2023 年 64 巻 2 号 p. 341-351
Almost 85 years have passed since the development of Al–Zn–Mg–Cu Extra Super Duralumin in Japan in 1936. This alloy was developed by Dr. Igarashi in response to the Japanese Navy’s order to Sumitomo to develop an alloy with tensile strength of 588 MPa (60 kg/mm2) or higher, exceeding Alcoa’s super duralumin 24S. This alloy was then adopted for use in the Zero fighter. Based on this alloy, Alcoa’s 75S was developed in 1943. This paper describes the history of the development of Extra Super Duralumin, starting with Duralumin. The paper also discusses the issues that are considered important for the future development of the aluminum alloys based on the historical review and our study. One is the relationship between quench sensitivity and age-hardening properties of Al–Zn–Mg alloys and the second is the occurrence of shear fracture in the Al–Zn–Mg–(Cu)–Zr alloys. This is the points to be noted in the development of high strength and high toughness Al–Zn–Mg–Cu alloys.
Fig. 9 Development of high-strength aluminum alloys for aircraft in the 20th century.
The purpose of this review is to describe the history of the development of the world’s strongest Al–Zn–Mg–Cu based Extra Super Duralumin (ESD) developed in Japan before World War II. Furthermore, the relevance of ESD to the alloy development after World War II will be clarified. Although ESD is a well-known alloy in Japan because it was used in the Zero fighter, it is almost unknown in other countries.
Before describing the history of ESD, it is necessary to know the alloy development before ESD. For this reason, the history of the development of Duralumin and super duralumin is also described. The development of these alloys is also related to the development of aircraft, which will also be discussed.
It was also discussed the issues that are considered important for the future development of the aluminum alloys based on the historical review and our study. One is the relationship between quench sensitivity and age-hardening properties of Al–Zn–Mg alloys. The second is the occurrence of shear fracture in Al–Zn–Mg–(Cu)–Zr alloys. This is a point to be kept in mind in the development of high-strength and high-toughness Al–Zn–Mg–Cu alloys.
For details of the history of development from Duralumin to ESD please refer to reports in Japanese journals and the report of National Museum of Nature and Science on the survey on the systematization of technology.1–9)
In 1901, A. Wilm was invited to Zentralstelle für wissenschaftliche-technische Untersuchungen in Neubabelsberg near Berlin, and the following year he was commissioned by a German weapons and ammunition manufacturer to develop an aluminum alloy to replace brass cartridge case and began research. He quenched Al–4%Cu alloy in the same way as steel and obtained tensile strength of 152 to 225 MPa (15.5 to 23 kg/mm2) and elongation of 5 to 7%, but it was not enough to replace brass. He continued his research and discovered the age-hardening phenomenon in 1906, in which the alloy hardened when left at room temperature (RT). He prepared 3 mm-thick sheets of an Al–4%Cu–0.6%Mn alloy to which 0.5%Mg was added and quenched them after heating in a salt bath at 520°C.10,11) The results showed that the hardness remained almost unchanged for up to 2 hours after quenching, increased over the next 4 days, and then remained constant. This heat treatment gave tensile strength of 390 MPa (40 kg/mm2) and elongation of 20 to 25%. The age-hardening curves for the Al–3.5%Cu–0.5%Mg alloy published in Wilm’s paper are shown in Fig. 111,12) and on January 11, 1907, a patent application was filed with the claim for “An aluminum alloy containing not more than 2% Mg and not more than 5% Cu, especially effective with 4% Cu and 0.25–0.5% Mg” (DRP204543, approved November 1908). Two related patents were subsequently filed.
Regarding the production of this alloy, a factory trial was made in 1908 at Dürener Metallwerke A.G. in Düren, a sister company of the German Weapons and Ammunition Manufacturing Company, but the laboratory director at the time was not interested in this invention. Wilm negotiated with the institute to industrialize this alloy with his own hands and resigned from the institute with the patent for this alloy in his name. Fortunately, Dürener Metallwerke obtained the right to use Wilm’s patent and succeeded in industrializing this alloy with the cooperation of Dr. R. Beck, a technical officer of Dürener Metallwerke.13,14) In 1909, Wilm and Dürener Metallwerke discussed a trade name for this new product, and Wilm initially suggested “Hartaluminium” with the German word “Hart” meaning hard. However, considering the international market, the name “Duralumin” was chosen, using the French word “Dur” which means hard.13)
2.2 Application of Duralumin to airshipsDuralumin was first used for airships. In 1914, this alloy was registered as a standard for the German Navy’s Zeppelin airships, and Duralumin was used from LZ26 in 1914,15) with 720 tons produced by 1916.16) Zeppelin airships were used when Germany bombed London and Paris in World War I. Inspired by this, military personnel in many countries began to build airships using Duralumin.
