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 is concerned with a new alloy in the 5000
• Ingots of the alloy can be converted to rolled sheet which can be formed into shaped components for use in vehicles.
• Non-heat-treatable alloys of the Al-Mg (5xxx) type are well suited to the application of automotive structural pressings to form a body- in-white structure.
• these alloys can have high formability allowing the complex structure pressings to be manufactured.
• Subsequent heat treatment during the car manufacture e.g. paint-bake ovens
• heat-treatable alloys these properties are then stable throughout the life of the vehicle, i.e. no artificial ageing takes place.
• the alloy AA5754 is a well known non-heat-treatable 5xxx series alloy, (2.6 to 3.6%wt Mg).
• the specification, given in Table 1 is broad and as such far too wide for the automotive industry.
• the Mg level must be controlled to tighter limits to maintain an acceptable spread of proof stress values in the final sheet.
• to render the alloy sufficiently formable it is usually based upon low Si and Fe (about 0.08%wt Si and about 0.2%wt Fe) requiring virgin smelter metal.
• Such alloys are not readily recyclable because during each remelting the Si and Fe levels increase and quickly exceed the level at which formability declines. There is a need for an alloy that can be recycled. This is particularly true of alloys intended for use in the mass production of automobiles. Alloys which require smelter metai obviously are not recyclable. Table 1
• This invention relates to the development of an alloy composition and processing route which gives rise to a higher strength 5xxx series alloy which is insensitive to SCC, and tolerant to high levels of Si and Fe in terms of formability.
• a characteristic of the current alloy is that because it can contain high levels of Si and Fe, it is therefore more recyclable.
• the present invention provides an alloy of composition in wt%:
• Al balance This is a relatively high-strength alloy; it has a 0.2% proof strength of 105 - 1 10 MPa, compared to 90 - 95 MPa for the standard AA5754 alloy containing 2.9wt% Mg.
• Components for load bearing structures in automobiles are press formed which involves stretch forming and deep drawing. Deep drawing is often the most important process, and this calls for a high r value, that is to say a high plastic strain ratio, that is uniform in the plane of the sheet. This need is met by the alloys of the invention.
• Mg is the principal solid solution strengthening addition in the alloy.
• the Mg content of the alloys of this invention which is relatively high at 3.0 - 3.5%, results in increased strength and formability.
• SCC stress corrosion cracking
• an upper limit of Mg is set at 3.3%.
• CASH solution heat-treated
• Mn is present at relatively high levels of 0.4 - 0.7% preferably up to 0.6% more preferably up to 0.5%. Homogenisation of the alloy results in precipitation of ⁇ -AIMnSiFe particles which give rise to additional dispersoid strengthening. Very high Mn levels are detrimental due to the formation of a coarse intermetallic phase MnAI 6 . The increased density of dispersoids causes a refinement of the O temper grain size and a resultant increase in strength.
• Cu may be present at levels up to 0.5% preferably up to 0.3%, more preferably up to 0.10%. At higher levels (e.g. up to 0.3%), Cu gives rise to significant strength retention after a paint bake cycle. Above 0.3% no additional benefit is obtained. Cu is an inevitable impurity in recycled scrap. Cu levels above 0.15% give rise to alloys having high r values but which may (unless the working conditions are rather closely controlled) be detrimental by virtue of very pronounced variation in the plane of the sheet (high ⁇ r). Si is present at 0.10 - 0.25% preferably up to 0.20% and improves strength. High Si and Mn have surprisingly been found to improve the r value of sheet and to promote uniformity in the plane of the sheet (low ⁇ r). But Si content as high as 0.3% gives rise to reduced ductility and reduced formability.
• Fe is specified at 0.18 - 0.30% preferably 0.20 - 0.30%. Fe contributes to dispersion strengthening, but at high concentrations lowers formability.
• the Si and Fe levels are set such that the alloy can be produced from recycled metal. Recycling increases the Si and the Fe levels in the charge. It also increases the Cu content.
• the new alloy of the invention is more tolerant of these impurities.
• Cr has similar effects to Mn and may be used in partial replacement of Mn.
• the (Cr + Mn) content is at least 0.4%.
• Cr is not deliberately added to the alloy, i.e. is present only as an incidental impurity at up to 0.05%.
• Ti may be added to refine the grain structure.
• Other alloying components may be present in minor concentrations up to 0.05% each, 0.15% total. Components deliberately added may include Zn and B. Other components would normally be present only as adventitious impurities. The balance of the alloy is Al.
• the invention provides rolled and annealed sheet of the alloy described.
• Rolled sheet for canstock is used in a hard as-rolled condition). The following paragraphs describe the processing steps used to produce that rolled sheet.
• Molten metal of the required composition is cast, typically by direct chill casting although the casting technique is not material to the invention.
• An ingot of the alloy is homogenised, preferably at a relatively high temperature of at least 500°C preferably 530 - 580°C particularly 550 - 580°C, for 1 - 24 hours.
