METHOD FOR THE MANUFACTURING OF PRODUCTS WITH ANODIZED HIGH GLOSS SURFACES FROM EXTRUDED PROFILES OF AL-MG-SI OR AL-MG-SI CU EXTRUSION ALLOYS

Abstract

Method for the manufacturing of products with anodized high gloss surfaces from extruded profiles of AlMgSi or AS-MgSiCu, where the alloys initially are cast to extrusion billets), containing in wt. % Si: 0.25-1.00 Mg. 0.25-1.00 Fe: 0.00-0.15 Cu: 0.00-0.30 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr: 0.00-0.10 Se: 0.00 -0.10 Zn: 0.00-0.10 Ti: 0.00-0.05., and including incidental impurities and balance A.L a) where the billet is homogenised at a holding temperature between 480 C. and 620 C. and soaked at this temperature for 0-12 hours, where after the billet is subjected to cooling from the homogenisation temperature at a rate of 150 C./h or faster, b) the billet is preheated to a temperature between 400 and 540 C. and extruded preferably to a solid shape profile and cooled rapidly down to room temperature, c) optionally artificially ageing the profile, d) deforming the profile more than 10% by a cold roiling operation, whereafter e) the profile is flash annealed with a healing time of maximum two minutes to a temperature of between 450-530 C. for not more than 5 minutes and subsequently quenched, and f) optionally the profile after flash annealing is further subjected to a cold deforming operation to remove residual stresses from cooling and adjusting dimensional tolerances, and g) the profile is finally aged.

Claims

1-12. (canceled)

13. Method for the manufacturing of products with anodized high gloss surfaces from extruded profiles of AlMgSi or AlMgSiCu alloys, where the alloys initially are cast to extrusion billet(s), containing in wt. % Si: 0.25-1.00 Mg: 0.25-1.00 Fe: 0.00-0.15 Cu: 0.00-0.30 Mn: 0.00-0.20 Cr: 0.00-0.10 Zr: 0.00-0.10 Sc: 0.00-0.10 Zn: 0.00-0.10 Ti: 0.00-0.05, and including incidental impurities and balance Al, a) where the billet is homogenised at a holding temperature between 480 C. and 620 C. and soaked at this temperature for 0-12 hours, whereafter the billet subjected to cooling from the homogenization temperature at a rate of 150 C./h or faster, b) the billet is preheated to a temperature between 400 and 540 C. and extruded preferably to a solid shape profile and cooled rapidly down to room temperature, c) optionally artificially ageing the profile, d) deforming the profile more than 10% by cold rolling operation, whereafter e) the profile is flash annealed with a heating time of maximum two minutes to a temperature of between 450-530 C. for not more than 5 minutes and subsequently quenched, and f) optionally the profile after flash annealing is further subjected to a cold deforming operation to remove residual stresses from cooling and adjusting dimensional tolerances, and g) the profile is finally aged.

14. Method according to claim 13, wherein the composition of the alloys measured in wt. % lies preferably within Si: 0.35-0.6 Mg: 0.35-0.6 and the following maximum levels (wt. %) of the following elements Fe: 0.09 Cu: 0.15 Mn: 0.06 Cr: 0.04 Zn: 0.03 Ti: 0.02, and including incidental impurities and balance Al.

15. Method according to claim 13, wherein the composition of the alloys measured in wt. % lies preferably within Si: 0.35-0.6 Mg: 0.35-0.6 and with the following maximum levels (wt. %) of the following elements Fe: 0.06 Cu: 0.12 Mn: 0.06 Cr: 0.04 Zn: 0.03 Ti: 0.02, and including incidental impurities and balance Al.

16. Method according to claim 13, wherein the profile in accordance with step c) optionally is overaged to a T7 condition at 200-230 C. for 1-5 hours.

17. Method according to claim 13, wherein the profile according to step d) is deformed more than 20%.

18. Method according to claim 13, wherein the profile according to step d) preferably is deformed between 30 and 50%.

19. Method according to claim 13, wherein the profile is flash annealed according to step e) with a maximum heating time of 20 seconds to a temperature of between 450-530 C. for more than 1 minute.

20. Method according to claim 13, wherein the flash anneal heating according to step e) is obtained by induction heating of the profile.

21. Method according to claim 13, wherein the flash anneal heating according to step e) is obtained by subjecting the profile to a salt bath or other convection or radiation heating means providing high heating rates.

22. Method according to claim 13, wherein the alloys are cast without the use of grain refiner, except in the start-up of the casting operation.

