Ji Shan Zhang

Effect of stabilizing treatment on the intergranular corrosion behavior of high strength Al-Mg alloys

In order to improve the intergranular corrosion resistance of high strength Al-Mg alloys, the effect of stabilizing treatment was systematically investigated. Microstructure evolutions of Al-Mg alloys after different stabilizing treatments have been studied by scanning electron microscopy and optical microscopy. Mechanical properties and corrosion resistance were measured. It was found that the mass loss of samples after sensitizing treatment decreased with an increase in the stabilizing temperature. It was suggested that the susceptibility to intergranular corrosion for high strength Al-Mg alloys has a strong relation to the stabilizing temperature that modify the morphology and distribution of precipitates. The precipitates continuously precipitated along the grain boundary when the stabilizing temperature was lower than 250°C, corresponding to a poor corrosion resistance. However, the precipitates randomly precipitated in the matrix as globular particles, and discontinuously precipitated at the grain boundary after stabilized at 250°C and 275°C, resulted in an improved intergranular corrosion resistance.

Mechanical Properties and Constitutive Behavior of as-Cast High Strength AA7xxx Alloys below Solidus Temperature

High strength AA7xxx alloys have been extensively used in aerospace industry. However, experience in cast house demonstrates that such alloys are particularly prone to cold cracking and ingot distortion during direct chill (DC) casting, which leads to big amount of scraping or even total rejection of ingot. Those stress induced defects are greatly affected by casting process, and fine tuning of casting parameters is critical to improve the quality and productivity. Recently, numerical modeling has been widely utilized in direct-chill casting for the purpose of thermal mechanical analysis and cracking evaluation. Parameter optimization has become convenient. The model needs the input of constitutive properties of the AA7xxx alloys, of which the microstructure should resemble those formed during DC casting. Unfortunately, these constitutive data are not yet available in literatures. In this study, the mechanical properties of two high strength AA7xxx alloys were measured at temperature range from solidus down to room temperature through on-cooling compressive tests and the effect of strain rate on stress-strain behavior was also studied. The results were fitted to extended Ludwik equation which has been proved to be able to properly describe the stress-strain response of aluminum alloys. The results of the current work shed some light on the evolution of mechanical behavior of high strength AA7xxx alloys during cooling from high temperature.

Experimental and Numerical Investigations of the Plastic Deformation during Multi-Pass Asymmetric and Symmetric Rolling of High-Strength Aluminum Alloys

For understanding the distribution of plastic deformation induced by asymmetric rolling (ASR), multi-pass ASR and symmetric rolling (SR) experiments combined with the finite element simulation were used for high-strength aluminum alloy in the present study. The influence of reduction per-pass on the shear / effective strain distributions were studied via different ASR processes. By measuring the shear angle (θ, the angle between the reference mark before and after rolling) of rolled sheets, redundant shear strain and equivalent strain were calculated. It is shown that with equal total thickness reduction for ASR and SR, ASR can induce much more shear deformation through the thickness. By calculating the evolution of redundant shear strain and total equivalent strain for different ASR routines, it indicates that small pass reduction could be much favorable to the strain accumulation than that of the large pass reduction under a same total reduction in ASR process. Also, the influence of shear stress on the strain distribution and the through-thickness strain distribution were studied and evaluated with FEM analyses.

Improvement of Mechanical Properties and Corrosion Resistance by Deformation Induced Precipitation in Al-Zn-Mg-Cu Alloy

A final thermomechanical treatment (FTMT) including peak aging and subsequent dynamic aging was proposed to prepare 7055 Al alloy sheets. The optimization was based on nine well-planned orthogonal experiments. Three main processing conditions in the thermomechanical treatment for obtaining the optimum synthetic properties of 7055 (i.e. preheating temperature, final rolling temperature and deformation degree) were investigated. It was shown that the final rolling temperature is the most important factor among the three parameters, and the optimum properties (yield strength: 651 MPa, ultimate tensile strength: 660 MPa) of 7055 Al alloy sheet can be gained with preheating at 140oC and 40% deformation at 170oC. With dynamic aging, grain boundary precipitates became discontinuous without much coarsening of matrix precipitates, while they were continuously distributed after T6 aging. The present optimal FTMT process can improve the intergranular / exfoliation corrosion resistance without sacrificing the strength compared to T6 tempering. The present FTMT process as a good alternative can produce high-strength Al alloy sheets with high strength and good corrosion resistance efficiently and economically.

Mechanical Properties and Microstructures of 5052 Al Alloy Processed by Asymmetric Cryorolling

Aluminum alloy sheets were asymmetrically rolled at room and cryogenic temperatures by imposing different velocity ratios of 1~1.5 between the upper and bottom rolls. After rolling, the stress-strain curves, microhardness as well as the microstructures of the rolled samples were characterized and analyzed. The experimental results showed that the asymmetric cryorolling could improve the grain refinement and offered (~12%) higher room temperature tensile strength than that processed by symmetrical rolling with velocity ration of 1.0 (~280 MPa). However, at cryogenic temperature, the strength of asymmetrically cryorolling sheet (with velocity ratio of 1.5) was 5.1%, which is less than that processed by symmetrical rolling.