• Homogenisation is preferably performed under conditions that result in the formation of a fine dispersoid of ⁇ - AIMnSiFe particles. If the homogenisation temperature is too low, it is possible that this may be produced as a coarser needle-like precipitate which exhibits growth with increased homogenisation time. These needles can break up during rolling to create voiding in the structure, resulting in possible reduced ductility.
• the homogenised ingot is then hot rolled and cold rolled, both under conditions which may be conventional.
• an interanneal is optional, preferably at a temperature of 300 - 400°C in batch operation or at 400 - 550°C in continuous operation.
• a final cold rolling treatment results in a thickness reduction preferably in the range 40 - 60% e.g. about 50%.
• a final annealing step preferably at 300 - 400°C for 0.05 - 5 hours in batch operation, or at 400 - 550°C in continuous operation, may be carried out on a batch basis, or as a continuous anneal and solution heat treatment.
• Annealing conditions should be such as result in a fully recrystallised grain structure i.e. one produced by high angle grain boundaries sweeping through the structure. Such alloys have good formability and high elongation to break.
• the resulting rolled sheet has the aforementioned combination of desired properties: high strength, insensitive to stress corrosion cracking and tolerant to high levels of Si and Fe in terms of formability.
• the sheet will be useful for forming into components to be joined together, e.g. by adhesive bonding or weld bonding or mechanical fastening to form structures e.g. load-bearing structures of motor vehicles.
• Example 2 The alloys used in Example 1 are set out in Table 2 below. Of these, STD is a typical AA5754 standard composition; 1 , 2, 3 and 4 are in accordance with the present invention. Table 2
• Figure 1 sets out the casting and processing schedule of the alloys described in Table 1.
• FIG. 2 to 14 is a bar chart comparing a particular feature between different alloys or different processing routes.
• the bulge height and thickness failure strains were determined using a hydraulic bulge testing machine that rigidly clamps a sheet of material using a draw bead section machined on a 175mm pitch circle. Sheet thickness was determined after bulging of the material using an ultrasonic probe, from which the failure strain was determined.
• Plane strain tension limit strains were determined by using a fixture that offered transverse restraint to the tensile specimens via the use of knife edges. (Technique reference: Sang H., Nishikawa Y., A Plane Strain Tensile Apparatus. J. Metals, 35(2), 1983, pp30-33).
• the r values were determined using JIS#5 tensile specimens, (50mm gauge length, 25mm width), the increased width giving rise to more accurate width strains and hence r values.
• R/t bend tests were carried out by bending the material according to ASTM Designation E 290 92. This apparatus was used to bend samples through approximately 150°, after which they were squeezed to a 180° bend in a vice. The outer surface of the bend was then examined for evidence of orange peel/cracking for the different radii used in the trials.
• SCC Stress Corrosion Cracking
• sensitivity was assessed via slow strain rate testing, (1x10 '7 per second).
• Specimens were pre- strained 20% followed by sensitisation at 150°C for varying times, and then tested under both dry conditions and immersed in a salt/peroxide solution (3%NaCI, 0.3%H 2 O 2 ). The elongation to failure for each test was recorded, and plotted for individual conditions as a ratio of the wet to dry performance. A ratio of one indicates no sensitivity to SCC.
• Figures 10 and 1 1 compare the r values of the sheets. 1 and 3 have the best combinations of high r value and little variation in the plane of the sheet ( ⁇ r). The Cu containing alloys had higher average r values but very pronounced variations ( ⁇ r) in the plane of the sheet.
• Figures 12 and 13 show respectively longitudinal R/t bend test data and transverse R/t bend test data.
• Example 2 Stress Corrosion Cracking Batch and Continuously Annealed Sheet Stress corrosion cracking was measured on experimental alloys rolled and processed on a commercial mill. Stress corrosion cracking is caused by the precipitation of a continuous film of AI 8 Mg 5 on grain boundaries and this process is substantially independent of the Si or the Mn contents of the alloy. The amount of these elements in the test alloys is therefore substantially irrelevant to the results obtained. The important element is Mg.
• composition and the process schedule for the alloys 5 and 6 are set out below:
• the comparison metals were: a commercial AA5182 alloy containing 4.5% Mg, a commercial AA5754 batch annealed alloy having a composition close to STD and alloy 1 from Example 1.
• Al alloy 7 had the composition in wt %: Mg 3.41

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Conductive Materials (AREA)

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• 1997
  • 1997-12-04 US US09/319,005 patent/US6544358B1/en not_active Expired - Fee Related
  • 1997-12-04 WO PCT/GB1997/003350 patent/WO1998024940A1/en not_active Ceased
  • 1997-12-04 DE DE69716949T patent/DE69716949T2/de not_active Revoked
  • 1997-12-04 JP JP52535698A patent/JP2001509208A/ja not_active Ceased
  • 1997-12-04 AT AT97947776T patent/ATE227353T1/de not_active IP Right Cessation
  • 1997-12-04 EP EP97947776A patent/EP0953062B1/en not_active Revoked
  • 1997-12-04 CA CA002273269A patent/CA2273269A1/en not_active Abandoned

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