Process for the batch heat treatment of flat and thin-walled light metal castings

Abstract

Method for the heat treatment of flat and thin-walled light metal castings (100), in particular for the heat treatment of flat and thin-walled aluminum castings, comprising the following steps: - arranging several light metal castings (100) in a charging frame (200); - preheating the light metal castings (100) arranged in the charging frame (200) in a fluidized bed heating chamber (310), wherein the charging frame (200) is immersed in the heating fluidized bed together with the light metal castings (100) arranged therein; - subsequent solution annealing of the light metal castings (100) arranged in the charging frame (200) in a solution annealing furnace (320) operated as a convection furnace, wherein the temperature reached during preheating in the fluidized bed heating chamber (310) is between 50 K and 100 K below the solution annealing temperature in the solution annealing furnace (320); - subsequent quenching of the solution-annealed light metal castings (100) arranged in the charging frame (200).

Description

[0001] The invention relates to a method for the batch-wise heat treatment of flat and thin-walled light metal castings, in particular for the heat treatment of flat and thin-walled aluminum castings, by solution annealing and quenching in a charging frame.

[0002] Light metal castings, especially aluminum castings, can be subjected to heat treatment to achieve specific component or material properties. For example, castings made of a heat-treatable aluminum alloy can be subjected to a so-called T6 or T7 heat treatment (comprising solution annealing followed by quenching and artificial ageing) to achieve an increase in strength and hardness.

[0003] For economical heat treatment, several light metal castings, forming a so-called charge, are usually arranged on a charging frame and then placed together with the charging frame into a furnace (or similar) and, if provided, subsequently into a quenching chamber (or similar) (so-called batch operation). The high total mass (component or workpiece mass + charging frame mass) sometimes results in very long heating and cooling times, which is both economically and energetically disadvantageous.

[0004] The closest DE 10 2011 119 002 A1 by the same applicant describes a method and apparatus for producing light metal castings. Heat treatment is carried out on the previously cast light metal castings using a fluidized-bed furnace. Advantageously, the fluidized-bed process for heat treating the light metal castings includes both solution annealing, quenching, and age-hardening of the light metal castings. Using the fluidized-bed process for heat treating the cast light metal castings results in a very short heat treatment time, compared to, for example, conventional convection furnaces.

[0005] DE 697 33 434 T2 describes a method for removing sand cores from castings, particularly aluminum castings, and, if necessary, also for heat-treating the castings simultaneously with or following sand core removal. This takes place in a fluidized-bed furnace equipped with a conveyor system that moves the castings through the furnace on a continuous or semi-continuous basis. In cases where sand core removal is followed by heat treatment of the castings, the heat treatment process is carried out in the same fluidized-bed furnace and / or in a heated volume following this furnace, or in a freeboard of this furnace above the bed of fluidized solids.

[0006] US 2011 / 0011501 A1 describes a heat treatment process that includes the steps of solution annealing, quenching, and age-hardening. Solution annealing and quenching can be performed in a fluidized bed.

[0007] DE 1 198 569 A describes the use of a fluidized bed bath for quenching metallic workpieces.

[0008] The invention is intended to avoid or at least reduce at least one disadvantage associated with the prior art.

[0009] This is achieved with the inventive method of patent claim 1. Further developments and embodiments of the invention emerge from the dependent patent claims, the following description of the invention and the drawing.

[0010] The method according to the invention for heat treating light metal castings comprises at least the following steps: - Arranging several light metal castings in or on a charging frame; - Preheating of the light metal castings arranged in the charging frame in a (first) fluidised bed heating chamber, as explained in more detail below; - subsequent solution annealing of the light metal castings arranged in the charging frame in a solution annealing furnace, as explained in more detail below; - subsequent quenching or quenching cooling of the solution-annealed light metal castings arranged in the charging frame.

