Materials Science Forum Vol. 1071

Abstract: The main alloying elements have a decisive influence on the type and quantity of the second phase of the Al-Zn-Mg-Cu alloy, and even on the dissolution of the second phase during solution treatment. The effect of Zn/Mg ratio on second phase dissolution during solution treatment of Al-Zn-Mg-Cu alloys was investigated by means of differential scanning calorimetry (DSC), scanning electron microscope (SEM) and electrical conductivity testing. The results showed that Mg (Zn,Cu,Al)₂ phase and Fe-rich phase existed in the as-deformed alloys. In addition, a small amount of Al₂CuMg phase was found in the low and medium Zn/Mg ratio alloy. The number of Mg (Zn,Cu,Al)₂ phases as the major second phase in the alloys was inversely proportional to the Zn/Mg ratio. Mg (Zn,Cu,Al)₂ phase essentially dissolved into the matrix after solution treatment at 465°C/2h. Increasing the solution temperature and time were both beneficial to the dissolution of the Al₂CuMg phase. With the increase of the solution temperature from 465°C to 475°C, the conductivity of the alloy showed a decreasing trend initially and then increased. As the solution time increased at 470°C, the electrical conductivity of the low-Zn/Mg ratio alloy decreased and then increased due to the more secondary phase. After the second phase was fully dissolved in the alloy, the electrical conductivity gradually increased with the increase of the solution time.

Abstract: The dissolution of second phase with relatively high melting point in as-cast Al-Zn-Mg-Cu alloys was closely related to Mg and Cu contents. In present work, second phases in three Al-Zn-Mg-Cu alloys with simultaneously enhanced Mg and Cu contents (named by LMC alloy, MMC alloy and HMC alloy as Mg and Cu contents progressively enhanced) were analyzed and the correlated dissolution during homogenization was investigated. The results showed that both Mg(Zn,Cu,Al)₂ phase and Cu-rich phase existed in as-cast alloys while HMC alloy possessed more eutectic phases. As homogenized by 470°C/24h, Mg(Zn,Cu,Al)₂ phase had dissolved completely, LMC alloy contained little Al₂CuMg phase and the amount of it for the three alloys was arranged as LMC alloy < MMC alloy < HMC alloy. As furtherly homogenized by a second stage at 480°C for 12h, no endothermic peak for Al₂CuMg phase was observed for LMC alloy and only Fe-rich phase existed. Meanwhile, Al₂CuMg phase still remained in MMC and HMC alloy. As the homogenization time prolonging to 36h, Al₂CuMg phase in MMC alloy dissolved completely while that still existed in HMC alloy. Adding a third stage at 490°C for HMC alloy, no Al₂CuMg phase could be observed for 24h. This gave rise to a method by incrementally grading homogenization temperature combined with prolonging soaking time to fulfill the dissolution of second phase for Al-Zn-Mg-Cu alloys with enhanced Mg and Cu contents

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: 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: The scanning electron microscope (SEM) and optical microscope (OM) were used to study the deformation of TC18 titanium alloy microstructure at 881°C and 896°C. And the basket weave structure fracture mechanism was discussed. The results have been shown: during deformation at 881°C, the TC18 titanium alloy β grain size of about 305μm and the discontinuous grain boundary α phase along the β grain boundary were obtained. With the deformation temperature rising to 896°C, the β grain growth of 510μm and the continuous grain boundary α phase along the straight β grain boundary were obtained. The TC18 titanium alloy fracture toughness decreased from 77.8 MPa·m^1/2 to 65.4 MPa·m^1/2 as the rising of deformation temperature from 881°C to 896°C. The average β grain size is about 305μm and the discontinuous grain boundary α phase leads to the higher fracture toughness of TC18 titanium alloy forging. The fracture mode of fine β grain and discontinuous grain boundary α phase is the transgranular fracture, while the coarse β grain and continuous grain boundary α phase is the intergranular fracture.

Abstract: Herein, Ti-6Al-4V alloy was remelting by high-energy laser, the influence of laser energy density of mobile laser source on the evolution of solidification structure of Ti-6Al-4V alloy was studied, and the changes of relevant service performance of Ti-6Al-4V alloy after laser-remelting were explored. The results showed that there were four distinct regions: overheated zone, melting zone, heat-affected zone and substrate zone in Ti-6Al-4V alloy after laser remelting. The overheated zone and melting zone were dominated by primary α phase and secondary α phase, and a heat-affected zone was dominated by β phase. With the increase of laser energy density, the depth of molten pool and the range of melting zone in Ti-6Al-4V alloy increased, and the crystal phase distribution and grain size changed significantly. Laser-remelting Ti-6Al-4V alloy could still maintain a good passive state in the potentiodynamic polarization curve test, and the optimal laser power during remelting was 1200 W. This study proved that the crystal phase composition and grain size during the solidification process after laser-remelting were a key factor affecting the service performance of Ti-6Al-4V alloy.

Abstract: In recent years, with the rapid development of modern petroleum industry, some conventional oil country tubular goods (OCTGs) have been unable to meet the requirements of ultra-deep oil and gas drilling and production. Titanium alloy OCTGs showed broad application prospects due to its high specific strength, Low elastic modulus and strong CO₂+H₂S+Cl^- corrosion resistance. However, large-scale applications of titanium alloy OCTGs were seldom reported, which is mainly attributed to the lack of anti-corrosive resistance and mechanical performance of titanium alloy OCTGs in the working conditions. In the present work, the research progress on properties and application of titanium alloy OCTGs were reviewed. The resistance to uniform corrosion, stress corrosion cracking, pit and crevice corrosion, mechanical properties of the titanium alloy OCTGs in air and downhole corrosion media were comprehensively discussed. Application status and prospect of titanium alloy OCTGs in oil and gas exploration were also summarized.

Abstract: The effect of pre-stretching and under-aging treatment on microstructure and fatigue crack resistance of Al-4.1Cu-1.27Mg-0.47Mn-0.02Ti alloy has been investigated in present work. The results showed that the fatigue strength corresponding to 10⁷ stress cycle numbers of the sample pre-stretched 4% and under-aged at for 2 hours reached up to 280MPa, 80MPa higher than that pre-stretched 0% and peak-aged at for 8 hours. Microstructural observations showed that fine and uniformly dispersed S’ plates were present in the sample treated by pre-stretching and under-aging, whereas, a coarse S’ plate was formed in the sample when treated by 0% pre-stretching and peak-aging. It is indicated that the pre-stretching and under-aging treatment promotes the dispersoid formation of strengthening particles, enhancing the fatigue crack resistance and resultant fatigue strength. Keywords: Al-Cu-Mg alloy, fatigue, pre-stretching, aluminum alloy casing pipe

Abstract: Ni/Al energetic structural materials were prepared by vacuum hot-pressing method and then treated by secondary treatment of cold rolling or cold isostatic pressing. The effects of secondary treatment on the surface morphology, the phase composition, density, exothermic properties and tensile properties of Ni/Al energetic structural materials were investigated. The results showed that the density, the reaction energy density and the sensitivity of energetic materials improved significantly by the secondary treatment of cold rolling or cold isostatic pressing. It was also found that, after cold rolling and cold isostatic pressing, the energy density increased from 780 J/g to 1089 J/g and 993 J/g, respectively, and the initial reaction temperature was advanced by 46 °C and 14°C simultaneously. This is related to the increased in the contact area between Al particles and Ni particles. Meanwhile, the tensile strength increased from 166.7 MPa to 254.8 MPa and 211.3 MPa, respectively.