23. Method according to claim 13, wherein the ageing, step g) is a one step or dual rate ageing operation to a final hold temperature between 160 C. and 220 C. and where the total ageing cycle is performed in a time span of between 3 and 24 hours.

24. Method according to claim 13, wherein the optical cold deforming after flash annealing, step f) is a rolling operation, a stretching operation or a forging operation.

Description

[0011] The invention will be further described in the following by way of examples and with reference to the drawings and figures where:

[0012] FIG. 1 is a photo of a quarter of a macro etched billet slice (228 mm in diameter) with abnormal grains.

[0013] FIG. 2 is a light optical micrograph showing a typical grain structure through the thickness of a thick solid shape extruded profile which is anodised and viewed in polarised light

[0014] FIG. 3 is a principal sketch of an industrial processing line for performing the cold rolling and the annealing process described in the present invention.

[0015] FIG. 4 shows light optical micrographs of samples from example 1 showing the grain structure in the middle of the cross section for the as extruded profile and for the samples that were cold rolled to give 10, 20, 40 and 60% reduction in the thickness prior to annealing. All samples are anodised and viewed in polarized light.

[0016] FIG. 5 shows grain structure in an as cast billet (95 mm diameter) without grain refiner, which was used in example 2 of the present application. Picture of a macro etched billet slice to the left and anodised sample viewed in polarized light in a light optical microscope to the right.

[0017] FIG. 6 are light optical micrographs showing the AlFeSi particles in a homogenised billet cast without grain refiner (upper picture) and in a homogenised billet cast with grain refiner (lower picture). The position of the samples in the billet is approximately half radius.

[0018] FIG. 7 is alight optical micrograph of an as extruded sample in example 2 of the application, showing the grain structure close to the surface. Anodised and viewed in polarized light.

[0019] FIG. 8 shows light optical micrographs of samples from example 2, showing the gram structure in the middle of the cross section for the as extruded profile and the samples that were cold rolled to give 20, 30, 40 and 50% reduction in the thickness poor to annealing. All samples are anodised and viewed in polarized light.

[0020] FIG. 9 shows further light optical micrographs of samples from example 2 of the present application, showing the grain structure in the middle of the cross section for samples that were cold rolled to 40% reduction in the thickness prior to annealing in air (upper) and in a salt bath (lower). Both samples are anodised and viewed in polarized light.

[0021] When the Fe content is below approximately 0.10 wt % the chance of getting abnormal grains (grains that grow and consume other grains that were formed during casting) in the billet during homogenisation becomes very high. Therefore a grain size of several centimeters is very common in billets of alloys with very low amounts of Fe. An example of abnormal grains in a homogenised billet with low Fe content is shown in FIG. 1.

[0022] The billet grain size will probably not affect the grain size in the extruded profile much if the extent of deformation is high, for example when extruding thin walled hollow profiles. For solid shapes, and especially for thick walled profiles, the billet grain size will most likely affect the grain size in the extruded profile. An additional challenge is that the billet temperature needs to be rather high in order to dissolve the Mg.sub.2Si particles, and a high billet temperature makes it more difficult to obtain a small gram size after extrusion.

[0023] In an extruded profile one usually sees a surface layer of mainly randomly oriented grains and typically one or a few grains in thickness. Underneath this layer one typically finds a region of larger grains. The thickness of this layer varies, and is usually thicker for a thick walled solid shape profile and thicker towards the back end of the extruded length. An example of a typical grain structure in a cross section of a thick walled industrially extruded profile can be seen in FIG. 2. Below the layer of larger grains the grain structure is typically more homogeneous. The grains in the homogeneous center region of the cross section are predominantly aligned in one direction, with a strong cube texture. This is often seen in s micrograph of the grain structure in the cross section by small differences in the colour of the grains.

[0024] More and more consumer electronics like mobile phones, tablets and lap tops are made of aluminium from extruded profiles. If the profile surface could have been used without any machining the grain structure in the anodised surface would probably be okay in most cases. However, very often them is a need to machine the extruded profile to make the shape and the dimensional tolerances of the final product. In that case the exposed surface can consist of grains from the coarse grain layer beneath the surface layer of the extruded profile. Due to this the entire coarse grain layer has to be removed before starting to machine the shape of the final product. The thickness of the layer that has to be removed due to coarse grains will very with the size of the profile and the extrusion conditions and is typicality in the range of 0.2 to 1 mm.

[0025] The present invention deals with the task to get a homogeneous grain structure with an average grain size below approximately 70 m irrespective of the Fe content, the grain size in the billet prior to extrusion and the extrusion conditions.

