Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/047—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with magnesium as the next major constituent

Definitions

• This invention relates to an aluminum alloy sheet which has excellent high-speed superplastic formability, and more specifically, to an Al—Mg alloy sheet which enables superplastic forming at high strain rate of 10 ⁇ 2 to 10 0 /s; and also to a process for forming the same.
• an aluminum alloy sheet containing 2.0 to 6.0% of Mg, 0.0001 to 0.01% of Be, and 0.001 to 0.15% of Ti, with Fe and Si as impurities being controlled each at 0.2% or less and the largest grain diameter of impurity-based intermetallic compounds limited to 10 ⁇ m or less is proposed in Japanese Patent Application Laid-Open No. 72030/1992. While such a product does show an elongation of 350% or more at a strain rate of 10 ⁇ 3 /s under a high-temperature deformation condition of 400° C., the elongation decreases as the forming speed increases and becomes insufficient at strain rates of 10 ⁇ 2 /s or higher.
• Another aluminum alloy sheet proposed in Japanese Patent Application Laid-Open No. 318145/1992, contains 2 to 5% of Mg, 0.04 to 0.10% of Cu, as well as optional small quantities of certain transition elements, Cr, Zr, or Mn; with Si and Fe as impurities being controlled at 0.1% or less, and at 0.15% or less, respectively; while controlling the crystal grain diameter at 20 ⁇ m or less and maintaining the grain diameter and the cubic ratio of transition metal-based intermetallic compounds within certain specific ranges.
• Such an alloy sheet also has a limited application range of strain rates in the order of 10 ⁇ 4 /s, and is not suitable for high strain rate superplastic forming at a higher strain rate.
• the present invention has been achieved as a result of diverse examination and exhaustive experiments concerning the relationships of superplastic formability with various alloy constituents and their quantitative combinations, in addition to those with impurity content and their distribution, as well as with crystal grain diameters of impurity-based intermetallic compounds, made in an attempt to overcome the aforementioned shortcomings of the Al—Mg superplastic aluminum alloy.
• the object of the present invention is to provide, by identifying a particular distribution and crystal grain diameter range for Al—Fe—Si compounds to be controlled based on restriction of Fe and Si as impurities, an aluminum alloy sheet that has excellent high strain rate superplastic formability with sufficient elongation in a forming process with a high forming speed such as at a strain rate ranging from 10 ⁇ 2 to 10 0 /s, as well as to provide a processing method for forming such an aluminum alloy sheet.
• the aluminum alloy sheet with excellent high strain rate superplastic formability in the present invention comprises 3.0 to 8.0% Mg, 0.001 to 0.1% Ti, small amounts of Fe and Si (as impurities), each 0.06% or less, the balance being Al and unavoidable impurities, wherein the number per square millimeter of grains of an Al—Fe—Si compound existing in the matrix structure of said alloy and having a diameter of 1 ⁇ m or above is 2000 or less, the mean crystal grain diameter is 25 to 200 ⁇ m and the elongation is 350% or more as worked at 350 to 550° C. and a strain rate of 10 ⁇ 2 to 10 0 /s, all of the foregoing constituting the basic features of the invention.
• this alloy may further comprise 0.05 to 0.50% of Cu in addition to Mg and Ti as described above; or may comprise either one or both of Mn or Cr not exceeding 0.10% each in addition to Mg and Ti as described above, or alternatively, in addition to Mg, Ti, and Cu as described herein.
• the processing for fabricating the aluminum alloy sheet with excellent high-speed superplastic formability in the present invention is characterized by working an aluminum alloy sheet prepared in accordance with the invention at 350 to 550° C. and a strain rate of 10 ⁇ 3 to 10 0 /s.
• Mg acts to recrystallize the alloy during the high-temperature deformation.
• the preferred content range is between 3.0 and 8.0%, below which the effect on promoting the recrystallization is insufficient while a content in excess of 8.0% acts to reduce hot workability of the material.
• Cu acts to improve the superplastic elongation of the Al—Mg alloy system.
• the preferred content range is between 0.05 to 0.50%, where a content below 0.05% fails to give sufficient elongation while a content in excess of 0.50% acts to reduce the hot workability.
• Ti acts to turn the ingot crystals into finer grains and to provide the alloy with a better superplastic formability.
• the preferred content range is between 0.001 to 0.1%, where a content below 0.001% will fail to give the expected effect and a content in excess of 0.1% will yield coarse compounds that hinder workability as well as ductility.
• Mn and Cr act to make recrystallized grains finer in the alloy recrystallization process that occurs during high-temperature deformation.
• the preferred content range is below 0.10% for each, where a content in excess of 0.10% will act to increase a constituent particle whose grain diameter is 1 ⁇ m or above to decrease the superplastic formability of the alloy.
• Fe and Si are impurities each at 0.06% or less.
• These impurities form an Al—Fe—Si compound that is insoluble and prone to precipitate along the grain boundary, increasing cavities and thereby impairing the superplastic elongation.
• the Fe and Si should each be controlled at 0.05% or less. It is also noted here that up to 50 ppm of Be may be added to prevent oxidation of the molten metal, just as in the case of ordinary Al—Mg alloys.
• the Al—Fe—Si compound present in the alloy matrix gives rise to the above mentioned problem, it is better to allow as little of such a compound as possible, and, in particular, the limit in terms of number per square millimeter of an Al—Fe—Si compound having a grain diameter of 1 ⁇ m or more should be 2000 or less, since particles in excess of 2000 per square millimeter will increase cavities and thereby impair the superplastic elongation.
