Patent Application
20180282849
The present invention relates to a method for producing structural components, in particular from AA 6xxx series alloys, which are extruded or rolled and subjected to further processing to obtain improved mechanical properties.
The aluminum extrusion process normally begins by heating cast and homogenized billets or logs to a desired extrusion temperature (depending on the alloy, typically: 400-520° C.). The aluminum alloy is at such a temperature still solid but malleable. The heated aluminum billet is then transferred to a container in an extrusion press. Then, a stem with a dummy block that seals towards the container presses from behind, forces the aluminum alloy through the opening(s) of an extrusion die, in turn resulting in a long length of an aluminum extrusion, emerging from the other side of the die.
In a modern extrusion plant, the front of the profile is gripped by a puller that applies a certain force depending on the alloy and cross sectional area of the profile. Typically, two pullers with a flying saw operate simultaneously and cut the profile in the stop mark between two extruded lengths. The extrusions are subjected to cooling at the runout table by water quenching or air-cooling. Water quenched profiles are typically cooled down by a quench box or standing wave to room temperature at the runout table, whereas air-cooled profiles are typically further cooled down at the cooling table after being transferred from the runout table. If the metal flow in the extrusion die is well balanced and the cross section is not too asymmetrical the profile will remain reasonably straight if the profile is cooled by air. For a water-quenched profile, it can be more challenging to avoid that the profile bends during the cooling operation. However, with a quench box where the water flow can be adjusted independently from all sides and along the length of the quench box, most profiles can be quenched without too much bending and warping. In either case, the puller will help keeping the profile straight after extrusion and cooling.
The cooled extruded lengths are then normally stretched to obtain a plastic deformation in the range of 0.3-1.0%. The purpose of such stretching is to have stress-relief and straight profiles. The long extrusions are cut to desired lengths and are then usually subjected to a heat treatment step called artificial ageing. This ageing treatment, which significantly increases the strength, is typically done at a temperature between 140 and 220° C., depending on what properties the aluminum profiles are going to have.
From EP 2 883 973 A1 is known a process of the above kind for obtaining extruded products made from a 6xxx aluminum alloy where the extruded profiles after extrusion are quenched to room temperature and then optionally stretched between 0,5 and 5% to obtain stress relief and straight profiles, as is stated in the description of the patent application.
Document WO2016/034607 describes an aluminium alloy extruded product obtained by following steps: a) casting a billet from a 6xxx aluminium alloy comprising: Si: 0.3-1.5 wt. %; Fe: 0.1-0.3 wt. %; Mg: 0.3-1.5 wt. %; Cu<1.5 wt. %; Mn<1.0%; Zr<0.2 wt. %; Cr<0.4 wt. %; Zn<0.1wt. %; Ti<0.2 wt. %, V<0.2 wt. %, the rest being aluminium and inevitable impurities; b) homogenizing the cast billet at a temperature 30° C. to 100° C. lower than solidus temperature; c) heating the homogenized billet at a temperature lower than solidus Ts, between Ts and (Ts−45° C.) and superior to solvus temperature; d) cooling until billet temperature reaches a temperature between 400° C. and 480° C. while ensuring billet surface never goes below a temperature substantially close to 350° C.; e) extruding at most a few tens of seconds after the cooling operation the said billet through a die to form at least an extruded product; f) quenching the extruded product down to room temperature; g) stretching the extruded product; h) ageing the extruded product, without beforehand applying on the extruded product any separate post-extrusion solution heat treatment, the ageing treatment being applied such that the product presents an excellent compromise between strength and crashability, with a yield strength Rp0.2 higher than 240 MPa, preferably higher than 280 MPa and when axially compressed, the profile presents a regularly folded surface having cracks with a maximal length of 10 mm, preferably less than 5 mm.
It is generally known, for instance from the publication “Properties for aluminum alloys”, Mr. J. Gilbert Kaufmann, ASM International, that many aluminum alloy products are given a small amount of cold work following solution heat treatment and quenching in order to minimize the internal residual stresses resulting from combination of working, holding at high temperatures and quenching rapidly. It is stated here that the amount of cold work given from stress relief treatment generally is in the range of 1 to 3% stretching for plate, rolled or extruded products and 3 to 5% compression for forgings. The amount of stretching for stress relief referred to here is much higher than normally used in a modern extrusion plant. Most likely, this is due to the T6 treatment with separate solutionizing followed by dropping a bundle of long profiles into a deep quench tank. In this case, the profiles will twist and bend much more than if a profile is quenched when it is held by a puller. For a T5 treatment much less stretching is used, normally in the range of 0.3-1.0% plastic deformation.
