WO2022120639A1

WO2022120639A1 - Aluminium alloy with improved strength and recyclability

Info

Publication number: WO2022120639A1
Application number: PCT/CN2020/134919
Authority: WO
Inventors: Minxia Li; Arvid Espedal; Xiao-Jun Jiang; Jan Halvor Nordlien
Original assignee: Hydro Extruded Solutions As
Priority date: 2020-12-09
Filing date: 2020-12-09
Publication date: 2022-06-16
Also published as: CN116568850A

Abstract

The invention relates to an aluminium based, corrosion resistant alloy consisting of 0,1-0,3 % by weight of silicon, 0,1-0,4 % by weight of iron, 0,5-1,0 % by weight of manganese, ≤ 0,02 % by weight of magnesium, ≤ 0.30 % by weight of zinc, ≤ 0,2 % by weight of chromium, 0.25 % by weight, preferably 0,05-0,20 % by weight of titanium, ≤ 0.05 % by weight Ni ≤ 0.05 % by weight Cu up to 0,05 % by weight of other impurities, each not greater then 0,05 % by weight and the balance aluminium and to extruded tubes produced from the alloy as well as methods for producing the tubes.

Description

Aluminium alloy with improved strength and recyclability

BACKGROUND

This present invention relates to an aluminium alloy, such as multiport extruded (MPE) tubing or round tubes in heat exchanger applications such as air conditioning condensers.

The object of the present invention is to provide an extrudable, drawable and brazeable aluminium alloy that has improved corrosion resistance and is suitable for use in thin wall, fluid carrying tube lines. It is a further object of the present invention to provide an aluminium alloy suitable for use in heat exchanger tubing or extrusions.

In EP1349965 an aluminium alloy composition comprising: <0.30 wt%Si, 0.20-0.50 wt%Fe, <0.05 wt%Cu, 0.5-1.2 wt%M n, <0.05 wt%Mg, <0.50 wt%Zn, 0.10-0.30 wt%Cr, <0.05 wt%Ti, <0.05 wt%Mg; the balance consisting of aluminium and unavoidable impurities is described. This alloy does not provide the strength and corrosion resistance required for MPE tubing in heat exchanger applications.

SUMMARY OF THE INVENTION

The introduction of aluminium alloy materials for automotive heat exchange components is now widespread, applications including both engine cooling and air conditioning systems. In the air conditioning systems, the aluminium components include the condenser, the evaporator and the refrigerant routing lines or fluid carrying lines. In service these components may be subjected to conditions that include mechanical loading, vibration, stone impingement and road chemicals (e.g. salt water environments during winter driving conditions) . Aluminium alloys of the AA3000 series type have found extensive use for these applications due to their combination of relatively high strength, light weight, corrosion resistance and extrudability. The AA3000 series alloys (like AA3102, AA3003 and AA3103) , however, suffers from extensive pitting corrosion when subjected to corrosive environments, leading to failure of the automotive component, in particular at high temperatures. To be able to meet the rising targets/requirements for longer life on the automotive systems new alloys have been developed with significantly better corrosion resistance.

Especially for condenser tubing, 'long life' alloy alternatives have been developed, such as those disclosed in US-A-5,286,316 and WO-A-97/46726. The alloys disclosed in these publications are alternatives to the standard AA3102 or AA1100 alloys used in condenser tubes, i.e. extruded tube material of relatively low mechanical strength. Due to the improved corrosion performance of the condenser tubing the corrosion focus have shifted towards the next area to fail, the manifold and the refrigerant carrying tube lines. Additionally, the tendency towards using more under vehicle tube runs, e.g. rear climate control systems, requires improved alloys due to the heavier exposure towards the road environment. The fluid carrying tube lines are usually fabricated by means of extrusion and final precision drawing in several steps to the final dimension.

The driver for aluminium alloy development is the recent trend for downgauge of the aluminium tubes for different applications as the tubes become thinner and lighter. Therefore, an alloy for MPE (Multi Ports Extrusion) application in a brazed heat exchanger should have significant higher strength and improved inherent corrosion resistance compared to the traditional AA3102 alloy. At the same time the alloy should have better extrudability and inherent corrosion resistance than the traditional AA3003/AA3103 alloy for round tube application, in addition to good mechanical properties and formability.

