Extruded 6XXX alloy product that is suitable for turning and has low roughness after anodisation

Abstract

The invention relates to extruded products suitable for turning, made from aluminium alloy with a composition (in weight %) of: 0.4-0.8 Si; 0.8-1.2 Mg; 0.20-0.4 Cu; 0.05-0.4 Fe; Mn0.10; Ti<0.15; Cr0.10; Bi0.8; Pb0.4; other elements <0.05 each and <0.15 remainder being aluminium, characterised in that the granular structure thereof is essentially recrystallized. The invention also relates to the method for the production of said products. The invention further relates to anodised turned mechanical parts obtained from extruded products according to the invention and to the production method thereof. The products of the invention are particularly suitable for the production of brake pistons and gearbox elements.

Claims

1. An extruded product for turning, made of an aluminum alloy consisting of a composition in wt % of Si 0.4-0.8; Mg 0.8-1.2; Cu 0.20-0.4; Fe 0.05-0.4; Mn0.10; Ti0.15; Cr0.10; Bi 0.4-0.8; Pb 0.2-0.34; other elements <0.05 each and <0.15 total, balance aluminum, wherein Cr+Mn0.10, wherein a rate of recrystallization at one quarter thickness of the granular structure thereof is greater than 70%, and wherein after mirror polishing and anodizing at a temperature of 30 C. with a solution comprising 180 g/l sulfuric acid and 14 g/l oxalic acid and 15 g/l glycerol to make an oxide layer 30 m thick, the product has a roughness R.sub.z on a generatrix parallel to the extrusion axis that is equal to or less than 1.7 m.

2. The extruded product according to claim 1, wherein the copper content is at least 0.23 wt % and/or the iron content is at least 0.20 wt %.

3. The extruded product according to claim 1, wherein anodizing time to obtain an anodic layer 30 m thick in a solution of 200 g/l H.sub.2SO.sub.4 at 5 C. is less than 30 minutes for a current density of 3 A/dm.sup.2.

4. The extruded product according to claim 1, wherein intergranular corrosion resistance thereof per test ISO 11846, method B, is such that a maximum corrosion depth on a cross-sectional cut of the extruded product is less than 200 m and a corresponding attacked surface area is less than 50%.

5. The extruded product according to claim 1, wherein said roughness R.sub.z on a generatrix parallel to the extrusion axis is less than 1.2 m.

6. The extruded product according to claim 1, wherein the copper content is at least 0.30 wt % and/or the iron content is at least 0.25 wt %.

7. The extruded product according to claim 1, wherein the silicon content is between 0.5 and 0.7 wt %.

8. The extruded product according to claim 1, wherein the magnesium content is between 0.9 and 1.1 wt %.

9. The extruded product according to claim 1, wherein the manganese content is at most 0.05 wt % and/or the chromium content is at most 0.08 wt %.

10. The extruded product according to claim 1, wherein an elastic limit thereof is at least 300 MPa in T6 temper and elongation is at least 10% in T6 temper.

11. The extruded product according to claim 1, wherein an elastic limit thereof is at least 330 MPa in T9 temper and elongation is at least 8% in T9 temper.

12. The extruded product according to claim 1, wherein the rate of recrystallization at one quarter thickness is greater than 90%.

13. The extruded product according to claim 1, wherein the extruded product is produced by a method comprising: a. preparing the aluminum alloy having said composition and casting in a form of a billet, b. homogenizing said billet at a temperature of at least 550 C., c. extruding said homogenized billet to produce an extruded product, an initial extrusion temperature being less than 550 C., d. placing said extruded product in a solution and quenching, optionally with water, with the placing in solution being done either at a temperature generated during (c), or in a separate heat treatment, e. artificially aging the product of step (d) at a temperature of between 150 and 200 C. for 5 to 25 hours.

14. The extruded product according to claim 13, wherein in (b), the temperature is at least 580 C.

15. The extruded product according to claim 13, wherein in (c), the initial extrusion temperature is less than 450 C.

16. The extruded product according to claim 1, wherein the extruded product is produced by a method comprising: a. preparing the aluminum alloy having said composition and casting in a form of a billet, b. homogenizing said billet at a temperature of at least 550 C., c. extruding said homogenized billet to produce an extruded product, an initial extrusion temperature being less than 550 C., d. placing said extruded product in a solution and quenching, optionally with water, with the placing in solution being done either at a temperature generated during (c), or in a separate heat treatment, e. straightening and/or cold deforming, optionally by traction and/or drawing, and/or naturally aging the product from step (d), f. artificially aging the product of step (e) at a temperature of between 150 and 200 C. for 5 to 25 hours, g. cold deforming the product of step (f), optionally by drawing.

