ALUMINUM ALLOY WITH IMPROVED STRENGTH AND DUCTILITY

The present disclosure concerns an aluminum alloy with improved strength and ductility. The aluminum alloy composition achieves a superior strength/ductility combination compared to existing Al—Mg—Si alloys. The aluminum alloy may be particularly suitable for applications requiring a yield strength of more than 240 MPa. The aluminum alloy comprises, in weight percent: 0.41-0.59 Si; ≤0.3 Fe; 0.08 to 0.30 Cu; 0.45 to 0.55 Mg; 0.08 to 0.20 Mn; and the balance being aluminum and inevitable impurities. The Mn can be replaced in the aluminum alloy by Cr to an equivalence of Mn and Cr at a ratio Mn=1.6Cr, wherein Mn+1.6Cr+Cu≥0.25.

TECHNICAL FIELD

This disclosure relates to the field of aluminum alloys such as Al—Mg—Si alloys, particularly aluminum alloys useful in the automotive industry.

BACKGROUND OF THE ART

Aluminum alloys are used in the automotive industry because they have desirable mechanical properties suitable for that industry. In automotive extrusion applications, increased strength is desirable to increase the energy absorption and to permit a reduction in wall thickness of a part allowing the weight of the automotive vehicle to be reduced. Good ductility is also a requirement to accommodate plastic deformation during part forming operations and to withstand the severe plastic deformation in crash situations without cracking, which can limit energy absorption. With existing Al—Mg—Si based extrusion alloys, strength can be increased by increasing the concentration of the major elements Mg and Si but in general, any strength increase is associated with a loss of ductility as measured by bend testing or fracture strain.

It would be desirable to produce an aluminum alloy with increased strength without compromising the ductility, particularly for use in the automotive industry.

SUMMARY

In one aspect, there is provided an aluminum alloy comprising, in weight percent: 0.41-0.59 Si; ≤0.3 Fe; 0.08-0.30 Cu; 0.45-0.55 Mg; 0.08-0.20 Mn, or Mn being replaced by Cr to an equivalence of Mn and Cr at a ratio Mn=1.6Cr; and the balance being aluminum and inevitable impurities, wherein Mn+1.6Cr+Cu≥0.25. In some embodiments, the aluminum alloy comprises up to 0.12 Cr wt. %. In some embodiments, the inevitable impurities comprise less than 0.1 wt. % of the aluminum alloy and each inevitable impurity is present at a maximum of 0.05 wt. %. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Ni. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Zn. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Ti. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % B. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % V. In some embodiments, the alloy comprises 0.43-0.55 Si. In some embodiments, the alloy comprises ≤0.25 Fe. In some embodiments, the alloy comprises 0.10-0.30 Cu. In some embodiments, the alloy comprises 0.47-0.53 Mg. In some embodiments, the alloy comprises 0.10-0.20 Mn. In some embodiments, the alloy comprises at least 0.10 Fe. In some embodiments, the alloy comprises up to 0.10 Cr. In some embodiments, the alloy comprises 0.48-0.52 Mg. In some embodiments, the alloy further comprises a grain refiner.

In one aspect, there is provided an aluminum alloy comprising, in weight percent: 0.41-0.59 Si; ≤0.3 Fe; 0.08-0.30 Cu; 0.45-0.55 Mg; up to 0.20 Mn; up to 0.12 Cr; and the balance being aluminum and inevitable impurities. In some embodiments, the inevitable impurities comprise less than 0.1 wt. % of the aluminum alloy and each inevitable impurity is present at a maximum of 0.05 wt. %. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Ni. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Zn. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % Ti. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % B. In some embodiments, the inevitable impurities comprise ≤0.05 wt. % V. In some embodiments, the alloy comprises 0.43-0.55 Si. In some embodiments, the alloy comprises ≤0.25 Fe. In some embodiments, the alloy comprises 0.10-0.30 Cu. In some embodiments, the alloy comprises 0.47-0.53 Mg. In some embodiments, the alloy comprises 0.08-0.20 Mn. In some embodiments, the alloy comprises at least 0.10 Fe. In some embodiments, the alloy comprises up to 0.10 Cr. In some embodiments, the alloy comprises 0.48-0.52 Mg. In some embodiments, the alloy further comprises a grain refiner. In some embodiments, the aluminum alloy comprises a combined Cu, Mn, and Cr concentration in weight percent being Cu+Mn+1.6Cr≥0.25. In some embodiments, the aluminum alloy comprises Cu+Mn≥0.25 or Cu+1.6Cr≥0.25. In some embodiments, the aluminum alloy comprises a combined Mn and Cr concentration in weight percent of up to 1.6Cr.