Industrial brazing of is mainly based on CAB (Controlled Atmosphere Brazing) technology for 3xxx alloys not containing Mg. The problem of the prior art alloys is that during the brazing cycle a transformation of microstructure results in big grains that will dominate the tube cross section.

In order to obtain a high strength after brazing a small grain microstructure is required. The controlling parameter for a stable microstructure is the number density and size of dispersoids. In order to provide the desired microstructure the processing conditions need to be set correctly.

The homogenisation temperature and time is important to get the right number and density of dispersoids. Furthermore, the deformation during extrusion must be controlled so that a low degree of deformation is obtained in order to create a final material with sufficiently small grains throughout the material of the tube wall.

Calibration sizing of MPE profile introduce a small deformation which is critical for grain structure transformation during brazing cycle. During the brazing cycle most of the dispersoids will dissolve or be reduced in size.

The object of this invention is to provide an extrudable, drawable and brazeable aluminium alloy that has improved high temperature resistance and is suitable for use in thin wall, fluid carrying tube lines. It is a further object of the present invention to provide an aluminium alloy suitable for use in heat exchanger tubing or extrusions. A still further object of the present invention is to provide an aluminium alloy with improved recyclability, which can be used both for multiport extrusions and drawn round tube.

The invention provides an extruded and drawn tube alloy having a mechanical strength similar to 3103/3003 type alloys, but with more resistance to microstructural changes during heat treatment.

The main manufacturing requirements for this type of product is maintenance of properties after brazing with CAB, before exposure to in-service elevated temperature. The alloy should have a Controlled Atmosphere Brazing compatibility and be easily produced (extrudable, processable) .

The alloy according to the invention provides the above advantages due to a careful selection of the alloy components according to the appended clams.

DESCRIPTION OF THE DRAWINGS

Fig. 1a shows the production route according to the invention, optionally followed by coating of the tubes with a Zn coating.

Fig 1b shows the production route process according to the invention, optionally followed by coating of the tubes with a Zn coating.

Fig 1c shows the drawn tube production route according to the invention.

Fig. 2 shows the process route for MPE cutting.

Fig. 3 shows the tensile strength of alloy A according to the invention as a function of temperature.

Fig. 4 shows the grain structure of an MPE tube according to the invention after different reductions in calibration.

Figure 5 shows the grain structure of MPEs with the composition according to Alloy A and B produced with the process according to the invention.

Figure 6 shows the corrosion resistance in SWAAT of a round tube produced from an alloy according to the invention (left bars) and a 3003 alloy (right bars) .

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an aluminium-manganese (Al-Mn) based alloy composition and, more particularly, it relates to an Al-Mn based alloy composition combined with a specific homogenization treatment for extruded and brazed heat exchanger tubing.

The invention relates to an aluminium based, corrosion resistant alloy consisting of 0,10-0,30%by weight, preferably 0.10-0.20%by weight of silicon, 0,10-0,40%by weight, preferably 0,10-0,20%by weight of iron, 0,50-1,0%by weight, preferably 0.60-0,80%by weight, more preferably 0.65-0,75%by weight of manganese.

≤0,02%by weight, preferably≤0,02%by weight of magnesium,

≤0.30%by weight, preferably 0,20-0,30%by weight of zinc,

≤0,20%by weight of chromium,

≤0,25%by weight of titanium,

≤0.05%by weight Ni;

≤0.05%by weight Cu,

up to 0,05%by weight of other impurities, each not greater then 0,05%by weight and the balance aluminium.

The aluminium alloy is cast as an ingot such as a billet and is subjected to a homogenization treatment at a temperature ranging between 550 and 600℃ to obtain a billet/ingot conductivity of>38%IACS (International Annealed Copper Standard) , preferably>39%IACS, most preferably 40-42%IACS.

The aluminium alloy is homogenized for two to eight hours and, in an alternative embodiment, for four to eight hours.

The homogenization treatment is followed by a controlled cooling step carried out at a cooling rate below approximately 150℃ per hour.