17. The extruded product according to claim 1, wherein the extruded product is produced by a method comprising: a. preparing the aluminum alloy having said composition and casting in a form of a billet, b. homogenizing said billet at a temperature of at least 550 C., c. extruding said homogenized billet to produce an extruded product, an initial extrusion temperature being less than 550 C., d. placing said extruded product in a solution and quenching, optionally with water, with the placing in solution being done either at a temperature generated during (c), or in a separate heat treatment, e. straightening and/or cold deforming, optionally by traction and/or drawing, and/or naturally aging the product from step (d), f. artificially aging the product of step (e) at a temperature of between 150 and 200 C. for 5 to 25 hours.

18. The extruded product according to claim 1, wherein the extruded product is produced by a method comprising: a. preparing the aluminum alloy having said composition and casting in a form of a billet, b. homogenizing said billet at a temperature of at least 550 C., c. extruding said homogenized billet to produce an extruded product, an initial extrusion temperature being less than 550 C., d. placing said extruded product in a solution and quenching, optionally with water, with the placing in solution being done either at a temperature generated during (c), or in a separate heat treatment, e. artificially aging the product of step (d) at a temperature of between 150 and 200 C. for 5 to 25 hours, f. cold deforming the product of step (e), optionally by drawing.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1: Observation of the samples after the standardized corrosion test conducted according to standard EN ISO 11846:2008 (method B).

DESCRIPTION OF THE INVENTION

(2) Unless stated otherwise, all indications concerning the chemical composition of alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed in wt % is multiplied by 1.4. The alloy designations are made in compliance with the rules of the Aluminum Association, which are known to a person skilled in the art. Unless indicated otherwise, the definitions of standard EN 12258-1 apply. Unless indicated otherwise, the metallurgical temper definitions of standard EN 515 apply.

(3) Unless indicated otherwise, the static mechanical specificationsin other words the ultimate strength Rm, conventional elastic limit at 0.2% elongation Rp0.2, and elongation at break A %are determined by a tensile test according to standard ISO 6892-1, with the sample and direction of the test being defined by standard EN 485-1.

(4) The turning capability is evaluated by a machining test as described in international application WO2013/170953 in paragraph [0039]. The test consists in determining the fragmentation ability of chips by measuring the number of chips in a given mass of collected chips, 100 g in the present case. The machining is done using an SP 12 CNC lathe and a rhombic insert with a basic shape of 80 sold under the registered trademark SANDVIK Coromant Coroturn 107 as item CCGX 09 T3 04-AL, designed for aluminum alloys. The machining parameters used are a rotational speed of 3000 rpm, a feed of 0.3 mm/revolution, and a cutting depth of 3.5 mm. The extruded products of the invention are suitable for turning, that is, when subjected to the test described in international application WO2013/170953, paragraph [0039], they yield a number of chips for 100 g of chips of at least 3000 and preferably at least 4000. The corrosion resistance was assessed according to standardized test EN ISO 11846:2008 (method B). The surface area of the samples was 20 cm.sup.2. The samples were prepared by degreasing with an organic solvent, immersion for 2 minutes in 5% sodium hydroxide at a temperature of 55 C., rinsing and immersion for 2 minutes in 2% nitric acid.

(5) The corrosion test consists in submerging the sample thus prepared in a solution containing 30 g/l NaCl and 10 ml/l concentrated hydrochloric acid (p=1.19 g/ml) for 24 hours at ambient temperature.

(6) Three roughness parameters measured according to ISO Standard 4287 are used: R.sub.max: maximum height of the roughness profile, i.e. the largest of the R.sub.zi values over the evaluation length R.sub.z: Average height of the R.sub.z profile, i.e. the arithmetic mean of the individual R.sub.zi values over the evaluation length R.sub.a: Mean roughness deviation, i.e. the arithmetic mean of all the ordinates of the profile over the evaluation length.

