Aluminum alloy

Abstract

In a first aspect, the invention provides aluminum alloy comprising the following composition, all values in weight %: Si 0.25-1.5 Cu 0.3-1.5 Fe up to 0.5 Mn up to 0.1 all other elements including Mg being incidental and present (if at all) then in an amount less than or equal to 0.05 individually, and less than or equal to 0.15 in aggregate, the balance being aluminum. In a second aspect, the invention provides a composite aluminum sheet product comprising a core layer and at least one clad layer wherein the at least one clad layer is an aluminum alloy comprising the following composition, all values in weight %: Si 0.25-1.5 Cu 0.3-1.5 Fe up to 0.5 Mn up to 0.1 all other elements including Mg being incidental and present (if at all) then in an amount less than or equal to 0.05 individually, and less than or equal to 0.15 in aggregate, the balance being aluminum. In a third aspect, the invention provides a method of making a joined structure of a steel component and an aluminum component made from the alloy and/or the sheet product of the invention.

Claims

1. A composite aluminum sheet product comprising a core layer and at least one clad layer, wherein the core layer is made from an alloy selected from the group consisting of AA6016, AA6016A, AA6014, AA6011, AA6111, AA6009, AA6010, AA6022 and AA6451, and wherein the at least one clad layer is an aluminum alloy comprising the following composition, all values in weight %: TABLE-US-00009 Si 0.25-1.5 Cu 0.3-1.5 Fe up to 0.5 Mn up to 0.1 all other elements including Mg being incidental and present then in an amount more than or equal to 0 and less than or equal to 0.05 individually, and more than or equal to 0 and less than or equal to 0.15 in aggregate, the balance being aluminum.

2. The composite aluminum sheet product of claim 1, wherein the aluminum alloy of the at least one clad layer comprises a lower limit of Si of 0.5 weight %.

3. The composite aluminum sheet product of claim 1, wherein the aluminum alloy of the at least one clad layer comprises an upper limit of Si of 1.25 weight %.

4. The composite aluminum sheet product of claim 1, wherein the aluminum alloy of the at least one clad layer comprises an upper limit of Cu of 1.25 weight %.

5. The composite aluminum sheet product of claim 1, wherein the aluminum alloy of the at least one clad layer comprises an upper limit of Mn of 0.08 weight %.

6. The composite aluminum sheet product of claim 1, wherein the aluminum alloy of the at least one clad layer comprises an upper limit of Mn of 0.05 weight %.

7. A joined structure wherein the joined structure comprises a steel component and an aluminum alloy component joined thereto and wherein the aluminum alloy component is made from a composite aluminum sheet product according to claim 1.

8. A joined structure according to claim 7 wherein the interface zone between the steel component and the aluminum alloy component is characterized by an FeAl layer adjacent the steel component.

9. The joined structure of claim 7, wherein the steel component is coated with zinc.

10. A method of making a joined structure wherein the joined structure comprises a steel component and an aluminum alloy component and wherein the steel and aluminum alloy components are joined by a thermal process that causes at least a part of the aluminum alloy component to melt and wherein the aluminum alloy component is made from a composite aluminum sheet product according to claim 1.

11. A method as claimed in claim 10 wherein the thermal process is laser welding.

12. A method of making a joined structure wherein the joined structure comprises a steel component and an aluminum alloy component and wherein the steel and aluminum alloy components are joined by a fluxless joining procedure comprising a thermal process, that causes at least a part of the aluminum alloy component to melt and wherein the aluminum alloy component is made from a composite aluminum sheet product according to claim 1.

13. A method as claimed in claim 12, wherein the thermal process is fluxless brazing, braze welding or fluxless laser welding.

Description

(1) In the following, the invention will be described in more detail by referring to examples and Figures which show the results of tests conducted on embodiments of the claimed invention. Neither the detailed description nor the Figures are intended to limit the scope of protection which is defined by the appended claims.

(2) FIG. 1 is a plot of a stress-displacement curve for an alloy according to the invention.

(3) FIG. 2 is a plot of the effect of Cu on the equilibrium solidus and liquidus temperature.

(4) FIG. 3 is a plot of the effect of Cu on calculated hot-cracking susceptibility.

(5) FIG. 4 is a plot of the effect of Cu on joint ductility.

(6) FIG. 5 is a plot of the effect of Cu on joint strength.

(7) FIG. 6 is a plot of the effect of Si on the equilibrium solidus and liquidus temperature.

(8) FIG. 7 is a plot of the effect of Si on joint ductility.

(9) FIG. 8 is a plot of the effect of Si on joint strength.

(10) FIG. 9 is a plot of the effect of Mg on bending and elongation.

(11) FIG. 10 is a plot of the effect of Mg on weld quality.

(12) FIG. 11 is a plot of the effect of Mg on joint ductility.

(13) FIG. 12A and FIG. 12B show two images of the interface produced when an AlSi10 alloy is welded to steel sheet including phase analysis.