In 1916, Alcoa received a request from the U.S. Navy for an alloy that was as strong as or stronger than the alloy used by Germany, and around the same time, the Navy sent Alcoa a part of a framework of a Zeppelin airship that had crashed in France. Based on this information, Alcoa commercialized alloy 17S (Al–4.0%Cu–0.6%Mg–0.7%Mn), similar to Duralumin, with tensile strength of 425 MPa (43 kg/mm2) and yield strength of 275 MPa (28 kg/mm2).17–19) Alcoa was obliged to supply 17S alloy sheets for the Shenandoah, an airship built by the Navy, and by the end of 1922, it was possible to produce 25,000 tons of high-strength alloy sheet annually, and 17S became the mainstay of the market.20)
On the other hand, in Japan, a part of a framework of a Zeppelin airship shot down in England, as shown in Fig. 2, was brought to Sumitomo Copper Plant (hereinafter referred to as Sumitomo) at the end of 1916 via the Naval Ship Administration Headquarters and the Chief Superintendent of the Osaka Navy. S. Sugiura of the Research Section of Sumitomo investigated this material the following year. Based on the results of the analysis of the framework and literature published in the Journal of the Institute of Metals (JIM) in the U.K., he started trial production at the plant, which was completed in 1919 and named “Sumitomo-keigin (keigin means light silver)”.21) The composition of this Sumitomo-keigin was Cu 4%, Mg 0.5%, Mn 1.0%, and Al remaining. Although it is not clear how the Navy requested Sumitomo to conduct this research, Sugiura stated, “There were few research institutes on metals at the time, and Sumitomo may have had an independent research department specializing in metals”.22)
In 1921, Sumitomo produced Duralumin industrially for the first time, and received an order from the Yokosuka Naval Arsenal for a total of about 1 ton of plate and tube rods as structural materials for the lifting bar of the SS-type soft type airship (No. 3 airship) designed by the British Vickers Company, which was produced domestically in the arsenal. Since the manufacturing technology of Duralumin at that time was very rudimentary, the Navy recommended T. W. Pagan to Sumitomo among the engineers they had invited to build flying boats. He had studied under W. Rosenhain at NPL (National Physical Laboratory) in the U.K. and had a grasp of aluminum manufacturing technology in the U.K. He was hired by Sumitomo for eight months from January 1922 and provided instruction in the casting, rolling, extruding, and drawing departments, as well as in the manufacturing of corrugated sheets and profiles.21) After 1923, Sumitomo acquired Duralumin manufacturing technology from Dürener Metallwerke as one of the reparations for World War I and laid the foundation for its manufacturing technology.
It had been a great mystery why duralumin age-hardens at RT. This was answered by W. Rosenhain and his group at the NPL in the U.K., who were researching aluminum alloys. They presented their research during World War I at a meeting of the Institution of Mechanical Engineers in August 1921.23) One of their research group, M.L.V. Gayler, based on the results of her study of age hardening of the Al–Mg–Si alloys, presented that the age hardening at RT of Al–Cu–Mg alloys also involved precipitation of Mg2Si.24) This was because the purity of the ingots at that time was poor and contained many Fe and Si impurities. Stimulated by her presentation, researchers in Europe and the U.S. began researching and developing alloys containing a large amount of Si in Al–Cu–Mg alloys as a super duralumin that exceeds the strength of Duralumin.
Alcoa also developed a super duralumin called 14S (Al–4.4%Cu–0.50%Mg–0.8%Mn–0.8%Si) in 1928. The age-hardening at RT of this alloy is equivalent to that of Duralumin, and, like Al–Mg–Si alloys, this alloy obtained a higher hardness after aging at high temperatures. Later, aging hardening at RT of Al–Cu–Mg alloys was also observed in experiments using high-purity Al with less Si, and it became clear that Si was not the cause of aging hardening at RT. Alloy 14S was not suitable for aircraft exterior panels, which require elongation in forming, but could be used in extruded and forged products that were aged at high temperature.
3.2 Super duralumin born from high-purity ingots25)Alcoa was the first in the world to establish a three-layer electrolytic process for making high-purity Al in the early 1920s and industrialized the production of ingots with a purity of 99.8% or higher.26) Experimental results using high-purity ingots revealed that the age-hardening at RT of Al–Cu–Mg alloy could be increased by increasing Mg from 0.5% to 1.5% in Duralumin. Alcoa commercialized this alloy in 1931 as 24S (Al–4.4%Cu–1.5%Mg–0.6%Mn). The reason why Mg could not be increased immediately after the development of Duralumin is thought that when Duralumin contains a large amount of impurity Fe or Si, they combine with Cu or Mg to form coarse compounds in the Al7FeCu2 and Mg2Si during casting. These compounds are not easily dissolved in solution treatment and reduce the solid solution of Cu and Mg, resulting in decreasing strength. This is thought to be the main reason why the strength of the alloy was not improved. Another possible technical limitation is the difficulty of casting and rolling due to the increased Mg content. Alcoa 24S is now 2024, which is still used for aircraft exterior panels. Today, the term “super duralumin” often refers to 2024.