(7) In the context of this invention, a granular structure such that the rate of recrystallization at one quarter thickness is greater than 70% and preferably greater than 90%, is called an essentially recrystallized granular structure. The recrystallization rate is defined as the surface fraction of a metallographic section occupied by recrystallized grains.

(8) The present inventors have observed that for known turning alloys, such as alloys AA6262, AA6064A, or AA6042, or the alloy described in international application WO2013/170953, the roughness after anodizing making it possible to obtain an oxide layer at least 20 m thick is much higher than the roughness before anodizing. Typically, even if a roughness such that R.sub.z<0.01 m is obtained after machining, the roughness after anodizing is at least 1.80 m or more. Consequently, the presence of numerous intermetallic compounds in this type of alloy generates substantial roughness during anodizing.

(9) The present inventors have observed that this problem is solved by controlling the composition of the alloy of the invention and the granular structure thereof.

(10) The extruded products of the invention suitable for turning are made of an aluminum alloy having a composition in wt % of Si 0.4-0.8; Mg 0.8-1.2; Cu 0.20-0.4; Fe 0.05-0.4; Mn0.10; Ti<0.15; Cr0.10; Bi0.8; Pb0.4; other elements <0.05 each and <0.15 total, balance aluminum.

(11) The simultaneous minimum values of silicon, magnesium, copper, and iron make it possible, in particular, to obtain extruded products suitable for turning. Alloys not having these minimum values, such as alloys 6063 or 6463, are not suitable for turning. Preferably, the copper content in this first embodiment is at least 0.23 wt %. In one embodiment of the invention, the copper content is at least 0.30 wt %. The iron content is preferably at least 0.20 wt % and advantageously 0.25 wt %. Advantageously, the composition is, in wt %, Bi 0.4-0.8 and Pb 0.2-0.4 and preferably Pb 0.2-0.34.

(12) Preferably, the silicon content is between 0.5 and 0.7 wt % and/or the magnesium content is between 0.9 and 1.1 wt %. The essentially recrystallized structure is obtained, notably, by controlling the manganese content and the chromium content. Preferably, the manganese content is at most 0.05 wt %. Preferably, the chromium content is at most 0.08 wt %. Advantageously, the sum of the chromium and manganese content is such that, in wt %, Cr+Mn0.15 and preferably Cr+Mn0.10. Controlling the zirconium content can also be important for obtaining the essentially recrystallized structure. Advantageously, the zirconium content is less than 0.04 wt % and preferably less than 0.03 wt %.

(13) The alloy and metallurgical structure of the extruded products of the invention are also advantageous, because the suitability thereof for extrusion is excellent, in particular the pressure required to initiate extrusion is lower, the extrusion rate is higher than for known allows, and no extrusion flaws such as hot tearing are observed.

(14) Contrary to what could have been expected, the extruded products of the invention have satisfactory static mechanical strength properties: the elastic limit thereof preferably being at least 300 MPa in the T6 temper and the elongation being at least 10%, and the elastic limit thereof preferably being at least 330 MPa in the T9 temper and the elongation being at least 8%.

(15) The present inventors have observed that an essentially recrystallized extruded product made of an alloy of the invention has an improved intergranular corrosion resistance. Consequently, the extruded products of the invention have an intergranular corrosion resistance per test ISO 11846, method B, such that the maximum corrosion depth on a cross-sectional cut of the extruded product is less than 200 m and the corresponding attacked surface area is less than 50%.

(16) In addition, the present inventors have observed that, surprisingly, an essentially recrystallized extruded product made of an alloy according to the present invention has improved roughness after machining and anodizing. Particularly, after mirror polishing and anodizing at a temperature of 30 C. with a solution comprising 180 g/l sulfuric acid and 14 g/l oxalic acid and 15 g/l glycerol in order to make an oxide layer measuring 30 m thick, the extruded product of the present invention has a roughness Rz on a generatrix parallel to the extrusion axis that is equal to or less than 1.7 m and preferably less than 1.2 m.