In one aspect, there is provided an aluminum product comprising the aluminum alloy of the present disclosure. The aluminum product can have a recrystallized grain structure. The aluminum product can be an extrusion billet. The aluminum product can be an automotive part.

It is also provided a process for making an aluminum product comprising the steps of casting the aluminum alloy as described herein to produce a cast aluminum product; extruding the cast aluminum product producing an extruded aluminum product; and cooling the extruded aluminum product.

In an embodiment, the aluminum alloy is casted by direct chill casting, continuous casting and/or semi-continuous casting. Preferably, the aluminum alloy is casted using a Properzi continuous casting, a twin roll caster, or a block caster. In another embodiment, the extruded aluminum product is cooled by using cooling fans, water spray or a water quench. In an embodiment, the cast aluminum product is extruded by a hollow extrusion. In a further embodiment, the process described herein further comprises the step of heat treating or homogenising the cast aluminum product before extruding.

DETAILED DESCRIPTION

The present disclosure concerns an aluminum alloy with improved strength and ductility. The aluminum alloy composition of the present disclosure achieves a superior strength/ductility combination compared to existing Al—Mg—Si alloys. In some embodiments, the aluminum alloy is particularly suitable for applications requiring a yield strength of more than 240 MPa. The Al—Mg—Si alloy of the present disclosure surprisingly achieves the improved strength and ductility combination through the controlled additions of 0.08-0.30 Cu and 0.08-0.20 Mn. The improved strength and ductility can for example be measured by comparison to a traditional 6XXX automotive extrusion aluminum alloy (e.g. AA6060 and AA6063). More specifically, the aluminum alloy of the present disclosure comprises in some embodiments, in weight percent, 0.41-0.59 Si, ≤0.3 Fe, 0.08-0.30 Cu, 0.45-0.55 Mg, optionally up to 0.12 Cr and 0.08-0.20 Mn, the balance being aluminum and inevitable impurities. It is encompassed that the content of Mn, Cr with a ratio of Mn: 1.6 Cr and Cu is at least 0.25 wt %. The alloy encompassed herein can be cast as an extrusion billet and extruded into a product, such as an extruded profile for an automotive application.

The aluminum alloy of the present disclosure has a Si content in weight percent with respect to the total weight of the aluminum alloy of from 0.41 to 0.59, from 0.42 to 0.59, from 0.43 to 0.59, from 0.44 to 0.59, from 0.41 to 0.58, from 0.41 to 0.57, from 0.41 to 0.56, from 0.41 to 0.55, from 0.41 to 0.54, from 0.41 to 0.53, from 0.41 to 0.52, from 0.41 to 0.51, from 0.41 to 0.50, from 0.41 to 0.49, from 0.41 to 0.48, from 0.41 to 0.47, from 0.41 to 0.46, from 0.41 to 0.45, from 0.42 to 0.50, from 0.43 to 0.50, from 0.42 to 0.48, from 0.43 to 0.47, or from 0.43 to 0.45. Silicon improves the strength of Al alloys by combining with Mg and Cu to give precipitation hardening and also promotes the formation of Al-Mn—Fe—Si dispersoids which can prevent slip localisation.

