with network structure shows excellent high-temperature properties. Ma et al. Reported for the first time a new type of nano AlN particle reinforced ultra-high strength aluminum matrix composite. In situ synthesized AlN particles are connected by twin bonding chains to form a three-dimensional network, which greatly strengthens the Al matrix as human skeleton. The tensile strength of the composite containing 16.4 wt% AlN particles can reach 190 MPa at 350 ℃. When Yang et al. Prepared tic nanoparticles reinforced Al matrix composites, it was found that tic nanoparticles along the α- The distribution of Al grain boundary forms a network structure, which makes the structure more stable α- The deformation behavior of Al matrix changes from local deformation to overall deformation at high temperature, and the ultimate tensile strength of the composite increases to 151 MPa at 350 ℃. Therefore, it is of great significance and broad prospects to make the second phase form an interconnected and dense network structure in three-dimensional space by purposeful structural control.

2. Creep properties

Creep refers to the phenomenon that metal materials produce plastic deformation slowly under the long-term action of constant load in constant temperature environment. Creep has a significant effect on the service metal and its alloy at high temperature, so it is necessary to include creep property in the evaluation index of cast heat-resistant aluminum alloy. Jeong et al. Studied the creep properties of al-6si-0.3mg alloy before and after adding 2.76 wt.% Cu. It was found that more Al3 (Cu, Ni) 2 with high thermal stability precipitated with the increase of Cu content. At the same time, the creep fracture time of the alloy increased from 2.8 h to 23.8 h and the stress index increased from 5.3 to 6.6 at 25 ~ 400 ℃ and 20 ~ 130 MPa. Yao et al. Studied the effect of La on the microstructure and creep properties of Al Cu cast aluminum alloy. The results show that the addition of La makes al11la3 with good thermal stability precipitate at the grain boundary, changes the shape and gap of dendrite, and increases the temperature of the alloy θ The results show that the creep resistance of the alloy is 3 ~ 5 times higher than that of the alloy without La addition.

3. Heat exposure characteristics

Due to the long service life of heat-resistant aluminum alloy castings in high temperature environment, the long-term heat exposure characteristics of the alloy should also be included in the high temperature performance index. The results show that the tensile strength of al-11.9si-3.5cu-1.7ni-0.8mg alloy decreases and the elongation increases with the increase of exposure temperature and time, but it tends to be stable at last; The heat exposure temperature determines the intensity of strength and elongation changes. The microstructure and properties of al-12si-0.9cu-0.8mg-xni alloy were studied by Feng et al. After being exposed at 350 ℃ for 200 h, it was found that the morphology of eutectic Si changed from long and thin flakes to spherical and short rods, while the morphology of eutectic Si changed to spherical and short rods ε- There is no change in al3ni ε- Al3ni has good thermal stability, and its tensile strength reaches 116 MPa at 350 ℃. Based on JMatPro thermodynamic calculation, we designed a system with δ- The evolution of Al Si Cu Ni alloy strengthened by Al 3cuni and Si was studied at 350 ℃ for 200 H δ- At the same time, the tensile strength at room temperature decreased from 186 MPa to 164 MPa, and the tensile strength at 350 ℃ decreased from 61 MPa to 46 MPa.

4. Fatigue performance

Fatigue refers to the phenomenon that metal and its alloy produce permanent damage in a certain part under cyclic load, resulting in crack initiation to complete fracture. Generally, the external stress causing fatigue fracture does not reach the tensile strength or even lower than the yield strength. The results show that the fatigue life of T6 state is the longest and that of as cast is the shortest; The as cast temperature sensitivity is the strongest at 300 ~ 350 ℃; The temperature sensitivity of as cast quenched and aged is the strongest at 350 ~ 400 ℃; The fatigue crack growth rate of all alloys first increases and then decreases with the increase of cycle times, which shows a non-standard ladder curve pattern. Feng et al. Studied the effect of Ni addition on the low cycle fatigue of al-12si-0.9cu-0.8mg alloy at 350 ℃. It was found that the fatigue peak displacement decreased with the increase of Ni content, and the peak displacement was the smallest when the Ni content was 4 wt.%. However, the longest fatigue life appears in the sample with Ni content of 2.5 wt.%, when the alloy has higher fatigue strength coefficient (198.29 MPa) and lower fatigue strength index (- 0.1295), it shows the best low cycle fatigue performance. Huang Chaowen et al. Studied the room temperature fatigue properties of T6 heat-treated 211z. X aluminum alloy under the action of stress ratio r = – 1 and frequency f = 20 Hz × The conditional fatigue limit of the alloy is 154.25 MPa. When the adjusted survival rate P increased from 50% to 99.9%, the probability fatigue limit decreased from 147.13 MPa to 140 MPa.