Method for and equipment for suppressing discoloration of Al—Mg products

Abstract

Method and means for suppressing discoloration during thermal treatment of a product of a magnesium containing aluminium alloy, the alloy contains in wt. % Mg: 0.45-12.0, with a preferred range of 0.45-6.0 wt %. The product, being either an extrusion billet, a sheet ingot, a cast product, or a forged product is heated to a temperature T where it is prone to surface discoloration and oxidation, wherein during the thermal treatment it is exposed to a suppressing atmosphere comprising 0.5-5.0% CO.sub.2 gas with a preference for 0.5-1.5% CO.sub.2 gas.

Claims

1. A method for suppressing discoloration during thermal treatment of a product of a magnesium containing aluminium alloy, the alloy containing in wt. % Mg: 0.45-12.0 where the product, being either an extrusion billet, a sheet ingot or a cast product, is heated to a temperature T where the product is prone to surface oxidation, wherein the product during the thermal treatment is exposed to a suppressing atmosphere comprising 0.5-5.0% CO.sub.2 gas.

2. The method according to claim 1, wherein the rest of the suppressing atmosphere comprises natural air.

3. The method according to claim 1, wherein the rest of the suppressing atmosphere comprises a mix of natural air and exhaust gases from combustion of natural gas or other gas compositions.

4. The method according to claim 1, wherein the suppressing atmosphere comprises 0.5-1.5% CO.sub.2 gas.

5. The method according to claim 1, wherein the suppressing atmosphere comprises approximately 1.0% CO.sub.2 gas.

6. The method according to claim 1, wherein the suppressing atmosphere comprises 1.0% CO.sub.2 gas and 99% air.

7. The method according to claim 1, wherein the alloy contains 0.45-6 wt % Mg.

8. The method according to claim 1, wherein the method further comprises exposing the cast product to various degrees of forming or machining between casting and the thermal treatment.

9. The method according to claim 1, wherein the temperature T is between 450° C. and the melting point of the alloy.

10. The method according to claim 1, wherein a holding time of up to 15 hours at the temperature T is applied.

Description

(1) The invention will be further described in the following by way of examples and with reference to the drawings and figures where:

(2) FIG. 1 is a sketch showing an example of a layout of a batch homogenisation furnace, seen from one side,

(3) FIG. 2 is sketch showing an end view of the homogenisation furnace shown in FIG. 1,

(4) FIG. 3 is a sketch showing a top view of the homogenisation furnace of FIG. 1,

(5) FIG. 4 is a sketch showing an example of a layout of a continuous homogenisation furnace, seen from one side,

(6) FIG. 5 is a sketch showing a top view of the furnace shown in FIG. 4,

(7) FIG. 6 is a photo taken of two end cuts of one extrusion billet homogenized in normal atmosphere,

(8) FIG. 7 is a photo taken of two end cuts of one extrusion billet homogenized in an atmosphere containing ca. 1% CO.sub.2,

(9) FIG. 8 is a photo taken of a sample exposed to 1% CO.sub.2 and Air,

(10) FIG. 9 is a photo taken of a sample exposed to 2% CO.sub.2 and Air,

(11) FIG. 10 is a photo taken of a sample exposed to 3% CO.sub.2 and Air,

(12) FIG. 11 is a photo taken of a sample exposed to Air.

(13) The present invention relates to suppressing discoloration or oxidation of solidified products of magnesium containing alloys where the alloy can contain magnesium in the range from 0.45% Mg up to 12% Mg, and more particular in the range 0.45-6% Mg.

(14) The thermal treatment temperature T can be in the interval 450-610 degrees Celsius.

(15) Further, according to the invention, the surface of the product is exposed to an atmosphere that contains 0.5-5% CO.sub.2, and more particular in the range 0.5-1.5% and as preferred concentration of approximately 1%.

(16) The mechanism that makes the protective layer by means of CO.sub.2 gas concentrations as described here is the same for all alloys having a Mg content as described here. This mechanism restricts diffused Mg from getting in contact with oxygen in the atmosphere, and therefore it hinders the formation of Mg-oxide and consequently that the surface becomes dark. Due to this mechanism it is not the Mg content as such that is decisive, but that the protective layer itself is formed.