[0026] Solid shape profiles which are blanks for consumer electronics will be more or less flat, but could possibly have some features in the cross section in order to save material and machining Such profiles are therefore very well suited for cold rolling after extrusion. By cold rolling a profile by a minimum of 10% followed by flash annealing a new recrystallization process will take place. With sufficient deformation and a proper annealing process the resulting grain structure will be homogeneous over the cross section with a much more random orientation of the grains than in the as extruded profile. The grain size will in addition to the alley content, depend on the degree of cold deformation, the annealing temperature, the heat up conditions and the time at the annealing temperature. In an alloy with very few Fe and no dispersoid particles the recrystallization will take place at a low temperature, most likely during heating to the annealing temperature. One issue will then be to avoid grain growth at the annealing temperature when there are almost no particles in the material to pin the grains.

[0027] The annealing temperature should preferably be above the solvus temperature for Mg.sub.2Si particles in order to avoid particles that can reduce the strength and the gloss of the anodised material. In addition, the time at this annealing temperature should be as short as possible in order to avoid grain growth. Therefore, the material should be processed through extrusion in a way that Mg.sub.2Si particles are avoided. This means sufficiently high billet temperature in combination with a high enough exit temperature from extrusion and fast cooling of the profile after extrusion. With no particles in the material prior to cold rolling and annealing there is no need for a holding time for the material at the annealing temperature.

[0028] The consequence of annealing at temperatures below the solvus temperature will be that MgSi containing precipitates larger than approximately 0.3 m may form. These particles will contribute to a reduction in the gloss and in the strength of the material. The amount of this reduction will depend on the actual time-temperature history during the flash annealing and cooling operation and the composition of the alloy.

[0029] An industrial process to perform the cold rolling and the annealing process could be done as shown schematically in FIG. 3. The cold rolling station should be followed by a station for performing fast hasting to the annealing temperature. Using induction heating is probably the best way to do this. With enough power and induction coils that fit the shape of the profile and good process control, it should be possible to heat the material to a temperature around 500 C. (depending on the composition and thereby the solvus temperature of the alloy) within a very short time and with sufficient accuracy in temperature.

[0030] In order to avoid precipitation of MgSi containing particles larger than approximately 0.3 m the profile needs to be cooled rather rapidly down to room temperature. The reason for this is described in a previous section. Thus, preferably according to the present invention, the profile is flash annealed wild a heating time of maximum two minutes to a temperature of between 450-530 C. for not more than 5 minutes and subsequently quenched.

[0031] After the annealing operation one option could be a second cold rolling operation to remove residual stresses from the quenching operation. An alternative to cold rolling to remove residual stresses would be to stretch the material in way similar to what is done after extrusion, or performing a cold forging operation en blanks from the flash annealed and cooled material.

[0032] Further, to obtain a more homogeneous distribution of deformation and more accumulated energy in the material the profile could optionally be subjected to ageing after extrusion and prior to cold deforming. Preferably the profile could be overaged to a T7 condition, for example at 200-230 C. for 1-5 hours.

[0033] After the annealing process the final ageing of the material can for example be done with the patented dual rate ageing cycle (U Tundal and O. Reiso, EP 1 155 161 B1) to get maximum strength with minimum amount of alloying elements.

[0034] The invention will be further described in the following by way of examples.

Example 1

[0035] Billets with diameter 95 mm were cast in a lab casting facility using the Hycast hot-top gas-slip technology (as described in EP 0 778 097 B1) and a TiB.sub.2 based grain refiner. The composition of the alloy is shown in Table 1.

TABLE-US-00001 TABLE 1 Chemical composition of the alloy used in example 1 Mg Si Fe Mn Cr Cu Zn Zr Ti B Al 0.354 0.539 0.110 0.001 0.001 0.001 0.002 0.001 0.012 0.002 98.95

[0036] The billets were homogenised at 575 C. for 2 hours and 13 minutes followed by cooling at a rata of approximately 400 C. per hour. Extrusion of the billets was performed at an 8 MN laboratory extrusion press with a 100 mm diameter container to a profile with 540 mm.sup.2 cross section. The billet preheating temperature was approximately 500 C. and the extrusion speed 20 m/min. After extrusion the profile was quenched in water.