Figure 3 shows the effect of Mg addition on the aging hardening of Al–4%Cu–Mg alloys at room temperature when high-purity Al is used, as reported by Alcoa researcher, W.L. Fink.27,28) However, it is not clear why the addition of Mg increases the strength of Al–Cu, and the aging rate increases with increasing Mg addition.
In Japan, as in the U.S. and Europe, Si-containing super duralumin was developed based on the use of domestic ingots with high impurities. However, the Japanese Navy switched to 24S, the same as Alcoa, because bauxite, which can produce ingots with fewer impurities, was expected to be stably available from the Dutch island of Bintan.25) Figure 4 shows the results of Dr. Igarashi’s research in 1935, the year Sumitomo started production of 24S, and the effect of Cu and Mg contents on the tensile strength of Al–Cu–Mg alloy sheets using 99.8% ingot.29,30) The optimum alloy composition is almost the composition of 24S. It is assumed that Alcoa probably had such data to determine the composition of 24S.
At the same time as researching and developing 24S, Dr. Igarashi was also testing the addition of Cr to improve corrosion resistance.30–32) In particular, it was found that the addition of 0.30%Cr to the cladding on 24S sheets prevented the diffusion of Cu from the core into the cladding compared with Alclad 24S-T432) shown in Fig. 5. Figure 5(a) shows the schematic figure of Alclad 24S (cladding of pure aluminum on 24S). (b) the microstructure of Cralclad on 24S. Cralclad was named by Dr. Igarashi.30,31) This was because, at the time, the diffusion of Cu into the cladding and the formation of diffusion zone shown in Fig. 5(a)32) during solution treatment was a problem that reduced the corrosion resistance of the cladding. Based on these test results, in December 1935, a new super duralumin was developed by adding 0.13% Cr to the core 24S and 0.23% Cr to the cladding, and it was found that the Cr addition was effective in suppressing diffusion into the cladding.30,31) Such research could be used for corrosion control of 24S sheets, which is still a problem today. It will need to be reviewed from today’s perspective.
In 1935, when Sumitomo started manufacturing 24S as a super duralumin, the Navy instructed Sumitomo to develop an alloy with a strength of 588 MPa (60 kg/mm2) or higher, exceeding Alcoa’s 24S. Dr. Igarashi, who took charge of this development, wrote an internal research report No. 3326 “Exploration of Strong Light Alloys, No. 1” (August 10, 1935) and declared the start of research. In this report, he described he began to examine Rosenhain’s E alloys (Al–20%Zn–2.5%Cu–0.5%Mg–0.5%Mn) in the U.K., Sander’s alloys (Al–8%Zn–1.5%Mg–0.5%Mn) in Germany, and Al–Zn–Mg alloys with the addition of a fourth element (Cu, Si, Fe, Ni, Mn, Sn, Cr, Mo, W and Li).
In the development of the new alloys, the February 1936 report stated that systematic experiments were conducted with different ratios of E alloy (E), Sander alloy (S), and super duralumin 24S (D) to investigate the possibility of an Al–Cu–Zn–Mg quaternary alloy with excellent performance, and the alloy compositions were determined based on strength and workability. Table 1 shows the relationship between the composition ratio, strength, and hardenability of D-alloy, S-alloy, and E-alloy.30) Hardenability is an indication of whether the alloy can be industrially manufactured. The hardenability was calculated by dividing (maximum hardness - annealed hardness) by annealed hardness, and the larger the hardenability, the better the workability. As a result, three alloys were selected, and their mechanical properties were evaluated in sheets and extrusions. In June of the same year, the above alloys were named ESD alloys after the E, S and D alloys. Elements such as Ni, Fe, Cr, Ca, Ti, and V were further added to the above alloys, and their mechanical properties and resistance to seawater were investigated. In July, the corrosion loss of the cladding sheet with Cr after immersion in corrosive solution was measured and it was reported that it was significantly improved compared to the core sheet as well as Cralclad on 24S.