(17) In FIG. 1 there is shown a sketch of a layout of a batch homogenisation furnace 10, seen from one side. A batch of billets 1 is arranged in the furnace. Further, the furnace has an electrical cabinet 2, a control cabinet for supply of CO.sub.2 3 and a CO.sub.2 tank 4.

(18) FIG. 2 is sketch showing an end view of the homogenisation furnace 10 shown in FIG. 1, with the batch of billets 1, inlet 6 for CO.sub.2 and a measurement arrangement 5 for CO.sub.2 gas concentration.

(19) FIG. 3 is a sketch showing a top view of the homogenisation furnace 10 of FIG. 1, disclosing a batch of billets 1, inlets 6 for CO.sub.2 gas, measurement arrangements 5 for CO.sub.2 gas concentration. Further, there is disclosed the electrical cabinet 2, the control cabinet for supply of CO.sub.2 3 and the CO.sub.2 tank 4.

(20) In FIG. 4 it is disclosed an example of a layout of a continuous homogenisation furnace 11, seen from one side, there is disclosed an electrical cabinet 2′, a control cabinet for supply of CO.sub.2 3′, a CO.sub.2 tank 4′, an inlet 6′ for CO.sub.2 gas and a measurement arrangement 5′ for CO.sub.2 gas concentration. Floor level is indicated at FL and a log inlet at LI and log outlet at LO. The furnace has a Heating Compartment HE and a Holding Compartment HO.

(21) In FIG. 5 it is disclosed a top view of the furnace shown in FIG. 4, where there is disclosed the electrical cabinet 2′, the control cabinet for supply of CO.sub.2 3′, CO.sub.2 tank 4′, inlet 6′ for CO.sub.2 gas and measurement arrangement 5′ for CO.sub.2 gas concentration. The log inlet is shown at LI and log outlet at LO. It is also disclosed the Heating Compartment HE and Holding Compartment HO.

EXAMPLE 1

(22) After casting of an extrusion billet or a sheet ingot of a magnesium containing aluminium alloy, the product is often subjected to a homogenization heat treatment in a homogenization oven. A common homogenization practice is to heat the alloy to a temperature in the range 560-590° C. and keep it at that temperature between 1-5 hours.

(23) During this treatment, CO.sub.2 gas can be injected into the homogenization oven in a manner that practically the whole surface of each individual product is exposed to a sufficient concentration of the suppressing atmosphere.

(24) The concentration of the suppressing atmosphere is controlled by one or more sensors connected to a controller such as a PLC that controls the outlet of a CO.sub.2 source in relation to the measured value(s) and the set gas concentration. The source can be constituted by pressurized CO.sub.2 containers or tanks.

(25) The concentration of CO.sub.2 can be adjusted to a level from 0.5% CO.sub.2 up to 5% CO.sub.2, where the rest is mainly natural air, at least for an electrically heated oven.

(26) For a gas fired oven, the suppressing atmosphere can be adjusted slightly to compensate for the particular composition of the gas therein, due to the exhaust gases from the combustion.

(27) For an induction oven, the procedure may be that the product is heated very rapidly followed by a suppressing CO.sub.2 containing gas is brought to flow onto the surface of the product.

(28) The CO.sub.2 concentration needed to suppress discoloration can also be obtained by for instance, placing charcoal or other carbon containing combustable material in the heat treatment furnace

(29) Practical Ways of Implementing the Method in a Casthouse

(30) Extrusion billets of the Al—Mg—Si type are normally homogenised in the casthouse before transportation to the extrusion plant. There are two common types of homogenisation furnaces; batch homogenisation furnaces and continuous homogenisation furnaces.

(31) Batch Homogenisation Furnace

(32) In batch type of homogenisation furnaces the common procedure for homogenization is to insert a load of billets into a furnace chamber, then heat the billets to the desired homogenisation temperature and keep the billets at this temperature in the furnace chamber for a desired length of time. After the holding time, the furnace billet load is removed from the furnace chamber and cooled. Cooling is usually done in a cooling chamber or in a cooling station where the furnace load is cooled rapidly in forced air.

(33) Casthouses may have several furnace chambers and cooling chambers. Since the heating and holding segment in the furnace chamber takes longer time than cooling in the cooling chamber the number of furnace chambers normally is larger than the number of cooling chambers.

