Papers by Author: Xi Wu Li

Abstract: The influence of Cu and Li content on the formation and evolution of the second phase in Al-Cu-Li alloys was investigated under as-cast conditions and homogenization treatment regimes. The second phase was characterized via SEM, EDS, XRD and DSC analysis. The results revealed that elevated Cu content promoted the formation of all three resolvable phases in the as-cast microstructures. Increased Li content significantly enhanced the Al₇Cu₄Li phases while suppressing the Al₂Cu phases. During the homogenization treatment, the low-melting-point AlCuMgAg eutectic structure exhibited preferential dissolution kinetics and higher temperatures facilitated the complete dissolution of both Al₇Cu₄Li and Al₂Cu phases. The double-step homogenization treatment of 495°C/24 h+515°C/24 h is universally applicable to all the investigated alloys in this study.

Abstract: Fe and Si elements can form non-dissolvable phases in Al-Zn-Mg-Cu alloys and furtherly affect the property of finished products while the influence of minor contents lacks sufficient investigations. In present work, 7085 alloy with different Fe and Si contents (named by LFS alloy, MFS alloy and HFS alloy as Fe and Si contents progressively enhanced) was investigated and corresponding microstructure and property during the whole fabrication were analyzed via OM, SEM and DSC observation and tensile property, fracture toughness and salty-spray corrosion property. The results showed that Fe-containing phase existed in as-cast microstructure of three alloys while part of Fe-containing phases of HFS alloy has Si element. After homogenization treatment, coarse eutectic phases had dissolved into the matrix while Fe-containing phase remained in the alloys. Similarly, Si element existed in some Fe-containing phases of HFS alloy. The order of area fraction of remaining Fe-containing phases for the three alloys was LFS alloy<MFS alloy<HFS alloy. After extrusion, Mg(Zn,Cu,Al)₂ phase and Fe-containing phase possessed the majority, the former one was eliminated by solution treatment while the latter one had no obvious change. Under an unified aging treatment, the strength, elongation and fracture toughness were degraded with the increase of Fe and Si contents. Besides, the salt-spray corrosion property of HFS alloy was significantly inferior to LFS alloy and MFS alloy. This gave rise to a reference for controlling Fe and Si impurity of 7085 alloy for considering overall performance.

Abstract: In present work, a high Zn-containing Al-Zn-Mg-Cu alloy with different grain sizes was fabricated by extrusion and related precipitation characteristics and mechanical property were investigated after uniform heat treatments. The results showed that precipitation characteristics for the three alloys were almost the same. Matrix precipitates were GPII zone and η' phase and possessed small size and dense distribution while grain boundary precipitates exhibited discontinuous distribution. The rank of strength and fracture toughness for the three alloys are SG>MG>LG. Tearing ridges had been found on all the fracture surface while only LG alloy possess obvious dimple characteristics. The a-N curve showed that crack length list is MG>LG >SG under a same cycle number. The da/dN-ΔK curve also proved that fatigue crack propagation (FCP) rate of MG alloy is slightly larger than that of LG alloy, both were apparently larger than that of SG alloy. The width of fatigue striations on FCP fracture surface also backed it. Besides, obvious transgranular cracking characteristics and apparent secondary cracks were found on the FCP fracture surface.

Abstract: In this study, the heat transfer coefficients of 7050 aluminum alloy under different water temperatures, polyalkylene glycol aqueous solution concentrations and quenching medium types are calculated by an iterative method, and the heat transfer coefficient of the aluminum alloy under different quenching medium parameters was compared and the difference was discussed in detail. The results show that with the increase of water temperature, the heat transfer coefficient of 7050 aluminum alloy gradually decreases; with the increase of polyalkylene glycol aqueous solution concentration, the heat transfer coefficient gradually decreases; the order of heat transfer capacity of quenching medium is disclosed among the studied medium types.

Abstract: The dissolution of the second phase during solution treatment was of great importance for achieving preferential properties via aging treatment for Aluminum-Lithium (Al-Li) alloys. The microstructure characteristics of an extruded Al-Li alloy and its second phase dissolution during solution treatment were studied, while related electrical conductivity and tensile properties after ageing were tested for verification. The results indicated that as the alloy solution was treated from 500°C to 520°C with a soaking time of 1.5h, the Cu-rich phase dissolved into the matrix continuously. The statistics of remained phase area fraction ascertained no obvious decrement from 510°C to 520°C and only Fe-containing phase with large size was detected. Meanwhile, tensile properties under the same aging regime declared a higher strength was obtained at 510°C. As the solution time varied from 0.5h to 5h at 510°C, the Cu-rich phase was detected in a soaking time of 0.5h while disappeared after 1.5h and only the Fe-containing phase was observed. Correspondingly, electrical conductivity exhibited a moderate growth while tensile strength obviously increased from 0.5h to 1.5h and then maintained a platform, which revealed a preferential solution regime of 510°C/1.5h. This gives a reference for the second phase dissolution during solution treatments and furtherly obtaining preferential solution regimes.