[0011] A fluidized bed heating chamber is a container (also known as a retort) which is filled to a certain height with heated granules (fluidized material) to form a heated fluidized bed. (Fluidized bed technology is well known in the art; for this purpose, reference is made to the relevant specialist literature.) According to the invention, the charging frame, together with the light metal castings arranged or accommodated therein, is immersed in the heated fluidized bed. By blowing in a gas (the term “gas” in the context of the invention refers to both air and other gases such as are commonly used in the fluidized bed treatment of metallic workpieces or components), the heated granules are fluidized and behave like a heating fluid (heating medium), resulting in close contact and good heat transfer between the granules and the light metal castings or the charging frame.Quartz sand or aluminum oxide corundum is preferably used as the heating medium. The fluidized-bed heating chamber preferably has a heating device for the granules or heating medium.

[0012] The light metal castings to be heat-treated are preferably aluminum, titanium, or magnesium castings, and in particular aluminum, titanium, or magnesium die-cast parts. This means that these light metal castings are primarily manufactured from a casting alloy containing essentially aluminum, titanium, or magnesium. According to the invention, the light metal castings are shell-like, i.e. flat and thin-walled, and thus particularly susceptible to distortion, castings which, for example, have a wall thickness of no more than 2.5 mm, preferably no more than 2.0 mm, and in particular no more than 1.5 mm at their thinnest point. The light metal castings are, in particular, body components, especially structural components, or chassis components for a motor vehicle.

[0013] The invention therefore provides that, as a preliminary step to solution annealing in the solution annealing furnace, batchwise heating or preheating of the light metal castings takes place in a fluidized bed or fluidized bed heating chamber. The good heat transfer (see above) results in rapid and homogeneous heating or preheating of the light metal castings (and also of the charging frame), which on the one hand reduces dimensional deviations and on the other hand results in economic and energy advantages. However, the solution annealing takes place in a solution annealing furnace, which according to the invention is designed and operated as a convection furnace (see below), so that there is no risk of the light metal castings distorting due to the mechanical action of the fluidized bed or the granulate. The invention can therefore improve the dimensional accuracy of the heat-treated light metal castings.The invention is therefore suitable for the heat treatment of thin-walled light metal castings (see above). Furthermore, the invention promotes high plant throughput.

[0014] The method according to the invention may further comprise the following steps: - Preheating of the light metal castings arranged in the charging frame (and previously quenched) in another (second) fluidised bed heating chamber; - subsequent artificial aging of the light metal castings arranged in the charging frame in an aging furnace.

[0015] Analogous to the previous explanations, heating or preheating of the light metal castings in a fluidized bed or in a fluidized bed heating chamber is intended as a preliminary stage for artificial aging in an ageing furnace, which results in corresponding advantages.

[0016] According to the invention, the solution annealing furnace is designed or operated as a convection furnace. Preferably, the aging furnace is also designed or operated as a convection furnace. This means that the heat transfer to the light metal castings (and the charging frame) occurs almost exclusively through (forced) convection.

[0017] According to the invention, the temperature reached during preheating (in the fluidized bed or in the fluidized-bed heating chamber) (which means at least the temperature of the light metal castings) is below the solution annealing temperature. This means that the light metal castings (and the charging frame) are not heated to the solution annealing temperature during preheating prior to solution annealing. To achieve this, the temperature of the granulate (fluidized material) in the fluidized-bed heating chamber is lower than the temperature in the solution annealing furnace. According to the invention, the temperature reached during preheating is between 50 K and 100 K (significantly) below the solution annealing temperature. This has a beneficial effect on the dimensional stability of the light metal castings. Furthermore, energy advantages also arise. The same preferably applies analogously to preheating prior to artificial ageing.

[0018] The quenching of the solution-annealed light metal castings arranged in the charging frame can take place in a water or oil bath or in a quenching chamber by blowing air or an air-water mixture. However, it is preferably provided that the quenching takes place in a fluidized bed cooling system or in a fluidized bed cooling chamber. This allows particularly rapid and homogeneous cooling of the light metal castings (and also of the charging frame). Analogous to the previous explanations, a fluidized bed cooling chamber is a container filled to a certain height with cool granules (fluidized material) that behave like a cooling fluid (cooling medium). Quartz sand or aluminum oxide corundum is preferably used as the granules or cooling medium. The granules or cooling medium can have a temperature below room temperature. The fluidized bed cooling chamber preferably has a cooling device for actively cooling the granules orCooling medium (see below). The granulate or cooling medium can also be actively cooled, for example, by blowing in a cold gas (e.g., nitrogen, or possibly liquid nitrogen).