The internal research report from August 1936 to February 1937 summarized the results of systematic experiments on stress corrosion cracking (SCC) of ESD alloys. SCC was also a serious problem with British E alloys and German Sander alloys and was the reason why they could not be put into practical use. At that time, SCC was called “season cracking”, the cause of which was unknown, and no evaluation method had been established. Dr. Igarashi et al. started by establishing evaluation methods. One of the methods is the U-bend test, as shown in Fig. 6, described in the internal research report No. 3939.4,8,9) In this report, it was described a specimen 25 mm wide and 180 mm long was bent at both ends and bound with copper wire at 15 mm from the end as shown in Fig. 6, and the specimens were left in a room with the distance between the two ends varied to change the stress applied to the bending tips, and then the occurrence of cracks was investigated. Fortunately, around 1928, Sumitomo had experienced difficulties in dealing with cracks that occurred in brass tubes used for condensers in destroyers, and the evaluation method used in this case is said to have been useful. As a result of these evaluation tests, it was found that the addition of Cr was very effective against SCC, as shown in Fig. 7.30) In February 1937, the alloy composition of ESD was determined to be Al–8%Zn–1.5%Mg–2%Cu–0.5%Mn–0.25%Cr, and tensile strength was 570 to 590 MPa, yield strength 470 to 510 MPa, and elongation 10 to 16% in the trial production of sheets in the plant.
Effect of Cr addition on season cracking (SCC) of Al–Zn–Mg–(Cu) alloys.30)
It is interesting to note that the patent was filed on June 9, 1936, as “Strong Light Alloy for wrought products”. It mentions Cr addition to prevent season cracking (corresponding to the “SCC”). As far as the internal research report is concerned, the chemical composition of the main alloy was almost determined in June 1936, and only the resistance to seawater was examined, but there was no evaluation of season cracking. However, it is assumed that the results of the 24S cladding sheet and brief evaluation tests of SCC showed that Cr was effective in suppressing SCC. I believe that the subsequent research was more like a search for a better alloy through systematic testing.
SCC is intergranular cracking that occurs at grain boundaries. It is especially likely to occur in T4 and in T6 temper, which have the highest strength. Cr and recently Zr are added to prevent SCC, but the addition of these elements tends to cause the development of subgrain boundaries (low-angle grain boundaries) even after solution treatment. Low-angle grain boundaries have narrower PFZs (Precipitate Free Zones) than high-angle grain boundaries, resulting in less grain boundary precipitation. PFZs tend to cause intergranular corrosion due to potential differences in regions with fewer solute atoms than those within the grain. Cr addition is thought to suppress intergranular corrosion, which is localized corrosion, by narrowing the width of the PFZ. Dr. Igarashi once said that he added Cr as the starting point of corrosion to let the whole surface corrode without localized corrosion. But if we consider this hypothesis based on current knowledge, it may have been correct in the sense that the addition of Cr suppressed the formation of PFZ and made the microstructure uniform over the entire surface, suppressing localized corrosion, regardless of the degree of corrosion. SCC was not completely prevented by the addition of Cr, but it was greatly reduced by the addition of Cr. Recently, hydrogen is said to be involved in SCC, but the relationship with SCC is not clear. The role of Cr on SCC still has many issues to be investigated.
4.4 Factors behind the development in a short period of timeAs for why the world’s strongest aluminum alloy was developed in less than a year, ahead of the rest of the world, the question was summarized by Terai, Senior Managing Director of Sumitomo Light Metal and the former director of the Technical Research Laboratories.33) I think the following factors can be considered.
First, the Navy’s needs and target values were clear. Second, as can be seen in Fig. 4, it was difficult to achieve strength by extending Super Duralumin, and the addition of Zn, as in E alloys and Sander alloys, was considered essential to obtain even higher strength, which is related to the high solid solubility of Zn. In this sense, Dr. Igarashi predicted that the answer would be found in changing the ratio of these alloys based on E, S, and D alloys. To develop the strongest alloy, one must have a good understanding of the characteristics of each alloy. His extensive research on various aluminum alloys in general, as well as Cu alloys and Mg alloys in terms of strength and corrosion resistance, prior to his research on ESD, was of great help.
Third, he was the first to establish an evaluation method for SCC and to find the conditions under which cracking occurs, and he was the first to discover the effectiveness of Cr addition as a result of his research. In his research, he always said, “Never take an immature theory as absolute. When in doubt, you must repeatedly and thoroughly investigate, asking why and how it happens. Progress is made and development is achieved only when facts are shown that contradict the expected results”.
Fourth, there was Goro Kitahara, who was a skilled experimenter. Most of Igarashi’s experiments were conducted by Kitahara. The combination of a researcher who has a bird’s-eye view of experimental results and can formulate a policy and a researcher who can perform excellent experiments is the strongest.
Fifth is the cooperation of the manufacturing site. Practical application cannot be achieved without the cooperation of manufacturers. Mutual trust is important, and manufacturing also responds to the demands of research by advancing manufacturing technologies, such as continuous casting technology, improved rolling workability, and increased rolling speed.