In the same article, there is a chapter on “Effect of Additional Cold Work Following Solution Heat Treatment”, which refers to studies on the effect of stretching on fatigue properties of alloys 2024, 6061 and 7075. None of these alloys shows any benefits of the stretching and for alloy 7075 possibly a negative effect.
In the ASM Specialty Handbook, “Aluminum and Aluminum Alloys”, edited by J. R. Davies there is a chapter on thermomechanical effects on ageing. The T3 temper refers to cold working after extrusion whereas the T8 refers to cold working after separate solutionizing. Here it is stated that alloys of the 2xxx series, such as 2014, 2124 and 2219, respond positively to cold working after quenching with respect to strength, whereas other alloys show no or little added strengthening for the same type of treatment. For 2xxx series alloys there are several T3 and T8 type of tempers while 7xxx series alloys, which do not respond positively to cold work following the solution treatment, no such tempers are standard.
Results of extensive experimentation with 7xxx alloys is further carried out and published by ASM (American Society for Metals), “Properties and Physical Metallurgy, John E. Hatch, where among other things is concluded that for 7xxx alloys “the attainable strength decreases progressively with increasing cold work up to at least 5%”. This effect is attributed to the dislocations that are causing heterogeneous nucleation of the η′-precipitates and thereby suppressing the more dense homogeneous nucleation of the η″-precipitates that gives a higher strength contribution. Cold working by cold rolling to higher levels than those used for stress relief purposes can provide hardness levels surpassing those provided by precipitation hardening effects only, but those are not used commercially.
Accordingly, it is desirable to have a method that allows efficient production of structural components from heat treatable aluminium alloys that not only produces said components with improved mechanical properties, but also enables an efficient production. Such a method is especially desirable as the alloys that allow improved mechanical properties of a structural component generally also offer more deformation resistance during the production of a structural component, for example during extrusion, and therefore result in an inefficient production process.
Accordingly, the invention provides a method for producing structural components from heat treatable aluminum alloys, in particular AA 6xxx series alloys, the components having improved crush properties and being particular applicable in crash zones of vehicles, such as longitudinals and crash boxes, the method including the following steps.
When producing the component by extrusion, the method according to the invention may include the following steps:
When producing the components from a rolled sheet, the method according to the invention includes the following steps:
As is apparent from the experimental data provided below, it has been found that the stretching of the structural member or the extruded profile produced according to the method according to the invention to obtain at least 1.5% plastic deformation greatly improves the crushperformance. It has further been found that the production efficiency of the structural member can be further improved when the method comprises a heterogenizing step (herein also referred to as “soft annealing”) after the homogenizing step and before the extrusion step. This allows precipitating Mg2Si from the Al-rich phase (α-phase) resulting in a depletion of Mg and Si from the Al-rich phase. This reduces the deformation resistance of the alloy and allows better extrusion performance. The stretching according to embodiments of the invention is carried out after the solutionizing step and before the aging (also before the optional pre-aging) for embodiments in which a structural member (e.g. a profile) is formed by extrusion. It has been found that when the process comprises heterogenizing, better properties of the profile are obtained if the process comprises solutionizing as well. For rolled material, stretching according to the invention is carried out after the solutionizing step and before forming a structural member (i.e. the rolled sheet metal is stretched) or after forming the structural member (i.e. the sheet metal that has been formed into the structural member is stretched). In other words, the structural member is optionally stretched for embodiments in which a structural member (e.g. a profile) is formed from rolled sheet metal, wherein the stretching is also in these embodiments carried out before the aging (e.g. before the pre-aging).
Homogenization may for example be carried out at a temperature between 520° C. and 590° C., e.g. at a temperature between 550° C. and 580° C., for a duration of more than 0 hour and less than 12 hours, wherein a value of 0 hours indicates that the alloy is heated to reach the homogenizing temperature and, when reaching the homogenizing temperature, is immediately cooled. According to embodiments, the homogenization is carried out for 1 to 4 hours. The temperature and time should be chosen so that the single phase region with respect to Al, Mg and Si in the phase diagram is reached so as to bring these (and further elements) into solid solution in the Al-rich phase. Further, homogenization may be carried out such as to precipitate intermetallic phases of elements that are not fully solvable in the Al-rich alpha phase.
According to embodiments of the invention, homogenization may be followed by a heterogenization step (also referred to as “soft annealing”). Said heterogenization step may immediately follow the homogenization (i.e. without any cooling below the heterogenizing temperature between the steps) or may be carried out separately (i.e., there may be cooling below the heterogenization temperature, e.g. to room temperature, between the steps). When the heterogenization is performed immediately after the homogenization, the process is more efficient and uses less energy. When homogenization and heterogenization are carried out separately, the process is more versatile. The cooling from the homogenization temperature to the heterogenizing temperature or, when homogenization and heterogenization are carried out separately, to room temperature, is, according to embodiments of the invention, performed using a cooling rate of between 25° C./hour and 500° C./hour. According to embodiments the cooling rate between homogenization and heterogenization temperatures is for example between 100° C./hour and 400° C./hour.