[0037] A 50 cm long piece from the front part of the extruded profile was cold rolled to give 10, 20, 40 and 60% reduction in the thickness. The samples that were cold rolled to different thicknesses were then annealed in a salt bath which had been preheated to 500 C. A hole was drilled into each of the samples to fit a thermocouple. The heating time to temperature was in the range 5-10 seconds, depending on the thickness of the sample. When a sample was put Into the salt bath a holding time of 10 seconds started when the temperature reached 490 C. After annealing the samples were quenched in water.

[0038] Prior to extrusion the billets had an even and small grain size. The as extruded sample in FIG. 4 shows a homogeneous grain size throughout the cross section. In this case there is no coarse grain layer below the surface. This is maybe because the sample is smaller then the sample shown in FIG. 2 and maybe also because it is taken from the front part of the extruded length. It is evident that the grains under the randomly oriented layer of grains in the profile surface area are predominantly aligned in one direction since the colour contrast between the grains is low.

[0039] As can be seen from the large colour contrast, the cold rolled and annealed samples show a much more random orientation of the grains than the as extruded sample. This confirms that these samples are fully recrystallized after annealing. The samples that were cold rolled to 10 and 20% reduction in thicknesses clearly have an uneven grain structure with the largest grains in the middle of the cross section. The samples that were cold rolled to 40 and 60% reduction m thicknesses hove an even grain structure throughout the cross section. The grain sizes of the samples shown in FIG. 4 (measured 250 m below the surface of the cross sections) are shown in Table 2.

TABLE-US-00002 TABLE 2 Average grain sizes as measured 250 m below the surface of the cross section. The as extruded grain size is very uncertain due to the very low contrast between the individual grains. 10% cold 20% cold 40% cold 60% cold As rolled + rolled + rolled + rolled + extruded annealed annealed annealed annealed ~87 m 79 m 60 m 44 m 33 m

Example 2

[0040] Billets with diameter 95 mm were cast in a lad casting facility using the Hycast hot-top gas-slip technology without using a grain refiner. A picture of a macro etched billet since is shown in FIG. 5 together with a micrograph showing an anodized sample viewed in polarized light in the light optical microscope. Towards the surface there am some relatively lame equiaxed grains, but a large pad of the cross section of the billet slice consists of feather crystals. The composition of the alloy is shown in Table 3.

TABLE-US-00003 TABLE 3 Chemical composition of the alloy used in example 2 Mg Si Fe Mn Cr Cu Zn Zr Ti B Al 0.380 0.473 0.092 0.002 0.001 0.001 0.006 0.000 0.004 0.000 99.00

[0041] The cast billets were homogenised at 575 C. for 2 hours and 15 minutes followed by cooling at a rate of approximately 400 C. per hour. Micrographs of the particle structure in the billets from the two different alloys in examples 1 and 2 are shown in FIG. 6. The material cast without grain refiner (upper picture) shows Fe containing particles (mainly -AlFeSi) that are smaller and mush more evenly distributed than the Fe containing particles (mainly -AlFeSi) in material cast with grain refine (lower picture). In the latter case the AlFeSi particles mainly are located at the grain boundaries. In both cases the Fe/Si ratio is very low, which makes -AlFeSi particles very stable in the homogenising process. A particle structure as shown in the material cast without a grain refiner would be beneficial in avoiding alignment of particles and possible visible dark lines in the extruded and anodized high gloss surface.

[0042] The billets where extruded at an 8 MN laboratory extrusion press with a 100 mm diameter container to a profile with a cross section of 540 mm.sup.2. The billet preheating temperature was approximately 500 C. and the extrusion speed 20 m/min. After extrusion the profile was quenched in water.

[0043] A 100 cm long piece from the back pad of the extruded profile was cold rolled to give 20, 30, 40 and 50% reduction in the thickness. The samples that were cold rolled to different thicknesses were then annealed in a salt bath which had been preheated to 500 C. A hole was drilled Into each of the samples to fit a thermocouple. When a sample was put into the salt bath the holding time of 10 seconds started when the temperature reached 490 C. After annealing the samples were quenched in water, in addition one sample of the material cold roiled to 40% reduction in thickness was held 5 minutes at 500 C. Yet another sample of the material cold rolled to 40% reduction in thickness was heated in an air circulating oven at a considerably lower heating rate to the annealing temperature than that obtained in a salt bath.

[0044] A micrograph of the as extruded sample is shown in FIG. 7. It seems like some of the grains below the surface are considerably larger than 100 m, which could give some unwanted effects in the surface appearance. Inside the surface region the grains are strongly aligned in one direction, which gives very little contrast between each individual grain in the micrograph.