The aluminum used to produce the alloy described herein can be a primary aluminum alloy or a recycled material. Fe is a natural impurity in primary aluminum and can also be found at increased levels in recycled material and the claimed ranges reflect the use of material from these two sources. The aluminum alloy of the present disclosure has a Fe content in weight percent with respect to the total weight of the aluminum alloy of up to 0.3, up to 0.25, up to 0.22, up to 0.21, up to 0.20, from 0.10 to 0.30, from 0.10 to 0.25, from 0.10 to 0.22, from 0.10 to 0.20, from 0.12 to 0.25, from 0.13 to 0.24, from 0.14 to 0.23, or from 0.15 to 0.22. The amount of Fe is limited to up to 0.3, preferably up to 0.25, more preferably up to 0.22 to avoid any negative impact on the mechanical properties of the aluminum alloy. Fe has a low solubility in aluminum and usually forms Al-Fe—Si type intermetallics or constituent particles during casting and homogenisation which can be detrimental to the surface finish of the profile and when present at high concentrations can be detrimental to ductility. For this reason an upper limit on Fe content is desirable as described herein, for example up to 0.3, up to 0.25 or up to 0.2.

The aluminum alloy of the present disclosure has a Cu content in weight percent with respect to the total weight of the aluminum alloy of from 0.08 to 0.30, 0.09 to 0.30, 0.10 to 0.30, 0.11 to 0.30, 0.12 to 0.30, 0.13 to 0.30, 0.14 to 0.30, 0.15 to 0.30, 0.16 to 0.30, 0.17 to 0.30, 0.18 to 0.30, 0.19 to 0.30, 0.08 to 0.29, 0.08 to 0.28, 0.08 to 0.27, 0.08 to 0.26, 0.08 to 0.25, 0.08 to 0.24, 0.08 to 0.23, 0.08 to 0.22, 0.08 to 0.21, 0.08 to 0.20, 0.10 to 0.29, 0.10 to 0.28, 0.10 to 0.27, 0.10 to 0.26, 0.10 to 0.25, 0.10 to 0.24, 0.10 to 0.23, 0.10 to 0.22, 0.10 to 0.21, 0.10 to 0.20, 0.11 to 0.19, 0.12 to 0.18 or 0.13 to 0.17. Cu promotes the formation of Al—Mg—Si—Cu ageing precipitates such as Q″ or Q′ in addition to the normal MgSi precipitates which can improve the dispersion of slip during plastic deformation.

The aluminum alloy of the present disclosure has a Mg content in weight percent with respect to the total weight of the aluminum alloy of from 0.45 to 0.55, 0.45 to 0.54, 0.45 to 0.53, 0.45 to 0.52, 0.45 to 0.51, 0.45 to 0.50, 0.46 to 0.55, 0.47 to 0.55, 0.48 to 0.55, 0.49 to 0.55, 0.50 to 0.55, 0.46 to 54, 0.47 to 0.54, 0.48 to 0.54, 0.47 to 0.53, or 0.48 to 0.52. Magnesium contributes to solid solution strengthening. The main role of magnesium is to combine with Si and Cu to provide precipitation hardening.

The aluminum alloy of the present disclosure has a Mn content in weight percent with respect to the total weight of the aluminum alloy of up to 0.20, up to 0.19, up to 0.18, up to 0.17, up to 0.16, up to 0.15, from 0.08 to 0.20, 0.08 to 0.19, 0.08 to 0.18, 0.08 to 0.17, 0.08 to 0.16, 0.08 to 0.15, 0.08 to 0.14, 0.09 to 0.20, 0.09 to 0.19, 0.09 to 0.18, 0.09 to 0.17, 0.09 to 0.16, 0.09 to 0.15, 0.09 to 0.14, 0.10 to 0.20, 0.10 to 0.19, 0.10 to 0.18, 0.10 to 0.17, 0.10 to 0.16, 0.10 to 0.15, 0.10 to 0.14, 0.11 to 0.16 or 0.12 to 0.15. Mn can contribute to the strength of the Al alloys by dispersoid strengthening and solid-solution hardening. Mn promotes the formation of Al-Mn—Fe—Si submicron dispersoid particles during homogenisation. These dispersoids act to disperse slip during plastic deformation which can delay the formation of internal stress concentrations and fracture events. Mn also promotes extrudability by facilitating the transformation of the beta Al—(Fe,Mn)Si constituent phase to alpha. However, an excessive quantity of dispersoids can increase the flow stress of the alloy at extrusion temperature and adversely affect extrusion speed along with inhibiting recrystallisation to form either coarse recrystallised or non-recrystallised grain structures.