(17) The extruded products of the invention are also advantageous in that for a so-called hard anodizing, the anodizing time is decreased, which is favorable for productivity. Consequently, an extruded product according to the invention is characterized in that the anodizing time to obtain an anodic layer 30 m thick in a solution of 200 g/l H.sub.2SO.sub.4 at 5 C. is less than 30 minutes for a current density of 3 A/dm.sup.2; in other words, the oxide growth rate is greater than 1 m/min. The subject matter of the invention is also the method for producing extruded products according to the invention.

(18) In the production method of the invention, an aluminum alloy having a composition according to the invention is prepared and then typically cast in the form of a billet. The billet is then homogenized at a temperature of at least 550 C. and preferably at least 580 C. The chosen homogenization temperature helps to achieve an essentially recrystallized granular structure. The homogenized billet is then extruded, with the initial extrusion temperature being less than 550 C. and preferably less than 540 C. An initial extrusion temperature of at least 450 C. is preferred. After extrusion, the resulting extruded product is placed in a solution and quenched, preferably with water, with said immersion in solution being done either thanks to the heat generated during the extrusion process, or in a separate heat treatment.

(19) Quenching at the extruder outlet while at extrusion heat, typically with water, is advantageous.

(20) Optionally, the extruded product is straightened and/or cold deformed, typically by traction and/or drawing, and/or the extruded product is naturally aged. Advantageously, cold deformation is sufficient, typically at least 7%, to influence the mechanical properties after artificial aging. Any natural ageing is typically for a few hours to a few days. The extruded product is then artificially aged at a temperature of between 150 and 200 C. for 5 to 25 hours in order to obtain a T6 or T8 age treated temper.

(21) It is possible after artificial aging to perform cold deforming, typically by drawing, so as to obtain a T9 temper.

(22) Yet another subject matter of the invention is a method for producing a turned and anodized mechanical part through the following steps,

(23) a. an extruded product according to the invention is prepared,

(24) b. the extruded product is machined to make a turned mechanical part,

(25) c. optionally, the resulting mechanical part is formed

(26) d. the resulting mechanical part is anodized, with the oxide thickness being at least 20 m

(27) In one embodiment, the anodizing is done at a temperature of between 0 and 10 C. with a solution containing 100 to 250 g/l sulfuric acid with a current density of 1 to 3 A/dm.sup.2 with an oxide growth rate greater than 1 m/min. Under these conditions, the extruded products according to the invention decrease the anodizing time compared to products according to the prior art.

(28) In another embodiment, the anodizing is done at a temperature of between 15 and 40 C. with a solution comprising 100 to 250 g/l sulfuric acid and 10 to 30 g/l oxalic acid, and 5 to 30 g/l of at least one polyhydric alcohol. Advantageously, at least one polyhydric alcohol is chosen from ethylene glycol, propylene glycol, or glycerol. Preferably, the anodizing is done with a current density of between 1 and 5 A/dm.sup.2, preferably between 2 and 4 A/dm.sup.2.

(29) Preferably, the thickness of the resulting anodic layer is between 15 and 40 m.

(30) The invention also relates to the turned and anodized mechanical parts obtained by the method of the invention. These mechanical parts are advantageous because they have both a roughness Rz on a generatrix parallel to the extrusion axis that is equal to or less than 2.3 m and preferably equal to or less than 1.7 m, and the intergranular corrosion resistance thereof according to test ISO 11846, method B, is such that the maximum corrosion depth on a cross-sectional cut of the extruded product is less than 200 m and the corresponding attacked surface area is less than 50%.

(31) The use of an extruded product according to the invention to make a turned mechanical part such as a brake piston or a gearbox part is advantageous.

EXAMPLES

Example 1

(32) In this example, two alloys having the composition specified in Table 1 were prepared.

(33) TABLE-US-00001 TABLE 1 Composition of the alloys (wt %) Si Fe Cu Mn Mg Cr Ti Zr Ni Pb Bi A 0.6 0.26 0.24 0.03 1.1 0.05 0.02 <0.01 <0.01 0.25 0.5 B 0.7 0.40 0.30 0.11 1.0 0.11 0.02 <0.01 0.01 0.38 0.7

(34) The alloys were cast in the form of billets measuring 254 mm in diameter, homogenized at 585 C., then extruded in the form of bars with a 15100 mm cross-section by direct extrusion, the initial extrusion temperature being 530 C. The pressure required to initiate extrusion was 140 bar for alloy A of the invention, significantly lower than the pressure required to initiate extrusion of alloy B, which was 160 bar. The extrusion rate was 8.3 m/min for the alloy A billet, whereas it was 7.2 m/min for alloy B. Tearing during extrusion was observed for alloy B, whereas these cracks were not observed for alloy A. Alloy A thus was more reliable than alloy B.