4.5 Application to the Zero fighterJiro Horikoshi, the chief design engineer of Mitsubishi Heavy Industries learned of the ESD invention and adopted it for the spar of the Zero fighter’s main wings. In 1938, Horikoshi, who had been trying to reduce the weight of the Zero fighter prototype, the Prototype 12 Fighter, to the nearest gram, came across the Sumitomo’s ESD while he was searching for an even better and lighter alloy than 24S, as follows;
“One day, Mr. Kimura, the engineer in charge of purchasing materials for the company, wandered over to Mr. Horikoshi’s desk and said, ‘Mr. Horikoshi, I hear that Sumitomo is on the verge of producing an extremely strong new alloy’. According to the story, they had developed a stronger alloy by slightly changing the composition of conventional super duralumin and were in the process of putting it into production on a trial basis. I was intrigued by this story. I contacted Sumitomo Metals and received a reply that they would like to show me the actual material while the person in charge explained it to me directly. I immediately flew to Sumitomo’s plant in Osaka, where Dr. Igarashi and Mr. Ozeki told me, ‘Sumitomo is responsible for guaranteeing the strength of 60 kg/mm2. Although it has not yet been adopted as a material standard by the Navy, the problem of season cracking has already been technically solved as far as extruded shapes are concerned’. After being shown the actual materials, I decided that I could use them. I also asked him about the points that needed to be considered when using the new alloy. For the time being, I calculated the rough weight of the wing spars, assuming that only the wing spars would be made of extruded shapes and found that the weight would be reduced by 30 kg. So, we requested the use of this new alloy to the Aeronautics Headquarters. The Aeronautics Headquarters had already taken notice of this alloy and was one step away from approving it. The Navy was rather pleased with the request and approved the use of this new alloy”.34–36)
The Prototype 12 Fighter was renamed the Zero Fighter, which succeeded in significantly reducing its weight, achieving spectacular results in the early stages of World War II. However, the U.S. military succeeded in unlocking the secrets of the Zero Fighter by capturing an intact Zero Fighter that crash-landed on Akutan Island in the Aleutian Islands in 1942. One of the secrets was the highest strength aluminum alloy, ESD. The U.S. military had Alcoa produce an equivalent alloy. Thus, Alcoa developed the alloy 75S (7075, Al–5.6%Zn–2.5%Mg–1.6%Cu–0.23%Cr) in 1943.
Dr. E. Hata, who was Managing Director of Sumitomo Light Metal and General Manager of the Research Department, wrote the following in the Technical Report of Sumitomo Light Metal. “After the war, an officer of the U.S. Army once came to the Nagoya Works to meet the 75S inventor. Dr. Igarashi had already left for Tohoku University. I had heard rumors that the U.S. had created the 75S after learning that ESD was used in the captured Zero Fighter, but it was only then that I learned that they officially recognized Dr. Igarashi as the inventor of the 75S”.37) This is a valuable testimony that proves that the U.S. military developed the 75S by analyzing the materials of the downed Zero Fighter.
Professor Emeritus Goro Ohira stated the following in the 75th anniversary commemorative publication of the Department of Metallurgical Engineering, Tohoku University (2000, p. 112).
“One day after Professor Igarashi’s arrival, a high-level engineer from the U.S. Naval Research Laboratories and a researcher from the Aluminum Research Laboratories of Alcoa suddenly arrived, asking questions about ESD. They asked the Professor Igarashi if he would come to the U.S. immediately. The professor’s only reply was, ‘What good would it do for a person from a losing country to go to a winning country? I refuse’. As a result, two or three engineers visited him once or twice a month, sometimes three times a month. I think this went on for almost two years. The professor had told me that they would probably come back to hear about this next one just because of the work he had done, and I admired that he was largely right in his prediction”.38)
Dr. Tanabe, who served as director of the research department at Sumitomo Metals until 1941, stated, “It is difficult to compare the superiority of the two alloys in detail, comparing 5.5% Zn in 75S with 8% Zn in ESD, it is undeniable that the former is superior in many respects, and the extruded shapes mainly used in Japan had tensile strength of 570 MPa (58 kg/mm2) or more as a standard. However, the tensile strength of 75S (530 MPa (54 kg/mm2)) would not make any difference in practical use. This is a common practice in Japan, where excessive strength is demanded. Although cladding sheets were rarely used in Japan, I am sure that they are used to a considerable extent in the U.S., and I would like to know the actual situation. In the U.S., 0.3 mm sheets are also stipulated, but we stopped at 0.5 mm, and although there are differences in composition and manufacturing methods, the U.S. is one step ahead in manufacturing”.39) He expressed his respect for Alcoa’s manufacturing technology. This was because Alcoa’s continuous casting and hot tandem rolling technologies were far ahead of those of Japan.