In some embodiments, the aluminum alloy of the present disclosure has a combined Cu and Mn content of at least 0.25, at least 0.26, at least 0.27, at least 0.28 or at least 0.29 weight percent. In some examples, the combined Cu and Mn content has at least 0.05 of Mn, at least 0.06 Mn, at least 0.07 Mn, or at least 0.08 Mn weight percent. In further a example, the combined Cu and Mn has at least 0.08 Mn. Indeed, the combination of Cu and Mn in the specified individual and combined minimal range was surprisingly found to improve the strength of the aluminum alloy while advantageously maintaining or improving the ductility. This is demonstrated in the Example section below.

In some embodiments, the Cr may act to replace or compliment Mn. Indeed, the dispersoid particles formed by additions of Cr and Mn have similar crystal cubic structures with similar lattice parameters and are mutually soluble in one another such that to some extent the two elements are interchangeable. Accordingly, the aluminum alloy of the present disclosure may have a Cr content in weight percent with respect to the total weight of the aluminum alloy of up to 0.12, up to 0.11, up to 0.10, up to 0.09, up to 0.08, up to 0.07, up to 0.06, from 0.03 to 0.12, from 0.03 to 0.11, from 0.03 to 0.10, from 0.03 to 0.09, from 0.05 to 0.12, from 0.05 to 0.11, from 0.05 to 0.10, from 0.05 to 0.09, from 0.06 to 0.12, from 0.06 to 0.11, from 0.06 to 0.10, from 0.06 to 0.09 or from 0.07 to 0.08. In some embodiments, the minimal content of Cr is at least 0.03 wt. %, at least 0.04 wt. %, at least 0.05 wt. %, or more than 0.05 wt. %. In other embodiments, Cr may not be included in the alloy, and accordingly the aluminum alloy would comprise less than 0.05 wt. %, less than 0.03 wt. %, or less than 0.01 wt. % of Cr. In some embodiments, Cr may be used instead of Mn and vice versa. In an embodiment Mn is being replaced by Cr to an equivalence of Mn and Cr at a ratio Mn=1.6Cr. In other embodiments, the alloy comprises no Cr when the Mn is at least 0.1 wt. %.

Since the Mn and Cr are interchangeable it is possible that only a portion of Mn is replaced by Cr (and vice versa). Accordingly the overall Mn, Cr, and Cu content in the present alloy can be set out as follows: Mn+1.6Cr+Cu≥0.25 wt. %. In some embodiments, the concentration of Mn+1.6Cr+Cu can be at least 0.26 wt. %, at least 0.27 wt. %, at least 0.28 wt. % or at least 0.29 wt. %. In such embodiments, the total content of Mn and Cr is defined as follows: Mn+1.6Cr≤0.20.

Since Mn and Cr can be interchangeable, in some embodiments, the aluminum alloy of the present disclosure can have a combined Cr and Cu content that can be defined as follows: Cu+1.6Cr≥0.25. In such embodiments, the Cr can completely replace Mn. In some embodiments, Cu+1.6Cr can be at least 0.26, at least 0.27, at least 0.28 or at least 0.29. Accordingly, in some embodiments the aluminum alloy can be defined as having Cu+1.6Cr≥0.25 with Mn being less than 0.05 wt. % or Mn+Cu≥0.25 with Cr being less than 0.05 wt. %.

The weight percentage concentration for the aluminum alloy are provided with the balance being aluminum and inevitable impurities. In some embodiments, each of the inevitable impurity is present at a maximum of 0.05 (and in some embodiments 0.03) and the total inevitable impurities comprise less than 0.10. In some embodiments, it is to be understood herein that the term “inevitable impurity” means that there was no deliberate addition.

In some embodiments, the inevitable impurities include Ni in a concentration of less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01. Ni has very low solubility in aluminum and forms undesirable constituent particles. Ni can be present as an impurity from the anodes in the reduction process.

In some embodiments, the inevitable impurities include Zn in a concentration of less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01. The presence of Zn may have a negative impact on the corrosion performance of the alloy.

In some embodiments, the inevitable impurities include Ti in a concentration of less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01.

In some embodiments, the inevitable impurities include B in a concentration of less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01.

