High strength products extruded from 6xxx aluminum alloys having excellent crash performance

Abstract

An aluminium alloy extruded product obtained by casting a billet from a 6xxx aluminium alloy comprising: Si: 0.3-1.5 wt. %; Fe: 0.1-0.3 wt. %; Mg: 0.3-1.5 wt. %; Cu<1.5 wt. %; Mn<1.0%; Zr<0.2 wt. %; Cr<0.4 wt. %; Zn<0.1 wt. %; Ti<0.2 wt. %, V<0.2 wt. %, the rest being aluminium and inevitable impurities; Wherein an ageing treatment is applied such that the product presents an excellent compromise between strength and crashability, with a yield strength Rp0.2 higher than 240 MPa, preferably higher than 280 MPa and when axially compressed, the profile presents a regularly folded surface having cracks with a maximal length of 10 mm, preferably less than 5 mm.

Claims

1. A method of producing an extruded product comprising: a) casting a billet from a 6xxx aluminium alloy, wherein the 6xxx alloy comprises: Si: 0.3-1.0 wt. %; Fe: 0.1-0.3 wt. %; Mg: 0.3-0.7 wt. %; Cu<1.5 wt. %; Mn: 0.1-1.0 wt. %; Zr<0.2 wt. %; Cr<0.4 wt. %; Zn<0.1 wt. %; Ti<0.2 wt. %, V<0.2 wt. %, the remainder being aluminium and inevitable impurities; wherein the content of eutectic forming elements (Mg, Si and Cu) is selected so as to present in equilibrium conditions a solidus to solvus difference greater than 5° C.; b) homogenizing the cast billet at a temperature 30° C. to 100° C. lower than solidus temperature; c) heating the homogenized billet at a temperature between Ts and (Ts−45° C.) and higher than solvus temperature for a time long enough to ensure a complete dissolution of precipitated eutectic phases; d) quenching with a water quench until billet temperature reaches a temperature between 400° C. and 480° C. while ensuring billet surface never goes below a temperature of about 350° C.; e) forming an extruded product; f) quenching with a water quench the extruded product to room temperature; g) optionally stretching the extruded product to obtain a plastic deformation of from 0.5% to 5%; h) ageing the extruded product to obtain a T7 temper such that: the tensile test samples from said extrusion product have a yield strength Rp0.2 higher than 240 MPa; when a hollow extrusion which has a rectangular cross-section, approximately 40*55 mm with wall thicknesses of about 2.5 mm is produced according to a) to h), to evaluate the crushability: crash test samples cut from said extrusion provide a regularly folded surface having cracks with a maximal length of 10 mm when axially compressed such that the crush distance is higher than half the initial cut profile length; the tensile test samples from said extrusion have a yield strength Rp0.2 higher than 240 MPa.

2. The method of claim 1, wherein in h), the crash test samples cut from a hollow extrusion provide a regularly folded surface having cracks with a maximal length of 5 mm when axially compressed such that the crush distance is higher than half the initial cut profile length.

3. The method of claim 1, wherein the ageing (h) comprises two successive steps: h1) naturally ageing the extruded product for a minimum of 1 hour, h2) artificially ageing the extruded product to T7 temper, to obtain said crash performance and strength.

4. The method of claim 1, wherein said 6xxx aluminium alloy comprises Cu: 0.05-0.4 wt. %.

5. The method of claim 1, wherein said 6xxx aluminium alloy comprises one or more of Ti: 0.01-0.1 wt. %; V 0.01-0.1 wt. %; or Nb 0.02-0.15 wt. %.

6. The method of claim 1, comprising (e) forming an extruded product by extruding a billet through a die after the quenching operation.

7. The method of claim 1, comprising (e) forming an extruded product by immediately extruding a billet after the cooling operation.

8. The method of claim 1, wherein the tensile test samples from said extrusion product have a yield strength Rp0.2 higher than 280 MPa.

9. The method of claim 1, comprising (h) ageing the extruded product without previously applying any separate post-extrusion solution heat treatment on the extruded product.