5.2 Differences between ESD and 75S (7075)The compositional comparison of Sumitomo’s ESD and Alcoa’s 75S is shown in Table 2 together with current AA international standards.39) Mn was treated as an essential element for both ESD and 75S. Alcoa’s first patent on Al–Zn–Mg alloys with the addition of Cr was U.S. Patent 2.240.940 (filed Sep. 28, 1940, granted May 6, 1941). However, no alloys with Cr addition were described as examples. Alcoa’s patent application was filed after Sumitomo’s U.S. patent was granted (July 18, 1939).
Table 2 shows Zn is almost below the lower ESD limit and Mg is above the upper ESD limit, which is outside the ESD composition. Is this coincidental or intended? Since Alcoa also investigated the strength properties of Al–Zn–Mg–Cu alloys at the same time, it is possible that this is a coincidence, but I believe it is more likely that the new alloys were investigated in the Japanese literature at the stage of determining the composition of 75S. A table in a paper published by Alcoa researcher E.H. Dix in 1949 shows the composition of ESD in comparison with 75S,40) and the reference cited is the Igarashi and Kitahara’s paper “On Extra Super Duralumin ESD and ESDC”, published in the September 1939 issue of the Journal of the Society for Aeronautical Sciences of Nippon.41) This paper describes all the chemical composition, mechanical properties, and SCC properties of ESD and cladding alloy ESDC. It is clear that Alcoa could have made something equivalent to ESD if it had followed the Japanese literature carefully. However, Alcoa announced the development of 75S in 1943, and no chromium-added Al–Zn–Mg–Cu alloys had been found before then. Alcoa probably did not examine Japanese literature until 1942, when the Zero was captured and pointed out to the US military.
J.T. Staley, who played a central role in Alcoa’s development of aircraft materials in the 1960s to 1980s, wrote in 1989 in “History of Wrought-Aluminum-Alloy Development” that Alcoa developed 7076 (Al–7.5%Zn–1.6%Mg–0.7%Cu–0.6%Mn) as a forging alloy in 1940, and used it for propeller blades because it had better wear resistance and fatigue resistance than the 2025 alloy (Al–4.25%Cu–0.8%Si–0.75%Mn). By 1938, the laboratory had solved the problem of SCC in plates, so X74S (Al–5.2%Zn–2.1%Mg–1.5%Cu–0.4%Mn) was used for some blades, but field tests on this alloy sheet showed that it was sensitive to SCC in the heat-treated temper of plastically deformed sheet. Therefore, the influence of trace elements with high melting points was investigated, and it was found that alloys containing 0.2 to 0.35% chromium were more resistant to SCC. In 1943, Alcoa developed 7075-T6, which is well known today, with increased strength and good resistance to SCC. He then stated, “Interestingly, chemical analysis of sheet from a downed Japanese Zero Fighter aircraft disclosed that the composition was almost the same as that of 7075”. The new alloy was soon used in the design of the fuselage.
This may be the first time that an Alcoa researcher has described the Zero Fighter.19) It is understandable that inventions usually reach the same conclusion as a result of similar research conducted at the same time, as seen in the inventions of molten salt electrolysis of aluminum by C. H. Hall and P. L. T. Héroult. However, there is almost no published data on what kind of research Alcoa conducted to develop the 7075.
After the war, GHQ (General Headquarters, the Supreme Commander for the Allied Powers) issued a prohibition on aviation and prohibited research, development, and manufacturing of aircraft in Japan. As a result, many researchers and engineers who were involved in aircraft development during World War II were forced to move to the development of rolling stock, automobiles, and motorcycles.
In the field of rolling stock, it was discovered that an Al–Zn–Mg alloy with excellent extrudability and weldability could be used for rolling stock, and this alloy was used in Shinkansen cars. This alloy is in the same vein as the Al–Zn–Mg alloy, HD alloy (named Honda Duralumin after Prof. Kotaro Honda), which was developed jointly by industry and academia under the auspices of the Army during World War II to improve the extrusion properties of ESD.
However, because the addition of Cr makes the alloy inferior in quench sensitivity, Dr. Yoshio Baba of Sumitomo Light Metal Industries, Ltd., and others were the first in the world to develop an Al–Zn–Mg alloy containing Zr with good strength that can be quenched even by air cooling and good weldability. This is 7N01 (now AA7204) registered with JIS (Japanese Industrial Standards) in 1970. Furthermore, alloy 7003 was developed by reducing the amount of Mg and increasing the amount of Zn in 7204 to further improve extrudability and became the first AA (The Aluminum Association) internationally registered alloy in Japan. This alloy made it possible to produce large, wide shapes. This alloy 7003 was widely used not only for Shinkansen cars but also for strength members for containers, vans, trailers, etc., and also for motorcycle and bicycle rims and automobile bumpers, etc., because of its capability for hollow extrusion.42) The development of this alloy was as original and world leading as ESD. The addition of Zr was also applied to aluminum alloys for aircraft, and most 7000-series alloys nowadays contain Zr.