Recrystallization Behavior of High-Strength AA 7075 Alloy Processed by New Short-Cycled Thermo-Mechanical Processing

Recrystallization behavior under different conditions (different temperatures and times) of AA 7075 alloy processed by new short-cycled thermo-mechanical processing was investigated to design a suitable recrystallization schedule. With acquiring recrystallization activation energy by DSC, the recrystallization behavior was successfully verified by theoretical calculation. Experimental results of recrystallization response and re-dissolution of precipitates during isothermal annealing reveal excellent agreement with DSC prediction. The results show that the obtained activation energy of recrystallization can be used to establish the relationship between recrystallization temperatures and times. It is proposed that an appropriate recrystallization treatment (703-753 K/1-5 min) could be used to acquire completely recrystallized grains with size <10 μm, contributing to better formability/ductility. The coarsening rate of these fine recrystallized grains is fairly low even though extending the solution treatment times at 753 K. Therefore, it indicates that the recrystallization dynamical equation would be a useful method to adjust recrystallization temperatures and times to satisfy various requirements of structures and properties.

Microstructures and Properties of Spray Formed CuCr25 Alloy

CuCr25 alloy contact materials were prepared by spray forming process, and subsequently hot forged and hot isostatically pressed to full densification. Microstructures of CuCr25 alloys were studied, and properties of the materials were measured . The research results show that the spray forming process is an ideal method to prepare CuCr25 alloys with typical rapidly solidified microstructure, excellent properties, fine chromium particles with size between 3~10μm uniformly distributed in the copper matrix. After hot forging and hot isostatic pressing, the density of the CuCr25 alloy could be 99%, electric conductivity of the CuCr25 alloy could be up to 26~29Ms/m, as the size of Cr particles increases to 10~15μm.

Hot Tearing Evaluation and Contraction Behaviors of Four AA7xxx Alloys

The hot tearing susceptibilities (HTS) of some AA7×××alloys, AA7050, AA7055, AA7085 and AA7022 were evaluated with constrained rod casting (CRC). Thermal contraction behaviors during solidification were measured as well in a T-shaped setup. The results showed that alloys with HTS from high to low were AA7055, AA7085, AA7050 and AA7022. Zn content in 7××× aluminum alloys seemed to play a major role with respect to the HTS index. Remarkable differences could be seen on thermal contraction behaviors within solidification range for each alloy. The rate and amount of thermal contraction for AA7055 was most prominent, followed by AA7085 and AA7050, while contraction curve of AA7022 was very flat together with least amount of thermal contraction. There was a well consistency between the amount of thermal contraction and HTS. Despite complex interactions of many variables in the formation of hot tear, thermal contraction behaviors within solidification range could give a quick evaluation of hot tearing susceptibility.

Effect of Cu Content on the Formability and Portevin-Le Chatelier Effect of Al-Mg Alloys

Al-Mg alloys developed for auto body sheets with different Cu contents were fabricated in the laboratory scale. The effects of Cu content on the microstructures, formability and Portevin–Le Chatelier(PLC) effect of the alloys were investigated by polarizied optical microscopy and room temperature tensile testing. It has been found that with increasing Cu content, there was little change of the strain hardening exponent, but the plastic strain ratio and limiting drawing ratio increased firstly and then decreased. A quantitative statistical analysis of the characteristics of the PLC effect was made, including the stress drop and the reloading time, which follow a common linear relationship with plastic strain, and the increase rate of stress drop and reloading time was bigger with more Cu content. A detailed discussion of the corresponding mechanism of Cu and Cu-containing precipitates on the dynamic strain aging(DSA) was made.

The Evolution of Microstructure and Texture during Thermomechanical Processing of Al-Mg-Si-Cu Alloy

In order to improve formability, it is practicable to control the texture through adjusting process parameters. This work describes the evolution of microstructure and texture during thermomechanical processing of Al-Mg-Si-Cu alloy. With the change of deformation conditions, both the microstructure and texture change dramatically. After hot rolling from 90 mm to 7.5 mm, H and E texture components in the surface layer become dominant due to non-uniform deformation. And then with the increasing of cold rolling deformation from 7.5 mm to 4.0 mm, the texture components gradually change from shear texture to typical fcc texture, i.e. Copper, S and Brass textures, and their intensities also increase. And these texture components transform to some uncommon texture components after intermediate annealing, including {013}<001>, {001}<130>, Goss texture, {556}<110> and {111}<110> texture, not as the typical recrystallization texture components. Continually giving a cold rolling deformation from 4.0 mm to 1.0 mm, not only Copper, S, Brass textures, but also Goss texture due to the lower deformation can be found in the alloy sheet. The high temperature solid solution treatment can result in the complete recrystallization and the formation of recrystallization texture, Cube, Goss and R texture, which results in the high formability of experimental alloy.