10. The method of claim 1, wherein in h), the crash test samples cut from a hollow extrusion provide a regularly folded surface having cracks with a maximal length of 1 mm when axially compressed such that the crush distance is higher than half the initial cut profile length.

11. The method of claim 1, wherein the ageing (h) comprises two successive steps: (h1) naturally ageing the extruded product for more than 48 hours; (h2) artificially ageing the extruded product to T7 temper, to obtain the said crash performance and strength.

12. The method of claim 1, wherein the content of eutectic forming elements (Mg, Si and Cu) is selected to present in equilibrium conditions a solidus to solvus difference greater than 20° C.

13. The method of claim 1, comprising (c) heating the homogenized billet at the temperature between Ts and (Ts−45° C.) for ten seconds to 120 seconds.

14. The method of claim 1, comprising (c) heating the homogenized billet at the temperature between Ts and (Ts−45° C.) for from 80 seconds to 120 seconds.

15. The method of claim 1, comprising stretching the extruded product to obtain a plastic deformation of from 0.5% and 5%.

16. The method of claim 1, comprising: casting a billet from a 6xxx aluminium alloy, wherein the 6xxx alloy consists of: Si: 0.3-1.0 wt. %; Fe: 0.1-0.3 wt. %; Mg: 0.3-0.7 wt. %; Cu<1.5 wt. %; Mn: 0.1-1.0 wt. %; Zr<0.2 wt. %; Cr<0.4 wt. %; Zn<0.1 wt. %; Ti<0.2 wt. %, V<0.2 wt. %, the remainder being aluminium and inevitable impurities.

17. The method of claim 1, wherein said quenching in (d) comprises water-spraying or water-bathing.

18. The method of claim 1, wherein said quenching in (d) follows a pre-defined operating route, wherein at least billet geometry, thermal conductivity of the alloy at different temperatures, and heat transfer coefficient associated with said quenching are taken into account.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(1) Crash test samples cut from the said profile provided with a regularly folded surface having cracks with a maximal length of 10 mm, preferably 5 mm, more preferably 1 mm, when axially compressed such that the crush distance is higher than half their length. Tensile test samples machined on the profile near the crash test samples having Rp0.2 higher than 240 MPa, preferably higher than 280 MPa

(2) Preferably, the ageing treatment is made in two successive steps: h1) naturally age the extruded product minimum 1 hour, preferably more than 48 hours in order to maximize material strength at peak age condition. h2) artificially age the extruded product to T6 or T7 temper, using either a one- or multi-step heat treatment applied at temperature(s) typically between 150° C. and 200° C. for a period of time between 1 and 100 hours defined to achieve the targeted combination of strength and crash performance.

(3) According to the invention, the aluminium alloy extruded product is obtained by casting a billet from a 6xxx aluminium alloy comprising: Si: 0.3-1.5 wt. %; Fe: 0.1-0.3 wt. %; Mg: 0.3-1.5 wt. %; Cu<1.5 wt. %; Mn<1.0%; Zr<0.2 wt. %; Cr<0.4 wt. %; Zn<0.1 wt. %; Ti<0.2 wt. %, V<0.2 wt. %, the rest being aluminium and inevitable impurities. The aluminium alloy according to the invention is of the AlMgSi type, which, compared with other such as e.g. AlZnMg alloys, provides good preconditions in the form of elongation and formability for energy-absorbing parts.

(4) Preferably, the Mg and Si contents are relatively low, i.e. both lower than 1.0%, to have an alloy easy to be extruded. Preferably, there is not Mg in excess. Advantageously, the Mg/Si weight ratio is largely lower than stoichiometric weight ratio corresponding to Mg2Si (1.73), typically lower than 1. More preferably, Mg content is not higher than 0.7 wt. %. Even more preferably, Mg content is not higher than 0.6 wt. %. In order to obtain an adequate level of strength, the alloy according to the invention contains also preferably copper and/or dispersoid-forming element additions such as Mn, Ti, Zr, Cr, V or Nb.