In the field of aircraft, other than the YS-11, Japan has developed few aircraft of its own, and it must be said that there are no aircraft alloys developed in Japan other than 2013, which was developed as an alternative alloy to 2024 alloy.43)
6.2 Development of aircraft alloys in the worldTable 3 shows first flight year and aircraft in which the new alloys began to be applied.9,44) The alloys, 7150 and 7055 improved strength and toughness by changing the composition of Zn, Mg, and Cu in 7075 and reducing impurities. These alloys have improved quench sensitivity and resistance to SCC by replacing a trace amount of Cr with Zr. The development of these alloys is the result of postwar research and development by Alcoa and others.
Figure 8 shows the relationship between yield strength and fracture toughness.44,45) In general, the fracture toughness decreases inversely with increasing strength, but this inversely proportional relationship has been increasing steadily since the end of World War II. This is because the toughness of materials has been improved by decreasing impurities and by controlling the microstructure through Thermo-Mechanical Treatment. In recent 7000 series alloys, Zn content has increased to 7–10%, and since the Zn content of ESD was 6–9%, it can be said that the world has finally reached the level of ESD in terms of alloy composition. Figure 9 shows the development of high strength and high toughness alloys for aircraft in the 20th century. It can be seen that ESD is prominent, although it was only used for a short period of time during war.
Development trends of high-strength aluminum alloys for aircraft in the 20th century.
What is important in the invention of Duralumin is why the Al–Cu alloys do not show significant age hardening at RT, while the Al–Cu–Mg alloys with Mg exhibit significant strength after about 4 days. Another major question is why increasing Mg, as in 24S, increases the aging rate and why strength is increased. In response to this, Prof. Sato et al. suggest that the strength increases due to the formation of fine nanoclusters and GP zones because of the strong attraction between Cu and Mg as a result of first principle calculations,46) but it is also necessary to consider the chemical bonding properties and phase stability with temperature change.
With regard to quenching, it has been believed that quenched-in excess vacancies play a major role in age hardening and that water quenching increases quenched-in vacancies and accelerates age hardening.11) However, as shown in Fig. 10, even if the Al–Zn–Mg alloy studied by us is cooled slowly as in furnace cooling (FC), room temperature age-hardening occurs as in water quenching (WQ), and strength at 120°C aging was almost equivalent to that of water cooling.47) This was pointed out more than 50 years ago by Dr. Baba.48) These results indicate that, it is necessary to reexamine whether quenched-in vacancy, vacancy cluster and their secondary defect in the atomic vacancy theory, really contribute to age hardening, including the existence of vacancies themselves.
Effect of cooling rate from solution heat treatment on Vickers hardness of an Al–6%Zn–0.75%Mg alloy aged at 20°C-10080 min pre-aging followed by 120°C-1440 min, 160°C-500 min and 200°C-50 min aging, (WQ: water quenching, AC: air cooling, FC: furnace cooing).47)
The strength of Al–Cu–Mg alloys decreases with slow cooling because the stable phase, such as S (Al2CuMg) phase, precipitates during cooling, reducing the number of supersaturated solid solution atoms.49) On the other hand, in the Al–Zn–Mg alloy, high strength was obtained by FC because the stable phase precipitated only slightly at the grain boundary even after FC due to the large solid solution limit. This is because, as in the water-cooled material, most of the solute atoms in the grains remain as a homogeneous supersaturated solid solution after FC, and strength almost equal to that of the water-cooled material is obtained by aging treatment.47) The alloy exhibits age hardening if there is some degree of supersaturated solid solution even after FC, as in the case of a softening process at 350–410°C for 2 hours followed by FC.50) The age hardening of Al–Zn–Mg alloys is difficult to explain by the conventional theory of age hardening. A very similar phenomenon is also observed in Al–Li alloys, which have a strength equivalent to T6 only by FC without high temperature aging.51,52)
The strength of a material is generally considered to be related to the bonding state and electronic structure of its atoms. Recently, Dr. S. Yamamoto et al. proposed that the chemical bonding theory (metallic bonding, electrostatic bonding, and covalent bonding), which focuses on valence electron, unoccupied valence electron and electronegativity can explain the strength of metallic material and atomic diffusion in metals.53) Age hardening is a hardening phenomenon caused by the addition of electrostatic bonding due to the difference in electronegativity to metallic bonding.54,55) The electrostatic bonding is also associated with the atomic population calculated by the molecular orbital method such as extended Hückel method.55,56) We believe that such an approach from chemical bonding based on such quantum chemistry is becoming necessary.