The heterogenizing step may for example be carried out at a temperature of between 350° C. and 450° C., for example between 390° C. and 430° C. A 6061 alloy has a solvus temperature of about 540° C., so, according to embodiments of the invention, the heterogenizing temperature may be at least about 90° C. lower than the solvus temperature of the invention. For the heterogenizing, an alloy may be held for 0 to 12 hours, for example for 1 to 12 hours, e.g. for 2 to 8 hours, at the heterogenizing temperature, wherein a value of 0 hours indicates that the alloy is slowly cooled from the homogenizing temperature, e.g. at 25° C./hour or less, all the way down to 350° C. or even below, e.g. to room temperature. After homogenizing or after homogenizing and heterogenizing, the billet is extruded or otherwise processed as described herein.
The stretching may be carried out so that the profile obtains at least 1.5% plastic deformation, e.g. more than 1.5% plastic deformation, for example 2% or more plastic deformation, for example 3% or more plastic deformation, for example 4% or more plastic deformation. Herein, stretching by x % may mean that a length before and after stretching differs by x % in the stretching direction after the stretching forces are relaxed. For example, a length that was 1 m before stretching may correspond to a length of 1.04 m after stretching by 4%.
After the stretching, ageing is carried out. The ageing may for example be performed as a one-step, two-step or a dual rate ageing process. In addition, the ageing may optionally comprise a pre-aging step. In this respect, it has been found that it is beneficial for the strength of 6xxx alloys with high contents of Mg and Si (e.g. 6061 or 6082) when the ageing is done as soon as possible after the solutionizing. There is a beneficial effect when ageing is carried out up to approximately 4 hours after the solutionizing, but the beneficial effect is the stronger the sooner the ageing is done after the solutionizing. However, the present inventors have discovered that a similar beneficial effect can also be achieved if only a short ageing cycle, herein referred to as pre-ageing, is started within 4 hours after solutionizing. After this pre-aging, the material may be held at room temperature, e.g. for up to several weeks, before further ageing is carried out. The use of pre-aging therefore allows to obtain the beneficial effects on strength that are achieved by carrying out ageing shortly after extrusion or solutionizing, while at the same time a more flexible production method is obtained.
As mentioned, the pre-aging step after the stretching that can further improve the mechanical properties of the profile. The pre-aging may for example be carried out at a temperature between 90° C. and 230° C. for a duration between 1 and 120 minutes, for example for between 1 and 7 minutes at a temperature between 140° C. and 160° C. However, depending on the alloy and the profile and the desired properties, also other temperatures and durations are possible.
According to embodiments, the pre-aging is started up to 15 minutes after the extrusion or the optional solutionizing is finished, although according to embodiments pre-aging may be started until up to 4h after the solutionizing is finished.
After stretching and optionally pre-aging, the profile may be artificially aged to the desired temper designation.
It has been found that the method according to an embodiment of the invention is particularly useful to produce extruded or rolled automotive parts where high strength and thin walls are wanted in order to save weight. This could for example be sills, which typically are extruded multi-chamber profiles. Such an automotive sill may for example be part of the vehicle body section below the base of the door openings of the vehicle body. A wall of a profile forming such an automotive part, e.g. a sill, can be rather thin. As the method according to embodiments of the invention allows the production of profiles with improved mechanical properties and allows, especially if heterogenization is used, to use favorable extrusion process parameters, thin-walled profiles with wall thicknesses smaller than 2.00 mm, e.g. smaller than 1.5 mm, and improved mechanical properties may be efficiently produced without defects.
The invention will be further described in the following by way of example and with reference to the drawings, where:
The choice of materials for a vehicle is the first and most important factor for automotive design and there is a variety of materials that can be used in the automotive body and chassis. The most important criteria that a material should meet are lightweight, economic effectiveness, safety, temperature stability, corrosion resistance, and recyclability in addition to meeting the demands with respect to mechanical strength requirements. With the present invention, the inventors aimed at optimizing the choice of aluminum alloy and method of manufacturing components of the alloy in relation to these criteria.
It was an objective of work in relation to the invention to test how stretching prior to ageing would affect the crush performance of a recrystallized and a non-recrystallized material and thus enable optimal selection of alloy and method of manufacturing.
Tests referred to in
The alloys were cast as ø95 mm billets at the applicant's casting lab, using casting parameters that are typical for these kind of alloys. Both alloys were homogenized at 575° C. for 2 hours and 15 minutes, and cooled by approximately 400° C. per hour down to room temperature.