In some embodiments, the inevitable impurities include V in a concentration of less than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01. V is detrimental to the extrudability of the aluminum alloy and is generally only an impurity from the reduction process.

In some embodiments, the inevitable impurities include Zr in a concentration of than or equal to 0.05, less than or equal to 0.04, less than or equal to 0.03, less than or equal to 0.02, less than or equal to 0.01.

In some embodiments, the aluminum alloy of the present disclosure comprises, consists essentially of or consists of any combination of the concentration ranges recited above for Si, Fe, Cu, Mg, Mn, Cu+Mn, Cr and the inevitable impurities.

In some embodiments, the aluminum alloy optionally further comprises a grain refiner, such as titanium, titanium boride, or titanium carbide to solidify aluminum alloys. In an embodiment, the grain refiner is in the form of Ti, TiB or TiC. When TiB is used as a grain refiner, this may result in a B content of up to 0.05 wt. % in the alloy. When TiC is used as a grain refiner, this may result in a C content of up to 0.01 wt. % in the alloy. The dissolved Ti in molten aluminum can enhance the formation of interfacial TiAl3 layer between TiB2/melt interface, which subsequently promotes nucleation of Al grains.

The present disclosure also provides a process for making a high strength aluminum product using the aluminum alloy of the present disclosure. In a first step, the process comprises casting the aluminum alloy of the present disclosure to obtain a cast aluminum product. The casting step can include, for example, direct chill casting, continuous casting and/or semi-continuous casting. A Properzi continuous casting may be used, which may be a wheel and belt casting process or a track & belt casting. The track & belt process replaces the casting wheel by a plurality of copper blocks. Other options including a twin roll caster may be used. The twin roll caster has two rolls that rotate and advance the mold continuously. The rolls may be chilled to aid in solidification of the molten aluminum alloy. Further options include a block caster that has blocks adapted to function as belts. The blocks may be chilled to aid in solidification of the molten aluminum alloy.

In a second step, the cast aluminum product is extruded into an extruded aluminum product. During extrusion the aluminum alloy is heated to a temperature where the alloy is malleable. A press container may be used in front of a die orifice. A hydraulic ram can be used to apply pressure to force the aluminum alloy to fill the container and through the die to make the desired shape. After the hot deformation (i.e., extrusion or Properzi hot rolling), the aluminum alloy may be actively cooled by the use of cooling fans and/or water spray or a full water quench. In preferred embodiments, a quench rate between 500 and 300° C. of 20° C./s is utilized. Most automotive extrusions are hollow to improve stiffness and crush performance. Accordingly, in some embodiments, the extrusion is a hollow extrusion such as a hollow automotive extrusion. Water quenching, usually spray quenching, at the press exit is generally beneficial to ductility as compared to air quenching for example and is thus preferred. The improvement of water quenching over air quenching can be associated with microstructural changes at grain boundaries.

Fully recrystallized grain structures offer advantages over non recrystallized grain structures (typically with a coarse recrystallized grain surface layer) for automotive applications. These include reduced sensitivity of strength to press quench rate, freedom from surface orange peel which can be an initiation site for fatigue and corrosion and local extrusion weld line ductility. In addition, the higher levels of dispersoid forming elements such as Mn and Cr required to produce a non-recrystallized grain structure typically can result in inferior extrudability and strength quench sensitivity. In some embodiments, the present aluminum alloy achieves a fully recrystallized grain structure to benefit from these advantages and the fully recrystallized structure is a feature of the product.

In one embodiment, the process further comprises after casting and before extruding, heat treating or homogenising the cast aluminum product. The heat treatment conditions can be for a time period of at least about 1, at least about 2, at least about 3, at least about 4 or at least about 5 h at a temperature between about 450 to about 600° C., about 500 to about 600° C., about 550 to about 590° C. or about 560 to about 580° C. In some embodiments, the aluminum product is an extrusion billet.

The strength and ductility of aluminum alloys can be determined with various suitable methods. However, the elongation to failure during a tensile test is not a useful parameter for predicting crash performance. On the other hand, the ductility of extrusions can be assessed by the bend testing of the German Association of the Automotive Industry (VDA 238-100) in the extrusion direction or transverse to the extrusion direction. Such bend testing can simulate the bending applied during folding in axial or lateral crush or strains applied during mechanical joining such as self piercing riveting. True fracture strain measured in a tensile test (Ln(initial cross section/final fracture area)) is a useful measure of ductility for such applications.