(35) The extruded products were quenched upon exiting the press. The resulting bars were stretched to 1% and then subjected to artificial aging to achieve a T6 temper.

(36) The resulting alloy A bar had a recrystallized granular structure at one quarter thickness, whereas the alloy B bar had a non-recrystallized granular structure at one quarter thickness. The mechanical properties of the resulting bars measured in the extrusion direction are shown in Table 2.

(37) TABLE-US-00002 TABLE 2 Mechanical properties obtained Rm R.sub.p0.2 Alloy (MPa) (MPa) A % A 327 306 12 B 370 348 13

(38) The resulting bars were suitable for turning.

(39) The bars then underwent the following preparation treatments: 2 mm machining, mirror polishing, then anodizing according to method (1) or method (2) described in Table 3

(40) TABLE-US-00003 TABLE 3 Description of anodizing methods 1 and 2 Oxide Pretreatment Current Temper- thick- Meth- before Anodizing density ature ness od anodizing electrolyte (A/dm.sup.2) ( C.) (m) 1 Degreasing with 200 g/l 3 5 30 Novaclean D708 H.sub.2SO.sub.4 2 Degreasing with 180 g/l 2 30 30 Novaclean D708 H.sub.2SO.sub.4 + 14 g/l oxalic acid + 15 g/l glycerol

(41) The roughness results are shown in Table 4.

(42) TABLE-US-00004 TABLE 4 Results of roughness measurements after anodization treatment. Anodiz- Anodiz- Current Temper- ing Mean Average Average ing density ature time Ra Rz Rmax Alloy method (A/dm.sup.2) ( C.) (min) (m) (m) (m) A 1 3 5 23 0.35 2.33 3.28 A 2 2 30 53 0.09 0.95 1.78 B 1 3 5 34 0.39 2.46 3.46

Example 2

(43) In this example, two alloys having the composition specified in Table 5 were prepared.

(44) TABLE-US-00005 TABLE 5 Composition of the alloys (wt %) Si Fe Cu Mn Mg Cr Ti Zr Ni Pb Bi A 0.6 0.26 0.24 0.03 1.1 0.05 0.02 <0.01 <0.01 0.25 0.5 C 0.7 0.37 0.32 0.12 1.0 0.12 0.03 <0.01 <0.01 0.35 0.7

(45) The alloys were cast in the form of billets measuring 254 mm in diameter, homogenized at 585 C., then extruded in the form of cylindrical bars and quenched upon exiting the press. The resulting bars were stretched to 1% and then subjected to artificial aging and drawn to produce bars measuring 14 mm in diameter.

(46) The resulting alloy A bar had a recrystallized granular structure at one quarter thickness, whereas the alloy B bar had a non-recrystallized granular structure at one quarter thickness.

(47) The resulting bars were suitable for turning.

(48) The corrosion resistance was assessed at the middle of the bar according to standardized test EN ISO 11846:2008 (method B). The results are shown in Table 6 and in FIG. 1.

(49) TABLE-US-00006 TABLE 6 Results of corrosion tests Surface of the cross- Surface of the bar sectional cut Maximum Relative Maximum Relative attacked attacked attacked attacked depth surface area depth surface area Alloy (m) (%) (m) (%) A 330 20 50 10 A 300 10 80 10 C 305 100 690 100 C 300 100 720 100 C 370 100 600 100

Example 3

(50) In this example, two alloys having the composition specified in Table 7 were prepared.

(51) TABLE-US-00007 TABLE 7 composition of the alloys (wt %) Si Fe Cu Mn Mg Cr Ti Zr Ni Pb Bi D 0.6 0.24 0.21 0.01 1.04 0.05 0.02 <0.01 <0.01 0.23 0.4 E 0.7 0.40 0.30 0.11 1.01 0.12 0.02 <0.01 <0.01 0.34 0.7

(52) The alloys were cast in the form of billets, homogenized, and then extruded in the form of bars measuring 30 mm in diameter.