The billets were then extruded to a 29×37 rectangular hollow profile with a wall thickness of 2.8 mm, as shown in
The extrusion was performed in a vertical 800-ton extrusion press with a 100 mm diameter container. The preheating temperature prior to extrusion was in the range 500-510° C. for all the extruded billets. The extrusion profile speed was 8.2 m/min for all billets. Immediately after extrusion, the profiles were quenched in water in a tube that was placed approximately 60 cm behind the die opening, and the cooling rate therefore was very high.
The profiles were then cut into approximately 100 cm lengths and stretched to different amounts of plastic strain (0%, 2% and 4%). All profiles, both the profiles that were un-stretched and stretched 2 and 4%, were aged at 200° C. The holding times at temperature were 1, 2, 4, 7 and 10 hours. The tensile results are shown in
The crush tests were performed mainly in accordance with the car manufacturer Volkswagen, VW TL 116 Norm. The difference was that the samples were only 100 mm to start with and then crushed down to approximately 35 mm. In the current tests, three parallel crush samples were tested at each condition.
Studying the results of the tests, 4% stretching appears to have a dramatic effect on the crush properties for the 6061 alloy used in the current test. This alloy only have 0.05 weight percentage of Cr, which is a too low amount to give a substantial number of dispersoid particles and thereby to prevent recrystallization of the profile after extrusion. This profile therefore has a recrystallized grain structure with high angle grain boundaries. In this respect,
As the current findings confirm that stretching has an effect on the crush properties of the tested 6061 alloy, it is also quite likely that stretching prior to ageing has a similar effect on other 6xxx alloy variants that give a recrystallized structure in the extruded profile.
Alloy 6110 contains 0.55 weight percentage Mn and 0.15 weight percentage Cr and therefore has many dispersoid particles (mainly α-AlFe(MnCr)Si type). Due to the high amount of dispersoid particles, the extruded profile of this alloy will normally have a non-recrystallized grain structure (cf.
As is apparent from
It is apparent from
It is thought that when the method according to embodiments is used, the number of dispersoid particles is low when Cr and Mn contents are low, and thus the dispersoid particles do not affect the deformation resistance very much. The material recrystallizes after extrusion and the grain structure in the profile is therefore very stable during the subsequent solutionizing process. The Mg/Si ratio of the alloys according to the invention may be close to Mg2Si (effective Si and in atomic percent), and the local eutectic melting point around of the particles may therefore be rather high. With excess Si the melting point drops significantly. The “effective” amount of Si is the total amount of Si present in the alloy (as e.g. obtained by chemical analysis) minus the amount of Si bound in primary constituent particles of the type AlFe(MnCr)Si and in possible dispersoid particles of the type Al(MnCr)Si. The melting point significantly affects the extrudability.
As the current findings confirm that stretching has an effect on the crush properties of the tested 6005A alloy, 6110 alloy and 6061 alloy, it is also quite likely that stretching prior to ageing has a similar effect on other 6xxx alloy variants that give a recrystallized or a non-recrystallized structure in the extruded profile.
The fact that recrystallized variants of 6xxx alloys can be used in high strength crush components of vehicles with demands on crush properties, opens up for a significant increase in the productivity at the extrusion plant and thereby reduced production costs for such components.
Even though the 6xxx alloys, based on the above observations related to improved productivity and improved crush properties may be the best choice for structural components in vehicles, some preferred 7xxx alloys as defined in the claims may also represent a good choice for such applications.
In this respect,
The above tests are performed with extruded hollow profiles. However, the method according to the invention may also be exploited for the production of structural hollow components based on sheet material as well as for the production of solid profiles formed by extrusion or other production means.
In this respect,
Accordingly, by combining a process that involves separate solutionizing of the profile after extrusion or rolling with uniform stretching of the profile by more than 1.5% plastic deformation in the axial direction, an efficient method for producing crush resistant parts, such as e.g. automotive sills, longitudinals or crash boxes, is obtained. Said method according to the invention may reduce variations in mechanical properties from the extrusion process. Further, the method may be carried out by less advanced extruders since it is not required to water quench the profiles after extrusion. That the extrusion process may be performed without water quenching may also increase the recovery from the extrusion process (there is less back end scrap produced). The solutionizing according to the invention may also increase the formability, in particular if it is performed directly before the forming operation. It has further been found that the heterogenizing according to the invention can greatly improve extrusion efficiency. In this respect, the heterogenizing may be carried out such that a material having a number density of Mg2Si particles that have a diameter of more than 3 μm of 1000 per mm2 or more in a cross section is obtained. In this respect,
| Number | Date | Country | Kind |
|---|---|---|---|
| 20151793 | Dec 2015 | NO | national |
| 20160252 | Feb 2016 | NO | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/082231 | 12/21/2016 | WO | 00 |