7.2 Development of high strength and high toughness alloyFuture material development will likely include performance improvement of conventional aluminum alloys and adoption of Al–Li alloys and CFRP composite materials, and it is expected that each material will be separated based on cost and performance. In terms of performance improvement of conventional Al–Zn–Mg–Cu alloys, we believe that there is still potential to improve their performance. Dr. Igarashi once said that 785 MPa (80 kg/mm2) could be produced. However, as shown in Fig. 8, this makes it difficult, because as strength increases, toughness, and ductility decrease. One of the causes of reduced ductility and toughness is the cast microstructure, which is thought to be due to the segregation of impurities and highly concentrated components and their compounds that remain at the grain boundaries even after rolling and heat treatment. The solution to this problem is to break the cast microstructure by applying huge strain to the casting microstructure, such as by torsion, to disperse the segregation and the compounds at the grain boundaries within the grains.
Figure 11 shows the effect of impurities on fracture toughness of alloys 7075 and 7150 (Al–6.2%Zn–2.4%Mg–2.2%Cu–0.12%Zr).57) The toughness of 7075 improves with reduced impurities, while 7150 does not necessarily improve with T651. Figure 12 shows SEM images of fracture surfaces of 7075 and 7150-T651 plates manufactured using 99.9% high-purity ingots after fracture toughness tests.57) Both plates show shear fracture, but the Zr-added 7150 plate has a smooth fracture surface, while the Cr-added 7075 plate has numerous fine dimples at intervals of 0.1 to 0.5 µm. These dimples are thought to correspond to the dispersion of Cr-based compounds; 7075 has a large amount of E phase (Al18Mg3Cr2) compounds with a diameter of 0.1 to 0.2 µm, which was precipitated by the homogenization process of the ingot. This compound is not consistent with the matrix. On the other hand, in 7150, fine Zr-based compounds with 0.01–0.02 µm diameter are dispersed and consistent with the matrix. Therefore, as shown in Fig. 13, when the cooling rate of alloys containing Cr based compounds slows down, solute atoms, Zn, Mg, and Cu precipitate at the interface of the Cr based compounds, and PFZs with depleted solute concentration are easily formed around the compounds.57) On the other hand, this is less so in Zr based compounds. This is the reason why the quench sensitivity of 7075 is high and why it is difficult to obtain strength when the cooling rate is slow.
Effect of impurities on fracture toughness of alloys 7075 and 7150.57)
Fracture surfaces of 7150 (a) and 7075 (b) -T651 plates after fracture toughness test.57)
Precipitation around (a) Zr compounds in 7150 and (b) Cr compounds in 7075 after air cooling (1°C/sec) and aged at 120°C-24 h and 154°C-20 h.57)
However, the formation of PFZs around the compounds provides stress relaxation during deformation, which is useful for dimple formation. In general, shear deformation is likely to occur when there are many solute atoms in solid solution, such as T4, which is age-hardened at room temperature after quenching, or T6, which is peak-aged at high temperature.58) When shear zones are formed, solute atoms diffuse and segregate into the shear zones during deformation, making them more likely to fracture. For this reason, overaging such as T7351 and T73651 (currently T7451) is used to reduce the solute atoms and coarsen the precipitates, and as a result, suppress shear failure. The highest strength T651 has relatively more solute atoms and as a result, shear zones occur easily. The reason why the toughness of 7150-T651 was not improved may be due to the absence of PFZ around the Zr-based compound. The addition of Zr to 7150 was to refine the grains and reduce the quench sensitivity. Although it is difficult to achieve both high toughness and high strength by suppressing shear fracture and quench sensitivity, we believe that this is an issue that must be considered to achieve high strength.
The strength target of 60 kg/mm2 was achieved by increasing the zinc content. In addition, the addition of chromium also suppressed SCC. These two points are the major features of ESD, and this was the first development in the world achieved in Japan, which was said to have been lagging behind. Alloy development in the postwar world has progressed in the direction of zinc-rich alloys, such as this ESD. In this sense, ESD was an advanced development. Since the development of aircraft was banned in postwar Japan and few aircraft were developed, it was not possible to develop alloys beyond ESD. Alloy development in Japan shifted from aircraft to rolling stock, automobiles, and motorcycles. If aircraft development progresses in Japan in the future, we are confident that the development of high-strength, high-toughness alloys that exceed ESD will be realized.
I thank Mr. S. Hirano, Director, Managing Executive Officer of UACJ Corporation and Chief Executive of Research & Development Division, for his support of this presentation.
