HIGH STRENGTH PRODUCTS EXTRUDED FROM 6XXX ALUMINIUM 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. An aluminium alloy extruded product obtained by: a) 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 the content of eutectic forming elements (Mg, Si and Cu) is selected so as to present in equilibrium conditions a solidus to solvus difference higher than 5° C., optionally 20° 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 lower than solidus T.sub.S, between T.sub.S and (T.sub.S −45° C.) and superior to solvus temperature for a time long enough to ensure a complete dissolution of precipitated eutectic phases; d) cooling until billet temperature reaches a temperature between 400° C. and 480° C. while ensuring billet surface never goes below a temperature substantially close to 350° C.; e) extruding at most a few tens of seconds after the cooling operation the said billet through a die to form at least an extruded product; f) quenching the extruded product down to room temperature; g) optionally stretching the extruded product to obtain a plastic deformation optionally between 0.5% and 5%; h) ageing the extruded product, without beforehand applying on the extruded product any separate post-extrusion solution heat treatment, the ageing treatment being applied such that: The Tensile test samples from the said extrusion product have a yield strength Rp0.2 higher than 240 MPa, optionally higher than 280 MPa when a hollow extrusion which has globally a rectangular cross-section, approx. 40*55 mm with a wall thicknesses close to 2.5 mm is produced according to a) to h), to evaluate the crushability the crash test samples cut from the said extrusion provides a regularly folded surface having cracks with a maximal length of 10 mm, optionally 5 mm, when axially compressed such that the crush distance is higher than half the initial cut profile length. The Tensile test samples from the said extrusion have a yield strength Rp0.2 higher than 240 MPa, optionally higher than 280 MPa.

2. An aluminium alloy extruded product according to claim 1, wherein the crash test samples cut from a hollow extrusion which has globally a rectangular cross-section, approx. 40*55 mm with a wall thicknesses close to 2.5 mm provides a regularly folded surface having cracks with a maximal length of 5 mm, optionally 1 mm when axially compressed such that the crush distance is higher than half the initial cut profile length.

3. An aluminium alloy extruded product according to claim 1, wherein the ageing h) comprises two successive steps: h1) naturally ageing the extruded product minimum 1 hour optionally more than 48 hours; h2) artificially ageing the extruded product to T6 or T7 temper, in order to obtain the said crash performance and strength.

4. An aluminium alloy extruded product according to claim 1 wherein Mg<1.0 wt. % and Si<1.0 wt. %.

5. An aluminium alloy extruded product according to claim 1 wherein Mg<0.7%, optionally 0.6 wt. %.

6. An aluminium alloy extruded product according to claim 1 wherein said 6xxx aluminium alloy comprises Cu: 0.05-0.4 wt. %.

7. An aluminium alloy extruded product according to claim 1, wherein said 6xxx aluminium alloy comprises Mn: 0.1-1.0 wt. %.

8. An aluminium alloy extruded product according to claim 1, wherein said 6xxx aluminium alloy comprises Ti: 0.01-0.1 wt. % and/or V 0.01-0.1 wt. % and/or Nb 0.02-0.15 wt. %.

9. An aluminium alloy extruded product according to claim 1 to manufacture automotive, rail or transportation structural components, optionally crash management systems.

Description

EXAMPLE

[0042] 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.

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 —

[0043] 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.

[0044] 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.

[0045] Profiles A-2, A-3 and B-2 were obtained by following a conventional route: [0046] Homogenising cast billets at a temperature close to 575° C.; [0047] Heating the homogenised cast billets to a temperature close to 460° C.; [0048] 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. [0049] Quenching the extruded material with an intense cooling device (water quench) down to room temperature. [0050] Stretching 1% [0051] 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.

[0052] Profiles A-1 and B-1 were obtained by following a route according to the invention. [0053] Homogenising cast billets at a temperature close to 575° C. [0054] Heating the homogenised cast billets to a temperature close to 575° C. [0055] 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.; [0056] 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.; [0057] Quenching the extruded material with an intense cooling device (water quench) down to room temperature. [0058] Stretching 1% [0059] 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.

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

[0060] 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.

[0061] 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.