(5) In some embodiments of the invention, copper is added with a content higher than 0.05% to have a strengthening effect and lower than 0.4 wt. % to keep a chance to have a solidus to solvus difference higher than 5° C., preferably higher than 20° C.

(6) From U.S. Pat. No. 6,685,782, it is known that a peritectic alloying element, such as vanadium has a positive effect on the crash performance of the 6xxx-series aluminium alloys. Therefore, in some embodiments of the invention, peritectic alloying elements are advantageously added, solely or in combination, typically Ti with a content higher than 0.01 wt. % and preferably lower than 0.1 wt. %, Nb with a content higher than 0.02 wt. % and preferably lower than 0.15 wt. % or V with a content higher than 0.01 wt. % and preferably lower than 0.1 wt. %. Other peritectic alloying elements such as Mo, preferably with content lower than 0.2%, or even Hf and Ta, can be added.

(7) By applying the overheat and quench steps c) and d) of the invention on dispersoid containing alloys including, but not limited to, Mn, Cr, Ti and Zr, especially if homogenized at low temperatures as suggested in homogenisation step b) of the invention, the manufacture of high strength extruded products is enabled, which have a better crash performance, probably because they have large non-recrystallised areas displaying fibrous structure with more retained deformation texture, than when using the conventional separate post extrusion solution heat treatment, the latter enabling material with high strength but inevitably leading to post deformation recovery and recrystallisation.

(8) The cast billet according to the invention is homogenised. Because of the heat treatment of step c), the homogenisation treatment may be carried out—typically between 3 and 10 hours—with a quite low homogenisation temperature, i.e. with T.sub.H between 30° C. and 100° C. lower than solidus. Typically, the cast billet is homogenised at a temperature between 480° C. and 575° C. The homogenised billet is then cooled down to room temperature.

(9) The homogenised cast billet to be extruded is heated to a temperature Th slightly below the solidus temperature Ts to be solution heat treated. According to the invention, this temperature is between Ts−45° C. and Ts. The heating temperature is significantly higher than the conventional heating temperature, which is generally 50° C. to 150° C. lower than Ts. Therefore step c) is called “overheat” by reference to the conventional practice. The billets are preferably heated in induction furnaces and hold at Th during ten seconds to several minutes, typically between 80 and 120 seconds, i.e. for a time long enough to ensure a complete dissolution of precipitated eutectic phases.

(10) The billet is then cooled preferably by water-spray or water-bath until its temperature reaches 400° C. to 480° C., while ensuring that the billet surface never goes below a temperature substantially close to 350° C., preferably 400° C. Some trials seem to show that the temperature of the billet surface can be lower than 400° C., even if precipitation of some constituent particles, in particular hardening particles such as Mg.sub.2Si or Al2Cu, can at least partially occur. We assume that these particles, if any, will be dissolved during extrusion because they are located in the periphery of the metal billet, which feeds the narrow area extending along the dead zone that is formed close to the die during the extrusion. The material issuing from the periphery of the billet flows through this area and is prone to very intense shear stresses. As a result of the very high shear strain rates imposed and the heat generated in this area, the particles, if any, are probably dissolved during the extrusion, such that the surface of the profile exiting from the die is free of the said particles.

(11) Anyway the billet must be cooled, preferably quenched with a high cooling rate, by controlling the mean temperature of the billet and checking that the surface temperature is higher that a temperature close to 350° C., i.e. largely higher than the ambient. This implies that the cooling step d) has to follow an operating route, which should be pre-defined, for example by experimentation or through numerical simulation in which at least the billet geometry, the thermal conductivity of the alloy at different temperatures and the heat transfer coefficient associated with the cooling means are taken into account.

(12) FEM simulation of the cooling of a Ø 254 mm diameter billet with a heat transfer coefficient of 1 kW/m.sup.2/° K shows that the cooling should be stopped after approximately 40 s to avoid that the billet surface is below 400° C. At that time, the temperature of billet core is still near 530° C. but 40 seconds later, the temperature is again almost homogeneous in the billet, i.e. approximately 480° C. in the core and near the surface, because of the high thermal conductivity of the aluminium alloy.