(14) FIG. 13A and FIG. 13B show two images of the interface produced when an alloy according to the invention is welded to steel sheet including phase analysis.

(15) FIG. 14 is a plot of the stress-displacement curves for two composite sheets after joining to steel, one according to the invention and another according to the prior art.

EXAMPLE 1

(16) Table 1 lists the compositions of alloys cast in the form of small ingots, each ingot measuring 20150200 mm.

(17) TABLE-US-00006 TABLE 1 Sample Si Fe Cu Mn Mg 1 0.50 0.30 0.46 <0.01 <0.01 2 0.51 0.19 1.02 <0.01 <0.01 3 0.51 0.31 1.48 <0.01 <0.01 4 0.005 0.20 0.99 <0.01 <0.01 5 0.98 0.20 1.02 <0.01 <0.01 6 1.48 0.20 0.98 <0.01 <0.01 7 2.97 0.20 1.00 <0.01 <0.01 8 0.51 0.20 0.98 <0.01 0.26 9 0.51 0.20 0.99 <0.01 0.50 10 0.51 0.21 1.02 <0.01 2.00

(18) All alloys contained less than 0.05 of other elements individually and less than 0.15 in total, the balance being aluminum. Samples 8, 9 and 10 are comparative, and not within the scope of the claims below.

(19) The ingots were homogenized in an air furnace at 550 C. for 6 hours, hot-rolled to 10 mm and cold rolled to 1 mm. The sheet samples were annealed at 430 C. for 1 hour to cause recrystallization. A final leveling operation was applied to the 1 mm sheet.

(20) Sheet samples were then joined by a fluxless laser welding process to a 1 mm sheet of low-alloyed steel coated with a 7 m zinc layer (hot dip galvanized) using an Nd-YAG laser with a constant power of 3 kW. The joining geometry was flange welding (Kehlnaht) with a laser angle of 60 and no gap between the two sheets. The laser speed was 4 m/min for all alloy combinations.

(21) The compositional effect of the different elements on the equilibrium solidus and liquidus temperatures was calculated using commercial thermodynamic software from JMatPro coupled to in-house database. The hot cracking susceptibility was also calculated on the basis of thermodynamics calculation of the solid fraction evolution through the solidification interval. In both cases, nominal alloy compositions were used.

(22) All samples of joined sheets were subjected to dye penetrant inspection (DPI) to assess the visual integrity of the joints. The quality of the joint under DPI was based on a simple ranking system from 1 to 4, with 1 being good, 4 being bad (containing a large number of hot-cracks or/and coarse porosity).

(23) The nature and distribution of intermetallics produced in the interface zone was evaluated by conventional SEM and EDX analysis.

(24) The joined samples were also subjected to lap shear tensile testing to assess joint fracture strength and ductility. It is not appropriate to use conventional stress-strain curves in such figures because the test configuration means that the tensile stress, and thus plastic deformation, is not constant throughout the specimen. The results of tensile tests on lap shear joints are presented as equivalent stress in the aluminum section against grip-to-grip distance during the test, (described herein as standard travel). The equivalent stress within the aluminum part of the joined sample is the nominal force divided by the cross-sectional area of the aluminum section. The standard travel is an indication of the ductility of the joint.

(25) Some samples were subjected to 3-point bending tests to evaluate formability. The formability of the samples was measured using a bend test based on DIN 50111, but with slight modifications to the procedure. In this test a 60 mm60 mm piece of sheet, with a prior pre-straining of 10% (uniaxial stretching), was placed over two cylindrical rolls, the rolls being separated by a distance equal to twice the sheet thickness. Each roll diameter was 30 mm. Under load, a tapered punch bar of width 100 mm pushes the sheet into the gap between the rolls. The punch force is measured as well as the displacement. At the point of plastic deformation, (i.e. the start of cracking), the load necessary to deform the sheet falls, the punch force reduces and the test is automatically stopped. The sheet thus tested is deformed into a V shape and the internal angle of the V is measured. In this test a lower angle translates into better formability of the sheet. This test, (hereinafter referred to as the modified DIN 50111 test), is preferable to other formability tests because the results do not depend so much, if at all, on operator judgment.

(26) Samples 1-3 illustrate the effect of Cu on the performance of the alloys. Samples 2 with 5-7 illustrate the effect of Si on performance. Samples 2 with 8-10 illustrate the effect of Mg on performance.

(27) FIG. 1 shows the stress-displacement curve for sample 2 after joining. The standard travel of the test piece, proportional to elongation is very high, indicating a ductile fracture mode which was also apparent in the fracture surface.

(28) Effect of Cu. FIG. 2 shows the effect of increasing Cu content to a base composition of Al0.5Si on the solidus of the alloys. Adding Cu reduces the solidus temperature and improves wettability. FIG. 3 shows the effect of Cu on hot-cracking susceptibility with hot-cracking more likely as the Cu content increases up to 1.5%. FIG. 4 shows the effect of Cu on joint ductility. FIG. 5 shows the effect of Cu on the joint fracture strength. Increasing Cu from 0.5 to 1.0% increases fracture strength but it falls again slightly if the Cu content is increased towards 1.5%. From FIGS. 3, 4 and 5 we can see that the Cu content should be not be >1.5% and is preferably up to 1.25%.