In some embodiments, the aluminum product of the present disclosure has an Ultimate Tensile Strength (UTS) of at least 250, at least 255, at least 259, or at least 265 MPa. In some embodiments, the aluminum product of the present disclosure has a yield strength of at least 230, at least 235, at least 240, more than 240, at least 245, at least 250, or at least 255 MPa. In some embodiments, the aluminum product of the present disclosure has a longitudinal VDA bend angle of at least 105, at least 109, at least 115, or at least 1190 for a thickness of 2.5 mm. In some embodiments, the aluminum product of the present disclosure has a transverse VDA bend angle of at least 58, 59, 60, 61, 62, 63, 64 or 65°. In some embodiments, the aluminum product of the present disclosure has a true fracture strain of at least 0.65, at least 0.66, at least 0.67, at least 0.68, at least 0.69, at least 0.70, at least 0.71, at least 0.72, at least 0.73, at least 0.73, at least 0.74, at least 0.75 or at least 0.75 when the aluminum product has a thickness of 2.5 mm.

Example

The alloy compositions listed in Table 1 were direct chill cast (DC cast) as 101 mm diameter billets. A 5% Ti-1% B grain refiner was added prior to casting. The billets were homogenised for 2 hrs at 580° C. followed by forced air cooling at 450° C./hr between 500 and 200° C. These were extruded into a 50×20×2.5 mm hollow box profile using a billet preheat temperature of 500° C. at an extrusion exit speed of 13 m/min. The die was constructed to position the extrusion welds on the 20 mm faces of the profile such that property evaluation could be conducted on the wide 50 mm face away from any extrusion welds. After exiting the die the profile was water spray quenched at a rate of 150° C./sec between 500 and 300° C. The quench rate was measured using a clip-on contact thermocouple attached to a high frequency logger with a WiFi transmitter. The cooled profiles were stretched to give a permanent set of 0.5% and aged for 8 hrs/177° C. after a room temperature delay of 24 hrs. Tensile testing was conducted in the longitudinal direction according to ASTM E8 and the fracture area projected in the tensile direction was measured using an image analysis technique to permit calculation of the true fracture strain using the expression Ln(initial cross section area/final fracture area). A higher value of fracture strain is indicative of superior bendability. VDA bend testing was conducted in the longitudinal (bend axis perpendicular to extrusion direction) and transverse (bend axis parallel to the extrusion direction) orientations using a punch radius of 0.4 mm, a roller spacing of 2× the material thickness i.e. 5 mm and a load drop of 60 N. The complementary bend angle was measured after the load was removed, such that an increasing bend angle is indicative of superior ductility or bendability.

Alloy compositions

Weight percent

I.D.
Alloy type
Si
Fe
Cu
Mn
Mg
Cr
Ti
V

Alloy A represents a typical commercial AA6063 alloy used for automotive and other applications and alloy K represents a typical commercial higher strength AA6063 with an increased level of silicon. The Mn additions of 0.02-0.08 wt. % in these types of alloy were made to facilitate the transformation of the beta Al—(Fe,Mn)Si phase to alpha to thereby improve extrudability. Compositions B-J and L represent incremental separate and combined additions of Mn and Cu.

Table 2 presents the mechanical property results for each composition where VDA-L is the longitudinal bend angle test, VDA-T is the transverse bend angle test, El is the elongation, Ef is the true fracture strain, YS is the yield strength and UTS is the ultimate tensile strength.