The homogenized ingot is reheated to a temperature ranging between 450 and 520℃ a rate of 70-100deg C/m of billet length, and extruded into tubes. The press container temperature is set to 350-450 deg C and the billet extruded through the die. In one embodiment, the extruded tubes have a wall thinner than 0.5 millimeter. The extrusion step can be followed by a drawing step in which the tube height is reduced by no more than 5%. The extruded or drawn tubes can be brazed to heat exchanger components such as manifold, internal and external corrugated fins, etc.

The homogenized aluminium alloy combines high extrudability with a uniform fine surface grain structure for improved corrosion resistance.

During homogenization of Al-Mn alloys, manganese is taken into solid solution or precipitated as manganese rich dispersoids depending on the homogenization temperature and the manganese content of the alloy. In the Al-Mn based alloy composition and homogenization treatment of the invention, the resulting ingot has a microstructure with sufficient manganese out of solution to reduce the high temperature flow stress and extrusion pressure, but with manganese rich dispersoids in the correct form, i.e. size and interparticle spacing, to inhibit recrystallization during a furnace braze cycle, while still providing reduced flow stress. The dispersoid size should be>100 nm and the dispersoid densitiy>100000 dispersoids/mm ²

The controlled homogenization cycle for the Al-Mn based alloy of the invention improves extrudability and prevents coarse grain formation during brazing.

The extrusion pressure is controlled by two factors and, more particularly, the level of manganese in solid solution and the contribution of strengthening from manganese rich dispersoids. When there is more manganese in solid solution the conductivity is lower and the extrusion pressure is higher.

However, at low temperatures, another mechanism is operating. More particularly, dispersion strengthening by the dense manganese rich dispersoids occurs through the Orowan strengthening mechanism. The optimum situation for extrusion pressure is at intermediate homogenization temperature where the combined effect of the two mechanisms is minimized. It is therefore possible to define a preferred conductivity in the homogenized billet of>38%IACS for optimum extrudability and microstructure.

With a combination of aluminium alloy composition and homogenization temperature according to the invention, there is sufficient manganese out of solution to reduce the high temperature flow stress and extrusion pressure, but with manganese rich dispersoids in the correct form, i.e. size and interparticle spacing, to inhibit recrystallization of the extruded tube during a furnace braze cycle, while still providing reduced flow stress.

In case of multiport extrusions the final grain size after brazing should preferably be<100 um,preferably<50 um, but the important feature is that there is more than one single grain occupying the whole cross section of the intermediate wall of the MPE (i e the walls separating the fluid lines from each other) .

The present invention provides an aluminium-based alloy, consisting of 0,10-0,30%by weight of silicon, 0,10-0,40%by weight of iron, <0,02%by weight of magnesium, 0,50–1,0%by weight of manganese, ≤0.30%by weight of zinc, ≤0,20%by weight of chromium, <0.25%by weight of titanium, ≤0.05%by weight Ni, ≤0.05%by weight Cu up to 0,05%by weight of other impurities, each not greater then 0,05%by weight and the balance aluminium.

The Mn/Fe ratio should preferably be larger than 2, to ensure a beneficial chemistry of intermetallics for a corrosion resistant alloy.

The reason for limitation of the individual alloying elements will now be described.

The silicon content is between 0,10-0,30%by weight, more preferably between 0,10-0,20%by weight. It is important to keep the silicon content within these limits in order to control and optimise the size distribution of AlMnFe/AlMnFeSi-type particles (both primary and secondary particles) , and thereby controlling the strength and the grain size of the final product.

In general, a low iron content is desirable for improved corrosion resistance, as it reduces the amount of iron rich particles which generally creates sites for pitting corrosion attack. A lower content of Fe could be difficult to achieve from a cast-house standpoint of view, and also has a negative influence on the final grain size (due to less iron rich particles acting as nucleation sites for recrystallization) . It is however expensive to completely remove Fe from the alloys and some iron may also give a positive effect on the final grain size. The iron content of the alloy according to the invention should be between 0.10-0.40%by weight, preferably 0.10-0.20%by weight.

The content of magnesium should be below 0,02%by weight due to its negative effect on extrudability. Additions above 0,02%by weight are also incompatible with good brazeability in inert gas brazing. The content of magnesium should preferably be below 0.01%by weight.

