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Aluminum-Lithium Alloys: Chapter 7. Mechanical Working of Aluminum-Lithium Alloys - Kindle edition by G. Jagan Reddy, R.J.H. Wanhill, Amol A. Gokhale.
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Renowned coverage of metals and alloys, plus other materials classes including ceramics and polymers. Updated coverage of sports materials, biomaterials and nanomaterials. Covers new materials characterization techniques, including scanning tunneling microscopy STM , atomic force microscopy AFM , and nanoindentation. Easy to navigate with contents split into logical groupings: fundamentals, metals and alloys, nonmetals, processing and applications. Detailed worked examples with real-world applications.

Rich pedagogy includes extensive homework exercises. Preface About the authors Acknowledgments Illustration credits Chapter 1 Atoms and atomic arrangements 1. Powered by. You are connected as. Connect with:. Thank you for posting a review! We value your input. Ma et al. They found that the severe localized corrosion of the alloy of T8 condition was associated with localized plastic deformation occurring during the pre-age cold working and heterogeneous precipitation of the T 1 Al 2 CuLi phase during subsequent artificial aging.

Lin et al. Chen et al. Other related research [ 9 , 10 , 11 , 12 ] has also reported that the quantity and distribution of some Li-, Mg- and Cu-richer precipitates, e. As a locally rapid melting and rapid solidifying process, the distribution of alloying elements and the microstructure homogeneity of the laser beam welding joints are definitely different from the Al—Li matrix alloy.

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This will affect its corrosion resistance in practice correspondingly. Therefore, it is important to figure out the relationship between the microstructure and its corrosion resistance. The microstructure, micro segregation of alloying elements, the phase constitution, and corrosion resistance of the welded joints in 3. Chemical composition of the Al—Li alloy and ER wire mass fraction. The Al—Li alloy sheets were welded in the form of a butt joint without groove.

Before welding, the surface of the sheets was polished and chemically cleaned to remove part of the oxide and oil contamination formed previously. The welding direction was parallel to the rolling direction of the sheet. The focused laser beam diameter was 0. The laser power used during the welding was 3. After welding, the samples were cut with wire-electrode cutting from every representative region of the joint. For microstructure observation, cross-section samples vertical to the welding direction were cut from the joints.

To obtain the phase constitution of the weld metal, the weld was cut with wire-electrode cutting into two parts along the central line, and four parts were arranged together to get a thickness of about 8 mm, which was used for the XRD testing.


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The used step size was 0. The sensitivity to common elements is theoretically 0. During analysis, an accelerating voltage of 10 kV was used and the distribution of Cu, Si, Mg, Ag and Fe was measured. A disc with a diameter of 3 mm for transmission electron microscopy TEM observation was polished to 0. TEM observations were operated at kV. To compare the corrosion behavior of different regions of the joint, samples perpendicular to the welding direction were cut and inserted with resin, leaving the cross-section out for corrosion testing.

Before corrosion, the surface was polished and cleaned with ethyl alcohol to keep the surface smooth and clean. The corrosion resistance of the different regions of the joints was estimated by static complete immersion tests and electrochemical measurements. Static complete immersion corrosion tests were carried out in 3. As for electrochemical corrosion testing, samples were cut from the substrate, the heat-affected zone HAZ area and the weld metal, respectively. Samples cut from the substrate were used as the substrate testing samples, and the position of the samples cut was far away from the weld, with a distance of more than 10 mm.

The samples cut from the weld metal along the weld central line were used as the weld metal testing samples. To get HAZ testing samples, the joints were cut from the fusion line down to the middle height and small blocks were obtained, and the distance of the samples was about 1 mm to the central line of the weld. The arrows show the testing surface of each sample.

Electrochemical measurements of open circuit potential OCP and polarization curve of each sample were conducted to estimate the corrosion resistance. The corrosion cell containing mL of an electrolyte was combined with a typical three-electrode configuration at room temperature. A saturated calomel electrode SCE was used as the reference electrode and a platinum plate was used as a counter electrode CE.

The welded joint specimens were employed as working electrodes WE.