(13) For billets having higher diameters, the cooling means should have higher cooling power or, if the same cooling means is used, cooling should be made in several steps including intense cooling, cooling stop when surface temperature is near 400° C., holding the billet few seconds such that the core and the surface temperatures are close each to the other and start a new similar cooling step as long as the mean temperature of the billet is higher than 480° C.

(14) For billets having lower diameters, cooling means can be used, which has lower cooling power or, if the same cooling means is used, cooling should be stopped after a shorter time, which can be estimated by an appropriate numerical simulation.

(15) As soon as the billet temperature reaches a temperature between 450° C. to 480° C., i.e. a few tens of seconds after the cooling operation is stopped, the billet is introduced in the extrusion press and extruded through a die to form one or several solid or hollow extruded products or extrudates. The extrusion speed is controlled to have an extrudate surface exit temperature higher than 430° C., preferably 460° C., but lower than solidus temperature Ts. The exit temperature may be quite low, because, as a result of steps c) and d), alloying elements forming hardening precipitates are still in solution in the aluminium lattice. The exit temperature should be high enough to merely avoid precipitation. Practically, the targeted extrudate surface temperature is commonly ranging from 500° C. to 580° C., to have an extrusion speed compatible with a satisfying productivity.

(16) The extruded product is then quenched at the exit of the extrusion press, i.e. in an area located between 500 mm and 5 m of the exit from the die. It is cooled down to room temperature with an intense cooling device, e.g. a device projecting sprayed water on the extrudates. The extrudates are then stretched to obtain a plastic deformation typically between 0.5% and 5% or even more (up to 10%), in order to have stress-relieved straight profiles.

(17) The profiles are then aged without beforehand applying any separate post-extrusion solution heat treatment to achieve the targeted strength and crash performance. The ageing treatment is made in two successive steps. First a natural ageing step of minimum 1 hour, preferably more than 48 hours, is applied in order to maximize material strength at peak age condition. Then a one- or multiple-step artificial aging treatment is applied at temperature(s) ranging from 150 to 200° C. for a prescribed period of time, between 1 to 100 hours, depending on the targeted properties. The alloy and the process according to the invention are particularly well suited to obtain T6 temper or T7 tempers, in order to achieve Rp0.2 higher than 240 MPa, preferably higher than 280 MPa while displaying an excellent crash performance characterised by crushed samples, the surface of which is regularly folded without any crack or with cracks having a maximum length of 10 mm, preferably 5 mm, more preferably 1 mm.

(18) The crash performance is evaluated on all described alloy and temper combinations using an identical extrusion shape. It corresponds to a hollow extrusion which has globally a rectangular cross-section, approx. 40*55 mm with a wall thicknesses close to 2.5 mm. Crushed samples are cut to a given length. It is preferred to use a length between 3 and 10 times, more preferably 4 and 7 times the radius of gyration of the profile cross-section. Cut profile are then axially compressed, typically by using a hydraulic press having flat dies. For crash samples made from crushable aluminium alloy materials, the compression force is increasing at the beginning of the test, until the beginning of folding; when the folding starts, the compression force is substantially constant, slightly varying during progressive folding. The crush distance is reached when the compression force increases significantly. The crush distance is generally higher than half the length of cut profile. The general aspect of the crushed sample and its folded surface are observed once the crush distance is reached. The level of the crash performance is given by measuring the maximal length of the cracks appearing on the folded surface.

(19) Another object of the invention is the use of an aluminium alloy extruded product according to the invention to manufacture parts of structural components for automotive, rail or transportation applications, such as crash boxes or crash management systems.

Example

(20) Hollow profiles made from two 6xxx aluminium alloys (A, B) were extruded by following two different process routes: the current prior art route and the route according to the invention. The chemical compositions of these alloys are shown on Table I. Alloy A is an AA6008 alloy. Alloy B is an AA6560 alloy.