Manganese increases the corrosion potential with around 5 mV per 0.1 wt%Mn in commercial aluminium alloys for multiport extrusions and adds to the corrosion protection of the tube. However, above 0.8 weight%the effect on the corrosion potential is minor, while the increase of extrusion pressure is significant. The manganese content should be 0.5-1.0%by weight, preferably 0,6-0,8%by weight, more preferably 0.65-0.75%by weight.

Zinc has a strong positive effect on the corrosion resistance by promoting lateral corrosion, and if added to the alloy one may avoid having to coat the tubes with Zn to obtain a corrosion resistant tube whereby a more recycle friendly product is obtained. Zn however lowers the corrosion potential of the tube material and needs to be balanced to the Mn content. A high content of Zn will reduce recyclability and in view of the polluting effect of zinc by “infecting” the furnace wall lining and the level of this element should be kept<0.3%by weight. The amount of zinc is preferably 0.20–0.30%by weight.

Chromium adds to the desired mechanical strength and corrosion resistance after heat treatment (such as brazing) . During the brazing process the smaller dispersoids of the AlMnFeSi alloy are dissolved to a greater extent than that for alloys comprising Cr and mechanical properties are degraded. Introducing Cr into the particles will stabilize the microstructure and effects of heat treatment (brazing, annealing) are more predictable. Additions of chromium, however, decreases the extrudability due to the formation of coarse primary particles and influences negatively the tube drawability. The content of chromium should be≤0.20%by weight, preferably≤0.05.

The elements titanium improves the corrosion resistance. The content of titanium should be≤0,25%by weight. Further optimizing of the corrosion resistance can be obtained by adding titanium between 0.05-0.20%by weight.

A low content of Cu and Ni is critical for corrosion resistance, therefore the content of these elements should be below≤0.05%by weight, preferably<0.02%by weight, more preferably<0.01%by weight. Copper also has a negative effect on extrudability, even for small additions.

Examples

Extruded tubes were prepared in a traditional way by DC casting of aluminium alloys according to Table 1 into extrusion ingots.

The ingots were Homogenized at 600℃ with soak time in the range of 8 hours.

The extrusion process for manufacture of MPE tubes was set up as follows: The invention alloy billets with a composition according to below were heated to a temperature of 460-550 deg C.

Table 1

A heating taper of 70-100deg C/m of billet length was used during ramp up. The dies were pre-heated at 460-510 deg C. and soaked 2 to 10 hours before extrusion. The press container temperature was set to 350-450 deg C based on billet temperature setting, where after the billets were extruded through the die and shaped to MPE tubes.

The MPE tubes were sprayed with Zinc by arc spray with a load of 4-13g/m ² on both flat surfaces when the tubes were hot coming out of the press for better corrosion resistance. The tubes were coated with zinc arc spray coatings before cut-to-length. Tubes are cooled by water quench and dried in a hot air blower, after which the tubes are coiled for next process step. The extrusion process flow chart is shown in fig 1 a.

If coated by flux or braze coatings the tubes are cooled by water quench immediately after extrusion and dried in a hot air blower, after which the tubes are coiled and coated with the flux or braze coatings by roll coating process. MPE extrusion and coating flow chart is shown in fig 1 b.

MPE cutting

The coiled tube is moved to cutting machine to cut to the desired length. The important part of this process is sizing for keeping fine grain size of the invention alloy. For keeping fine grain size on webs of MPE the reduction of the tube height is not more than 0.6mm (corresponding to 5%of the tube height) . The tubes are cut open at tube ends, the cut being 2/3 wall depth, and the walls pulled apart tube to get big opening. MPE cutting process flow chart is shown in figure 2.

Tensile testing

The mechanical properties of the tubes produced were tested according ISO 6892-1at different temperatures. The result in figure 3 shows that the properties of the tubes according to the invention did not degrade significantly up to temperatures of 180 deg C.

Grain structure analysis

The grain structure of MPEs with the composition according to Alloy A produced with different reductions in calibration is shown in figure 4. As can be seen large grains start to appear in the intermediate walls of the MPE tube when the reduction exceeds 7%.