• the original mean crystal grain diameter of the aluminum alloy sheet It is essential to regulate the original mean crystal grain diameter of the aluminum alloy sheet within a range of 25 to 200 ⁇ m. If the original mean crystal grain diameter is below 25 ⁇ m, the original crystal grains will be recreated when recrystallization occurs during high temperature deformation, making it difficult to obtain a recrystallized structure with clean crystal grains as a result of a recrystallization process to obliterate the grain boundary with precipitation of the aforementioned insoluble compounds. If the original mean crystal grain diameter exceeds 200 ⁇ m, the shearing deformation within the crystal grains becomes more prominent with increasing deformation rate, causing the crystal grains to rupture more easily, thus suppressing the superplastic elongation.
• the preferred range of the strain rate during the forming process is between 10 ⁇ 3 to 10 0 /s, where a rate below 10 ⁇ 3 /s will cause the crystal grains to become coarser, reducing elongation, while a strain rate exceeding 10 0 /s creates a shearing deformation within the crystal grains causing cracks, or forms precipitation along the grain boundary, reducing elongation.
• an aluminum alloy material with the above mentioned composition is melted, cast, and homogenized according to a conventional method. It is preferable to carry out the homogenizing process at a temperature between 450 to 550° C. At temperatures below 450° C., Mg or Cu that are formed along the grain boundary or the cell boundary of the ingot by segregation will not be fully dissolved and may contribute to cracks in a subsequent hot rolling step. Conversely, at temperatures exceeding 550° C., the Al—Mg or Al—Mg—Cu crystallization products will cause a eutectic fusion thereby giving rise to cracks during the hot rolling process.
• the ingot is hot-rolled to obtain a structure suitable as a forming material. While the required starting temperature for hot rolling is between 250 to 500° C., it is preferable to start just under 400° C. If the hot rolling process is started at a temperature below 250° C., the deformation resistance is too high, making proper rolling difficult. If the rolling temperature is too high, this could alter the distribution form of the precipitation, thereby making it difficult to obtain the required crystal grain structure as well as proper distribution of precipitated compounds.
• a cold rolling is provided.
• an intermediate annealing may be provided as necessary.
• the final annealing of the cold rolled stock should be provided at a temperature between 350 to 550° C. If the annealing is performed at a temperature below 350° C., the isotropy created during the cold rolling process may not completely disappear; if higher than 550° C., a local melting may occur at the recrystallization boundary. As such, it is preferred to conduct the final annealing in a rapid annealing process such as continuous annealing.
• the Al—Fe—Si compounds present in the matrix are controlled within certain specific distribution while maintaining the crystal grain diameter within a certain specific range, resulting in such alloy structure and characteristics to produce cleaner grain boundaries with less compounds formed along these bounderies to suppress cavity formation.
• Recrystallized grains having an average diameter of 20 ⁇ m or less are formed during a high-temperature deformation, thereby achieving an excellent elongation of 350%, or more preferably, 380% or greater, in high speed forming at a strain rate of 10 ⁇ 2 to 10 0 /s in a temperature range of 350 to 550° C.
• Al—Mg based aluminum alloys having compositions as listed in Table-1 below were melted and cast into ingots via a DC casting method.
• the resultant ingots were homogenized at 530° C. for 10 hours to a thickness of 30 mm, and then hot rolled at 390° C. to a thickness of 4 mm.
• the sheets were subsequently cold rolled to a thickness of 2 mm and then rapidly annealed by heating rapidly to 480° C. and holding at this temperature for 5 minutes.
• Specimens prepared from the test materials produced in the above process were evaluated by a tensile test at a strain rate of 10 ⁇ 2 /s at 480° C.
• Table 1 lists the average crystal grain diameter for each specimen(as measured at the sheet surface), the number per square millimeter of grains of the AL—Fe—Si compound having a diameter of 1 ⁇ m or above, and the elongation measurement results. Note here that the grain count of the compound was made using image processing.
• Al—Mg based aluminum alloys having compositions as listed in Table 2 were melted and cast into ingots in the same manner as in Examples 1, and made into 2-mm thick test materials using the same process as in Examples 1. Specimens were then evaluated in the same tensile test under the same conditions. Table 2 lists the average crystal grain diameter, the number per square millimeter of grains of the AL—Fe—Si compound having a diameter of 1 ⁇ m or above, and the elongation measurement results.
• the average crystal grain diameter (as measured at the sheet surface) for all of these specimens was in the range of 50 to 60 ⁇ m, and the number per square millimeter of grains of the AL—Fe—Si compound having a diameter of 1 ⁇ m or above, likewise, was below 2000.
• TABLE 3 Tensile Test Elongation Material Temperature (° C.) Strain Rate(/s) (%) 16 450 10 ⁇ 2 480 17 180 10 ⁇ 2 540 18 480 10 ⁇ 1 410 19 520 10 ⁇ 2 450 20 350 5 ⁇ 10 ⁇ 3 380 21 580 10 ⁇ 2 30 22 480 5 ⁇ 10 ⁇ 4 280 23 480 2 ⁇ 10 0 80
• the present invention prvides an Al—Mg aluminum alloy sheet having excellent superplastic elongation in high speed forming such as at high strain rate of 10 ⁇ 2 to 10 0 /s at a high temperature, and a superplastic forming process using this aluminum alloy sheet shortens the forming time to improve productivity.

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• Crystallography & Structural Chemistry (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)
• Heat Treatment Of Steel (AREA)
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• 1995
  • 1995-08-23 JP JP23770795A patent/JP3145904B2/ja not_active Expired - Fee Related
  • 1995-12-12 DE DE69519444T patent/DE69519444T2/de not_active Expired - Fee Related
  • 1995-12-12 US US09/000,097 patent/US20010001969A1/en not_active Abandoned
  • 1995-12-12 WO PCT/JP1995/002564 patent/WO1997008354A1/ja not_active Ceased
  • 1995-12-12 EP EP95940435A patent/EP0846781B1/en not_active Expired - Lifetime

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