(29) Effect of Si. FIG. 6 shows the effect of increasing Si content to a base composition of Al1.0Cu on the solidus of the alloys. Adding Si reduces the solidus temperature and improves wettability. FIG. 7 shows the effect of Si on joint ductility. Increasing Si content up to 1.0% improves bond ductility but there is a rapid decline in bond ductility as the Si content increases to 1.5% and beyond. FIG. 8 shows that increasing the Si content leads to an increase in joint fracture strength up to a 1% addition but the fracture strength declines as more Si is added. From FIGS. 7 and 8, we can see that Si should be limited to no more than 1.5% and preferably no more than 1.25% to maintain good joint qualities in terms of ductility and fracture strength.

(30) Effect of Mg. FIG. 9 shows the effect of Mg content on bendability as measured using the modified DIN 50111 test. The effect on elongation is minimal. As the Mg content increases, the bendability of samples prestrained by 10% diminishes towards an Mg content of 0.5 but then improves again as the Mg content is raised further to 2%. FIG. 10 shows the effect of Mg content on visual weld quality after DPI. Additions of Mg from essentially no Mg to 0.5 Mg led to worse weld quality, (coarse porosity and the presence of weld cracks), but the weld quality improved again when 2% Mg was added. The effect of Mg on weld ductility is shown in FIG. 11 and increased Mg content lowers weld ductility. For these reasons the Mg content is limited to the amount of an incidental element or impurity.

(31) FIGS. 12A and 12B show SEM images of the interface seen with AlSi10 alloys (sample 0) joined to steel. In the interface produced with AlSi10 alloys the width of the interface is approximately 10 m and the region immediately next to the steel alloy comprises an intermetallic zone dominated by FeAl.sub.3 (high Al/Fe ratio, in atomic %). The brittle structure is evidenced by the high amount of micro-cracks in the layer. FIGS. 13A and 13B show SEM images and EDX spectra of the interface produced when sample 2 was joined to steel. The width of the interface is approximately 20 m and the image reveals a dense and crack-free intermetallic layer. EDX analysis clearly shows that the continuous intermetallic layer at the interface is composed of two phases with various Al/Fe ratios. A third region on the top of the layer, with intermetallics in the shape of needles and a higher Al/Fe ratio is present. The first two intermetallic types are close to the FeAl and Fe.sub.2Al.sub.5 stoichiometry, whereas the third type is close to the more brittle FeAl.sub.3. There are fundamental differences between the interfaces including the presence of an FeAl-type layer adjacent the steel component when the steel component is joined to the inventive alloy.

EXAMPLE 2

(32) Two composite sheet products were produced where the core layer was an AA6016 alloy and a single clad layer was applied of either an AlSi alloy typical of the prior art (Sample 11) or an AlCuSi alloy of the present invention (Sample 12). The clad layers in each sample accounted for 10%, (1%), of the total sheet thickness. The alloy compositions of each layer are shown in Table 2.

(33) TABLE-US-00007 TABLE 2 Sample Si Fe Cu Mn Mg Core alloy 0.61 0.18 0.15 0.05 0.67 11 clad 9.91 0.11 <0.01 <0.01 <0.01 12 clad 0.51 0.17 0.98 0.06 <0.01

(34) The ingots were homogenized in an air furnace at 550 C. for 6 hours, hot rolled to 10 mm and cold rolled to 1 mm. The sheet samples were solution heat treated at 540 C. for 40 s, rapidly cooled by air fans and then pre-aged by holding samples at 100 C. for 1 hr.

(35) Some samples were allowed to naturally age to the T4PX condition after being subjected to a 10% pre-strain, a simulation of a typical forming operation. Other samples were further aged to the T8X (paint-baked) condition by subjecting them to a 2% pre-strain followed by ageing at 185 C. for 20 minutes and yet more samples were prepared in the T62 temper by subjecting them to a heat treatment at 205 C. for 30 minutes. The mechanical properties for sample 12 in three different temper conditions are summarized in Table 3.

(36) TABLE-US-00008 TABLE 3 T4PX T8X T62 Rp0.2 Rm A80 DC bend angle Rp0.2 Rm A80 Rp0.2 Rm A80 MPa MPa % MPa MPa % MPa MPa % 112 227 25.6 15 218 281 19.1 242 289 13.5

(37) They were then joined to steel sheet under the same laser welding conditions as described in example 1. The joined parts were mechanically tested to evaluate the strength and ductility of the joint.

(38) The stress-strain curve of FIG. 14 shows the results for both samples 11 and 12. In the case of sample 12, the curve is for the product in the T8X condition. There is a dramatic improvement in the strength attained and the ductility for the product according to the invention compared with these qualities for a sample not in accordance with the claims below.