(34) Continuous Homogenisation Furnace

(35) A continuous homogenisation furnace is normally divided in two or three parts, a heating zone, a holding zone and possibly a cooling zone. The individual logs of extrusion billets are moved through the zones of the furnace. A normal layout for a furnace divided two parts is a first heating chamber and next to that a holding chamber as in FIGS. 4 and 5.

(36) One other common layout is to have the heating zone and the holding zone in the same chamber, with ample heating capacity in the heating zone and sufficient heaters to keep the metal temperature at the desired temperature in the holding zone.

(37) The cooling zone is normally in a separate chamber or area, the logs are transferred from the holding zone to the cooling zone when they have reached the end of the holding zone. After suppressed air cooling, some casthouses also utilizes a water curtain cooling to reach a final temperature below 60° C. before sawing.

(38) Practical Test

(39) Two loads of billets were homogenized in the continuous homogenization furnace as shown in FIGS. 4 and 5, where the first load was homogenized without modifying the atmosphere, i.e. in air. The second load was homogenized in an atmosphere containing ca. 1% A CO.sub.2 and the rest air. The two loads came from the same casting batch, i.e. it was the same metal alloy composition in both loads.

(40) The aluminium alloy of the billets was AA6063 containing Mg 0.7222 wt %, Si 0.5219 wt % and Fe 0.2015 wt %.

(41) The furnace was initially boosted to a CO.sub.2 concentration that in short periods was approximately 2% to ensure good distribution of the gas. Following this, the concentration was adjusted in a controlled manner down to approximately 1%. Total cycle time for each billet was 4 h 10 min, where 1 h 54 min was in a heating zone and 2 h 15 min in a holding zone.

(42) FIG. 6 is a photo taken of two end cuts of one extrusion billet homogenized in normal atmosphere, the end cuts are stacked one onto the other.

(43) It can clearly be seen that the surface of the billet is discoloured with major parts being black.

(44) FIG. 7 is a photo taken of two end cuts of one extrusion billet homogenized in an atmosphere containing ca. 1% CO.sub.2 and rest air. The end cuts are stacked one onto the other.

(45) The photo shows that the billet surface is light grey with no major discoloured areas.

(46) Small Scale Experiments

(47) To investigate the effect of various gases, and in particular the effect of CO2 concentrations on the surface appearance of as-cast billets, small scale ampoule experiments have been carried out. An AA6063 alloy was industrially cast, slices were cut from the ingot, and samples including the as-cast surface were machined from the ingot slice. A sample was placed in a quartz ampoule and the ampoule was filled with a selected gas and sealed.

(48) The gases used in the experiments included (1) air; (2) 1% CO2 and 99% air; (3) 2% CO2 and 98% air; (4) 3% CO2 and 97% air; (5) 4% CO2 and 96% air; (6) 5% CO2 and 95% air; (7) 50% CO2 and 50% air; (8) 100% CO2; (9) 100% Ar; (10) 100% N2; (11) 100% O2; (12) 100% CO; (13) 50% CO and 50% Ar; (14) 25% CO and 75% Ar; (15) 1% CO and 99% Ar.

(49) The ampoule samples were heated at a rate of 200° C./h to 575° C. and/or 580° C., held at this temperature for 2.5 hours and subsequently air-cooled. In Table 1 there is given some visual assessments for the samples.

(50) TABLE-US-00001 TABLE 1 Surface colour Air Partly black  1% CO2 and 99% air Not black  2% CO2 and 98% air Not black  3% CO2 and 97% air Partly black  4% CO2 and 96% air Partly black  5% CO2 and 95% air Partly black  50% CO2 and 50% air Partly black 100% CO2 Black 100% CO Black  50% CO and 50% Ar Black  25% CO and 75% Ar Black  1% CO and 99% Ar Black

(51) FIG. 8 shows a photo of a sample exposed to 1% CO.sub.2 and Air. The sample is not black.

(52) FIG. 9 shows a photo of a sample exposed to 2% CO.sub.2 and Air. The sample is not black.

(53) FIG. 10 is a photo of a sample exposed to 3% CO.sub.2 and Air. The sample is partly black.

(54) FIG. 11 is a photo taken of a sample exposed to Air. The sample is partly black.