[0045] FIG. 8 shows micrographs of the grain structure in the as extruded sample as well as samples that have been cold roiled 20, 30, 40 and 50% and thereafter annealed. As also seen in example 1, one can see from the large colour contrast that the cold rolled and annealed samples show a much more random orientation of the grains than the as extruded sample. The samples that were cold rolled to 20% reduction in thickness clearly have an uneven grain structure with the largest grains in the middle of the cross section. The sample cold rolled to 30% reduction in thickness has smaller grains and a more even grain structure, but the grains in the middle still are somewhat larger than those towards the surfaces. The samples that were cold rolled to 40 and 50% reduction in thicknesses have a smaller grain size and an even grain structure throughout the cross section. As also shown in Table 4 the grain size seems to be similar for the samples cold rolled to 40 and 50% reduction in thicknesses.

TABLE-US-00004 TABLE 4 Average grain sizes as measured 250 m below the surface of the cross section. The as extruded grain size is very uncertain due to the very low contrast between the individual grains. 20% cold 30% cold 40% cold 50% cold As rolled + rolled + rolled + rolled + extruded annealed annealed annealed annealed ~88 m 101 m 95 m 52 m 57 m

[0046] The sample that was cold rolled to 40% reduction in thickness and held at 500 C. for 5 minutes did not show any grain growth. The reason for this is probably that the number of AlFeSi-particles is high enough to prevent grain growth. With even lower Fe contents than 0.09 wt % a holding lime of 5 minutes at this temperature could cause grain growth in the sample

[0047] FIG. 9 shows that the sample heated in an air-circulating furnace (6-7 minutes heating time) has a more uneven grain structure and a slightly larger grain size than the sample that was rapidly heated (5-10 seconds) in a salt bath up to the solutionising temperature. The reason for this is probably linked to precipitation of MgSi particles at the grain boundaries, which are pinning the nuclei for new grams during the heat up process. In a sample which is slowly heated in air there is enough time for precipitation of MgSi particles to prevent the nuclei tor new grains from growing until the particles start to dissolve again, i.e., when the sample is approaching the solves temperature of the alloy. In this process some grains will probably start to grew earlier than others and therefore get larger, resulting in an uneven grain structure when the recrystallization process is complete.

[0048] Example 2 shows that it is beneficial to heat the cold rolled sample fast to the solutionising temperature to obtain en even grain size and that a holding time of only 10 seconds is sufficient to obtain a fully recrystallized grain structure.

[0049] Example 2 also shows that the final grain structure in the blanks could be perfect for providing attractive high gloss anodized surfaces even though the billet grain structure is regarded as being far from optimum when it is cast without grain refiner.

[0050] The main benefit of the present invention is a grain structure with an even grain size end a dose to random texture throughout the cross section o the profile irrespective of the grain size prior to cold rolling (and thus also of the grain structure of the billet). An extruded thick welled flat profile will in most cases have a coarse grain layer that has to be removed in order to obtain a smooth anodized surface with a minimum of defects in the final product. The amount of material that would have to be removed in the as extruded cross section is typically in the range 7-15%.

[0051] Moreover, the cold rolling will ensure a very accurate thickness and flatness of the profile, and for that reason considerably reduce the need for machining. An extruded profile will have much more variation in the thickness, typically 0.1 mm.

[0052] Since the grain size in the billet end the extruded profile is of little importance for the resulting grain size in the cold rolled and annealed blanks there is a possibility of casting the billets with a minimum or even completely without the use of a grain refiner. In order to avoid centre cracks in the billets in the startup of the cast it could be beneficial to add some grain refiner in the first metal in cast. The grain refiner itself could be a source for inclusions that can cause failures in the anodized surface. Another benefit of not using a grain refiner is that the melt cleaning with the use of ceramic foam filters will be more effective on other type of inclusions (Nicholas Towsey, Wolfgang Schneider and Hans-Peter Krug, A comprehensive study of ceramic foam filtration, 7th Australasian Asian Pacific Course & Conference, Aluminium Cast House Technology Theory & Practice, P Whiteley and J. Grandfield (TMS: 2001)

[0053] The possibility of reducing the Fe content and still obtain an adequate grain structure will significantly improve with the use of the present invention. The lower Fe content can either be used to improve the gloss, or to keep the current gloss but add a thicker and more wear resistant oxide layer to the anodized product. The latter will make the product more durable.

[0054] Even though there is extra cost associated with the cold rolling and annealing process to obtain the uniform and random grain structure, this will probably ho more than compensated for by the savings due to reduced machining and reduced material consumption.