(21) TABLE-US-00001 TABLE I Alloy Si Mg Mn Fe Cu Cr Zn Ti V A 0.60 0.53 0.08 0.24 0.14 0.009 0.03 0.024 0.071 B 0.47 0.54 0.06 0.2 0.18 0.002 0.01 0.035 —

(22) Homogenized cast billets having a diameter of 254 mm and a length of 820 mm were heated, introduced into an extrusion press and pressed to form mono-chamber hollow profiles, which have globally rectangular cross-section, approx. 40*55 mm with a wall thicknesses close to 2.5 mm. This geometry is representative of hollow profiles used in automotive industry to manufacture crash boxes and corresponds to a geometry suited to evaluate the crashworthiness. Profiles were cut at 200 mm length to form crash test specimens. This length corresponds to approximately 10 times the radius of gyration of said profile, calculated around the axis corresponding to the width direction of the rectangular shape. Tensile test specimens were machined in the hollow profiles near the crash test specimens.

(23) 200 mm long crash test specimens were then crushed between two flat dies by axial compression at a displacement speed of 320 mm/min using a hydraulic press until a displacement of 125 mm was achieved. Folds generated under compression load were then observed and measured. The crush distance reached was higher than 100 mm.

(24) Profiles A-2, A-3 and B-2 were obtained by following a conventional route: Homogenising cast billets at a temperature close to 575° C.; Heating the homogenised cast billets to a temperature close to 460° C.; Extruding the said billet with a surface exit temperature higher than 530° C. and lower than 580° C., in order to avoid incipient melting due to non-equilibrium melting of precipitates formed from solute elements (e.g. Mg2Si, Al2Cu) in profile hot-spots but still allows to dissolve part of the aforementioned phases that will later by re-precipitation during ageing contribute to hardening the alloy. Quenching the extruded material with an intense cooling device (water quench) down to room temperature. Stretching 1% ageing heat treatment at temperatures ranging from 150 to 200° C.; in particular A-2 and B-2 were heated during 7 h at 190° C., A-3 was heated during 8 h at 170° C.

(25) Profiles A-1 and B-1 were obtained by following a route according to the invention. Homogenising cast billets at a temperature close to 575° C. Heating the homogenised cast billets to a temperature close to 575° C. Cooling by water-spray until billet temperature reaches a temperature Td close to 430° C. while ensuring billet surface never goes below a temperature substantially close to 350° C.; A few tens of seconds after the cooling operation, extruding the billet with a surface exit temperature higher than 500° C. and lower than 580° C.; Quenching the extruded material with an intense cooling device (water quench) down to room temperature. Stretching 1% Ageing to T7 temper by a two successive-steps heat treatment.; in particular A-1 and B-1 were naturally aged during 48 h at ambient temperature and heated during 7 h at 190° C.

(26) TABLE-US-00002 TABLE 2 Rm Rp0.2 A% Base alloy Process Temper [MPa] [MPa] [%] Crash performance A-1 AA 6008 Invention T7 301 288 14.7 Regular folds Crack maximal length <5 mm A-2 AA 6008 Conventional T7 280 265 12.1 Regular folds Crack maximal length between 5 mm and 10 mm A-3 AA 6008 Conventional T6 296 277 14.1 Regular folds Crack maximal length between 25 mm and 50 mm B-1 AA 6560 Invention T7 283 267 14.9 Regular folds Crack maximal length <5 mm B-2 AA 6560 Conventional T7 270 253 12.5 Regular folds Crack maximal length between 5 mm and 10 mm

(27) The results of table 2 show that the process route according to the invention enables the manufacture of aluminium alloy extruded products having simultaneously better strength (Rm and Rp0.2) and crash performance than products obtained by a conventional route.

(28) At iso design, it is well observed that according to the invention, it is possible to obtain simultaneously strength and crashworthiness. By using a conventional route, it is possible to increase the strength at a level of the invention by adjusting the ageing conditions (case A-2 and A-3) but it is observed that it deteriorates the crushability: the length of defects increases.