After the corrosion tests, the corrosion morphologies were observed with an optical microscope and a scanning electron microscope. Optical microscopy images of different regions of Al—Li alloy laser welding joint. As with other aluminum alloys [ 13 , 14 , 15 ], pores are easily formed during the laser beam welding of the Al—Li alloy filled with ER wires, more pores are observed on the top of the weld and the size is large, compared to the pores on the low part of the weld, as shown in Figure 1 a,b.

It was hard to determine the kindness of the pores according to their distribution location and morphologies; however, according to the analysis of Song et al. Meanwhile, hydrogen is easy to solute into the molten pool at a high concentration and will remain in the weld in the form of pores, since there is not enough time allowing it to escape from the weld.

The evaporation of low-boiling point elements such as Li and Mg also has an influence on the formation of pores in the weld, lower Li and Mg element contents can effectively reduce the pores in the weld [ 17 , 18 ]. The microstructures of different regions in the cross-section of the weld are shown in Figure 1 c—e. It can be seen that distinct differences in the morphology and grain size of different regions existed in the weld.

Next to the fine equiaxed grain region was a columnar grain region with a sub-structure of cellular crystals in the weld. Coarse equiaxed grains with fully developed equiaxed dendrites formed in the center of the weld, because the cooling rate was slow here, and secondary dendrite arms were well-developed in this region, as shown in Figure 1 e. The fine grain structure reflects that the cooling rate and temperature gradient of the liquid metal in this region were high during solidification. Closer to the center of the weld, the cooling rate and temperature gradient were lower.

Coarse grains formed in the corresponding regions, which changed from fine equiaxed grains to columnar grains, consisting of cellular and dendrite, and equiaxed dendrite grain with a large size distributed in the center. Electron microprobe analysis was used to measure the distribution of alloying elements in the weld, and the results are shown in Figure 2.

NEW ALUMINUM LITHIUM ALLOYS FOR AEROSPACE APPLICATIONS

As shown in Figure 2 a, some white precipitates were distributed at the grain boundaries. Figure 2 b—d suggests that some alloying elements, for example Cu, Mg and Si, were segregated in the grain boundaries, while Figure 2 e,f present the even distribution of Ag and Fe in the weld. As known from the chemical composition of the ER Al However, the addition of Cu and Si also promoted the precipitation of secondary phases at the grain boundaries.

The phase constitution of the joint was analyzed by X-ray diffraction analysis and the results are shown in Figure 3. Fewer phases were found in the HAZ area, as shown in Figure 3 b, which shows the solution effect to this area by heat input during laser welding. Its uniform precipitation in base metal can effectively strengthen the alloy after solution and aging treatment.

Transmission electron microscopy was used to observe the morphology of the secondary phases in the weld, and some images are shown in Figure 4.

Aluminum-lithium Alloys: Processing Properties And Applications Hardcover

X-ray diffraction spectrum of the Al—Li alloy laser welding joint. TEM images of the fusion zone of the Al—Li alloy laser welding metal. Figure 4 exhibits the TEM images of grain boundaries and the inner grains of the weld. It can be seen that there were more secondary phases precipitates in the grain boundary area than in the inner grain area in Figure 4 b, as marked by the arrow in Figure 4 a.

The precipitation of the secondary phase needs enough time for their nucleation and growth. However, in welded conditions, as observed in this experiment, the number of secondary phases was low and their size was small, because there was not enough time for nucleation and growth. However, as shown in the TEM images in Figure 4 , less precipitates formed in the inner grain area, and its volume fraction was less than 0. This value was much lower than that in normal Al—Li alloy, as shown in reference [ 22 ]. The EDS measurements of these precipitations are listed in Table 2. The secondary particles at the grain boundaries were large and contained more Cu, but those in the inner grain area were small and contained less Cu.

As the distribution of Cu has an important influence on the corrosion resistance, and the segregation of Cu at grain boundaries will increase its corrosion tendency, thus it can be indicated that the corrosion resistance of the inner grain area was much better than the grain boundary area. Even though lesser precipitation will lead to lower strength, the homogeneous structure will result in higher resistance to corrosion.

EDS analysis of the chemical composition of the precipitation in positions marked out in Figure 4. Also known as Data Search, find materials and properties information from technical references. Visual and interactive search of NIST pure compounds database for chemicals and their properties.

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