(53) The extruded products were quenched upon exiting the press. The resulting bars were stretched to 1% and then subjected to artificial aging to produce bars with a T6 temper.

(54) Both alloys D and E were tested in the T6 temper and are different in their granular structure. The resulting alloy D bar had a recrystallized granular structure at one quarter thickness, whereas the alloy E bar had a non-recrystallized granular structure at one quarter thickness. The mechanical properties of the resulting bars measured in the extrusion direction are shown in Table 8.

(55) TABLE-US-00008 TABLE 8 Mechanical properties obtained Bar diameter Rm R.sub.po, 2 Alloy Temper (mm) (MPa) (MPa) A % D T6 30 330 298 17.9 E T6 30 359 341 12.6

Example 4

(56) In this example, two alloys having the composition specified in Table 9 were prepared.

(57) TABLE-US-00009 TABLE 9 composition of the alloys (wt %) Si Fe Cu Mn Mg Cr Ti Zr Ni Pb Bi F 0.6 0.23 0.27 0.04 1.06 0.05 0.02 <0.01 0.01 0.28 0.5 G 0.6 0.26 0.24 0.01 1.03 0.07 0.02 <0.01 0.01 0.24 0.4

(58) The alloys were cast in the form of billets measuring 261 mm in diameter, homogenized at 585 C., then extruded in the form of bars.

(59) The extruded products were quenched upon exiting the press. The resulting bars were stretched to 1% then subjected to artificial aging followed by cold deformation in order to obtain a product with a T9 temper. Alloy F was drawn so as to obtain a bar measuring 24.5 mm in diameter, and alloy G a bar measuring 26 mm in diameter.

(60) The resulting alloy F and G bars have a recrystallized granular structure at one quarter thickness.

(61) The mechanical properties of the resulting bars of temper T9, measured in the extrusion direction, are shown in Table 9.

(62) TABLE-US-00010 TABLE 9 Mechanical properties obtained Bar diameter Rm R.sub.p0.2 Alloy Temper (mm) (MPa) (MPa) A % F T9 24.5 352 344 9 G T9 26 357 346 9

Example 5

(63) In this example, an alloy having the composition specified in Table 10 was prepared.

(64) TABLE-US-00011 TABLE 10 composition of the alloys (wt %) Si Fe Cu Mn Mg Cr Ti Zr Ni Pb Bi H 0.6 0.24 0.21 0.01 1.04 0.05 0.02 <0.01 0.01 0.23 0.4

(65) The alloy was cast in the form of billets measuring 261 mm in diameter, homogenized, then extruded in the form of bars.

(66) One bar was quenched upon exiting the press, stretched to 1%, then cold deformed to obtain a final diameter of 24.6 mm, then subjected to artificial aging to obtain a product with a T8 temper.

(67) Another bar was quenched upon exiting the press, cold stretched to approximately 1%, then subjected to artificial aging followed by cold deformation in order to achieve a final diameter of 24.5 mm so as to produce a product with a T9 temper.

(68) The resulting alloy H bars have a recrystallized granular structure at one quarter thickness.

(69) The mechanical properties of the resulting bars of temper T8 and T9, measured in the extrusion direction, are shown in Table 11.

(70) The turning capability was assessed by a machining test as described in international application WO 2013/170953 in paragraph [0039]. The test consists in determining the fragmentation ability of chips by measuring the number of chips in a given mass of collected chips, 100 g in the present case. The weight of 50 chips was also determined. The results are shown in Table 11.

(71) The machining is done using an SP 12 CNC lathe and a rhombic insert with a basic shape of 80 sold under the registered trademark SANDVIK Coromant Coroturn 107 as item CCGX 09 T3 04-AL, designed for aluminum alloys. The machining parameters used are a rotational speed of 3000 rpm, a feed of 0.3 mm/revolution, and a cutting depth of 3.5 mm.

(72) TABLE-US-00012 TABLE 11 Results obtained after the turning suitability test described in international application WO 2013/170953, paragraph [0039] Number Weight Bar of chips of 50 Tem- diameter Rm R.sub.p0.2 in 100 chips Alloy per (mm) (MPa) (MPa) A % g (g) H T9 24.5 380 357 8 4272 1.1705 T8 24.6 348 321 15.6 4744 1.0539