Abstract: Homogenization treatment is usually indispensable to obtain a good microstructure pattern and brilliant final performance of Al-Cu-Li alloys. In the present study, the effect of different Mg contents on the microstructure of Al-Cu-Li alloys during homogenization was investigated utilizing optical microscopy (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis. The results indicated that the higher Mg content changed the type of grain boundary phase in the as-cast alloy. The eutectic phases in the low Mg alloy were dominated by Cu-rich phases while the high Mg alloy also had many Ag-containing Al₂CuMg phases. The difference in Mg contents did not affect the grain morphology, while the high Mg content in the Cu-rich phase caused a decrease in its melting point. Suitable homogenization treatments for the low and high Mg alloys are 520 °C/12 h and 495 °C/24h + 505 °C/48 h, respectively. This provides a reference for studies related to the effect of composition evolution on the dissolution of the second phase during the homogenization treatment.

Abstract: Pre-deformation is usually indispensable to obtain an appropriate balance between microstructure and mechanical properties of Al-Cu-Li alloys. In this paper, the effect of pre-deformation degree on the evolution of strength, fracture toughness, and precipitates during artificial aging processes of a novel Al-Cu-Li alloy was studied. The results indicated that the amount of T₁ phase increased remarkably while the precipitation of θ' phase was inhibited with the increase of pre-deformation degree. The change in the average size of T₁ phase indicated that the increase in nucleation sites suppressed the full growth of T₁ phases. The fracture toughness could be related to the strength difference between intragranular and intergranular. Based on the matching of properties, a reasonable pre-deformation parameter was proposed.

Abstract: The microstructures of as-cast and homogenized Al-Cu-Mg-Ag alloys with 1.98, 3.66 and 5.16 wt.% Cu contents (Alloy 1, Alloy 2, Alloy 3, respectively) were investigated by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and calculation of phase diagram. The results indicate that the second phases in the as-cast alloys consist of θ (Al₂Cu), ternary α (Al)-θ (Al₂Cu)-S(Al₂CuMg) eutectic and Fe-enriched phase. The invariant reactions are L→α (Al) + θ (Al₂Cu) at 542°C and L→α (Al) + θ (Al₂Cu) + S(Al₂CuMg) at 505°C during solidification. An increase of Cu content promotes the formation of θ (Al₂Cu) phase, therefore leading to the total second phases increasing in the as-cast alloys. The ternary eutectic completely dissolves into the matrix after the first-step homogenization at 490°C, and θ (Al₂Cu) phase dissolves subsequently after the second-step homogenization at 510°C. It is suggested that the proper homogenization treatments are 490°C/24 h for Alloy 1, 490°C/24 h + 510°C/24 h for Alloy 2 and 490°C/24 h + 510°C/48 h for Alloy 3.

Abstract: Second phase dissolution of Al-Zn-Mg-Cu alloys during solution treatment was closely associated with the content of main alloying elements. In present work, the phase characteristics of several Al-Zn-Mg-Cu alloys with various main alloying contents were investigated, and the second phase dissolution of these alloys during solution treatment was analyzed. The results showed that the extrusion alloys possessed abundant second phases, mainly including Mg(Zn,Cu,Al)₂ phase and Fe-rich particles. The DSC analysis proved that the larger endothermic peak corresponded to the alloy with larger main alloying content, and the XRD spectrogram also backed up the advantage of Mg(Zn,Cu,Al)₂ phase. After solution treated at 450°C, the residual phases remained in the alloys and the quantity of them were positively correlated with the main alloying content. With the increase of solution temperature, the electrical conductivity of the alloys showed a decremental trend, while the alloys with relatively low main alloying contents exhibited an inversion at the solution temperature of 475°C. The SEM observation demonstrated that no Mg(Zn,Cu,Al)₂ phase was observed in the alloys with relatively low main alloying contents while seldom still remained in the alloy with high main alloying content after solution treated at 470°C. After solution treated at 475°C, Mg(Zn,Cu,Al)₂ phase completely dissolved into the matrix for the alloy with high main alloying content. The statistics of residual phase quantity also proved this.

Abstract: Effects of different solution treatments on microstructure, texture and mechanical properties of 6A16 aluminum alloy sheet were investigated by electron back-scattered diffraction (EBSD) and tensile test. The results show that among the five solution treatments, with the increase of solution temperature and solution time, the average grain size increases and the yield strength of the alloy sheet gradually increases. The maximum elongation at break of the sheet is at 545 °C, and it increases with the increase of solution time. Therefore, the mechanical properties of the sheet are the best when the solution treatment is 545 °C for 5 min.