Results of the alloy testing

I.D.
Alloy type
MPa
MPa
%
degree
degree
Ef

The microstructures of the extrusions were assessed metallographically. All the microstructures exhibited a fully recrystallised grain structure which is desirable for this type of product for improved bulk ductility and local ductility at extrusion weld-lines. There was a general trend exhibited by aluminum alloys of decreasing ductility (as measured by VDA bend angle or fracture strain) with increasing yield strength, mainly as a result of the higher stress level applied to the microstructure promoting earlier initiation of fracture events. FIG. 1 is a plot of longitudinal VDA bend angle vs. yield strength after artificial ageing. The points are labelled with the alloy I.D. from Table 1. The baseline alloys A and K lie in a band along with alloys with incremental Mn additions J, H, B and L and alloy C with a combined addition of 0.08 wt. % Mn and 0.08 wt. % Cu. The Mn additions to alloy A slightly increased the bend angle but also decreased the strength which is undesirable. The addition of 0.20 wt. % Mn to baseline alloy K increased the bend angle slightly for a slight loss in yield strength. However, inventive alloys D, E, F, G and I with combined additions of Mn and Cu provided an increased strength compared to baseline alloy A for the same ductility or equivalent high strength to baseline alloy K for a significant improvement in ductility. FIGS. 2 and 3 present the other measures of ductility, transverse VDA bend angle and true fracture strain, also plotted against yield strength. The trends were very similar to those in FIG. 1 with alloys D, E, F G and I offering a superior combination of strength and ductility. The bend angle measured transverse to the extrusion direction is typically lower than that measured in the longitudinal direction as shown by comparison of FIGS. 1 and 2.

FIG. 4 plots the Mn contents of the experimental alloys vs. the Cu content, separating the compositions in the same way as was shown in FIGS. 1-3. The compositions exhibiting enhanced strength and ductility (solid points) can be distinguished from the compositions exhibiting baseline strength and ductility by a line corresponding to Mn+Cu>0.20 wt. %, preferably Mn+Cu≥0.25 wt. % as shown in FIG. 4, wherein the effective Mn is calculated using Mn+1.6Cr.

All the test material was water spray quenched after extrusion, which was a necessary step to achieve high ductility. Indeed, there is a significant improvement in ductility with increased quench rates associated with this type of cooling.

The alloy compositions in Table 3 were DC cast as 101 mm dia. billets. A 5% Ti-1% B grain refiner was added prior to casting. The billets were homogenised for 2 hrs/580° C. followed by forced air cooling at 450° C./hr between 500 and 200° C. These were extruded into a 50×20×2.5 mm hollow box profile using a billet preheat temperature of 500° C. at an extrusion exit speed of 13 m/min. The profile was spray quenched at the die exit at a rate of 150° C./s. The cooled profiles were stretched to give a permanent plastic extension of 0.5% and aged for 8 hrs/177° C. after a 24 hour room temperature delay. Mechanical testing including tensile and VDA bend testing was conducted in a similar manner to previous examples.

Alloy composition for testing for substitution of Mn by Cr

I.D
alloy type
Si
Fe
Cu
Mn
Mg
Cr
Ti
V

Alloy M was a repeat cast of alloy G which gave improved ductility as measured by VDA for a given yield strength in the example reported in the specification. Alloys N and O were produced to evaluate the effect of partial and complete substitution of Mn by Cr, keeping the Mg, Si and Cu contents fixed. As provided herewith, these two elements—Mn and Cr—can be interchangeable in the submicron dispersoid particles formed during homogenisation.

Table 4 presents the mechanical property results for the three compositions described in Table 3. The tensile properties and ductility values as measured by VDA bend angle were almost identical, indicating that partial and full substitution of Mn by Cr is effective at maintaining superior strength and ductility.

Mechanical properties of alloy compositions of Table 3

I.D
alloy type
Si
Cu
Mn
Cr
Mpa
Mpa
% El
°
°

The yield strength and VDA bend angle of the tested compositions are shown in FIGS. 1 (longitudinal VDA) and 2 (transverse VDA). In FIG. 1, alloys M, N and O gave similar yield strength values to the original alloy G but the longitudinal bend angles were slightly lower. This can be explained by slight variations in test conditions between extrusion tests. However, all three compositions gave superior performance to the band of baseline alloys, indicating that Cr can be substituted for Mn as exemplified herein. Similar trends were observed for transverse bend angle in FIG. 2. The results for the three compositions M, N and fell within the improved strength/ductility band as compared to baseline strength and ductility.

As demonstrated Cr can be used to partially or completely replace Mn as a dispersoid forming element.

While the present disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.