The results show that the aluminium alloy extrusion produced with the inventive alloy composition and process gives a significantly better as brazed strength and corrosion resistance than aluminium extrusions produced according to the standard procedure, while assuring good brazeability in CAB.

Extrusion of tubes

Alloy billets with the following compositions were extruded to round tubes:

A heating taper of 70-100deg C/m of billet length were used during ramp up. The dies were pre-heated at 460-510 deg C. and soaked 2 to 10 hours before extrusion. The press container temperature was set to 350-450 deg C based on billet temperature setting, where after the billets were extruded through the die. The extruded round tube according to the invention can be drawn by 2 or 3 draws to a total maximum reduction of 70%before annealing. The process flow chart is in Fig 3c.

Corrosion testing

Figure 6 shows the corrosion resistance in SWAAT of a drawn tube from an alloy according to the above specification (left bars) and a 3003 alloy (right bars) , both produced according to the invention with a final step of inline annealing at 500 deg. C and the final reductions of 58%on invention alloy and 72%on 3003 alloy. The SWAAT was done according to ASTM G85-A3. The alloy according to the invention showed a considerably better resistance to corrosion.

Claims

An aluminium based, corrosion resistant alloy consisting of

0,1-0,3%by weight of silicon,

0,1-0,4%by weight of iron,

0,5-1,0%by weight of manganese,

≤0,02%by weight of magnesium,

≤0.30%by weight of zinc,

≤0,2%by weight of chromium,

≤0,25%by weight of titanium,

≤0.05%by weight Ni

≤0.05%by weight Cu

up to 0,05%by weight of other impurities, each not greater then 0,05%by weight and the balance aluminium.
The aluminium based alloy according to claim 1, characterized in that it contains≤0,01%by weight of magnesium.
The aluminium based alloy according to claim 1 or 2, characterized in that it contains 0,6-0,8%by weight of manganese.
The aluminium based alloy according to claim 1 or 2, characterized in that it contains 0.65-0.75%by weight of manganese.
The aluminium based alloy according to claim 1, characterized in that it contains 0.10-0.20%by weight of silicon.
The aluminium based alloy according to any of the proceeding claims, characterized in that it contains 0.10-0,.30%by weight, more preferably 0,10-0,20%by weight of iron.
The aluminium based alloy according to anyone of the preceding claims, characterized in that it contains≤0.05 by weight of chromium.
The aluminium based alloy according to anyone of the preceding claims, characterized in that it contains 0.05-0.20%by weight or titanium.
The aluminium based alloy according to anyone of the preceding claims characterized in that it contains 0.20-0.30%by weight zinc.
The aluminium based alloy according to anyone of the preceding claims, wherein the content of Cu and Ni is<0.02, preferably<0.01 weight%.
The aluminium based alloy according to anyone of the preceding claims wherein the ratio Mn/Fe>2.
An extruded tube produced from the alloy of claims 1-11.
Extruded tube according to claim 12, where the tube is a multiport extrusion.
Extruded tube according to claims 12 or 13, which has a dispersoid size>100 nm and a density of>100000 dispersoids/mm ².
Extruded tube according to claim 12-14, wherein the final grain size after brazing is<100 um, preferably<50 um.
Extruded tube according to claim 12, where the tube is a drawn tube.
Extruded tube according to claims 16, which has a corrosion resistance in SWAAT of>20 days.
Extruded tube according to claims 12-17, where the electric conductivity of the alloy is>38%IACS (International Annealed Copper Standard) , preferably>39%IACS, most preferably 40-42%IACS.
Method for producing the tube of claim 13-15 or 18, characterized in;

- casting billets of the alloy composition according to claims 1-10

- homogenizing the billets at 550–600 deg C for 2-8 hours followed by cooling

- heating the billets to a temperature of 450-550 deg C at a rate of 70-100deg C/m of billet length

- setting the press container temperature to 350-450 deg C and extruding the billet through the die

- Reducing the tube height by no more than 5%in a final step
Method according to claim 19, characterized by the further step of cooling the tubes in water quench and drying the tubes, followed by coiling the tubes.