[0019] The method according to the invention is preferably carried out fully automatically. At least one robot or the like is preferably used for the transfer of a charging frame together with the light metal castings arranged therein. The transfer preferably includes all transport paths as well as the immersion or insertion and lifting or removal of the charging frame at the fluidized bed chambers and furnaces.

[0020] A system or device suitable for carrying out the method according to the invention comprises at least the following components, which are arranged in particular in a predetermined direction of flow for the charging frame or even arranged in series: - one, i.e. at least one, (first) fluidised bed heating chamber for preheating the light metal castings arranged in the charging frame (this fluidised bed heating chamber is intended as a preheating device); - one, ie at least one, solution annealing furnace for solution annealing the light metal castings arranged in the charging frame (and preheated in the fluidized bed heating chamber), this solution annealing furnace being designed as a convection furnace; - one, ie at least one, quenching device, such as a water or oil bath or a quenching chamber, for quenching the solution-annealed light metal castings arranged in the charging frame (and in the solution annealing furnace).

[0021] The quenching device is preferably designed as a fluidized-bed cooling chamber and, in particular, comprises a cooling device for actively cooling the cooling medium, in particular quartz sand or aluminum oxide corundum. This cooling device is, for example, at least one external cooling line surrounding the container or retort, e.g., in the form of a cooling coil or the like, through which a coolant flows and essentially functions as a heat exchanger. Furthermore, the fluidized-bed cooling chamber can comprise a control or regulating device for the cooling device, by means of which a specific cooling medium or cooling fluid temperature and thus a desired cooling gradient for the light metal castings can be achieved. This cooling gradient is preferably freely and continuously adjustable.

[0022] The system according to the invention may further comprise the following components: - one, ie at least one, ageing furnace for the artificial ageing of the light metal castings arranged in the charging frame (and previously quenched), this ageing furnace being designed in particular as a convection furnace; - one, i.e. at least one, other (second) fluidized bed heating chamber for preheating the light metal castings arranged in the charging frame (and previously) quenched and cooled before artificial ageing in the ageing furnace (this fluidized bed heating chamber is also provided as a kind of preheating device), wherein it is provided in particular that this (second) fluidized bed heating chamber is arranged in the direction of flow after the quenching device and / or before the ageing furnace; - at least one robot or the like for the transfer of a charging frame; and / or - a control device for controlling the system.

[0023] The method according to the invention and the system described above can also be applied or used for the heat treatment of workpieces or components made of a light metal wrought alloy.

[0024] The invention is explained in more detail below with reference to the drawing. Fig. 1 schematically illustrates a method according to the invention for the heat treatment of light metal castings.

[0025] The Fig.The method illustrated in Figure 1 is carried out using a system 300 comprising a first fluidized-bed heating chamber 310, a solution annealing furnace 320, a fluidized-bed cooling chamber 330, a second fluidized-bed heating chamber 340, and an aging furnace 350. The components 310 to 350 of the system 300 can be structurally arranged in the order shown, although this is not absolutely necessary. The solution annealing furnace 320 and, in particular, the aging furnace 340 are convection furnaces, which are designed, for example, as chamber furnaces. The fluidized-bed cooling chamber 330 and the two fluidized-bed heating chambers 310, 340 can have a container designed, for example, as an immersion tank, tub, or the like, which is filled, for example, with quartz sand or aluminum oxide corundum.

[0026] The light metal castings 100, which are in particular aluminum castings, are arranged in a charging frame 200 for heat treatment in the system 300. The charging frame 200 then passes through the individual components 310 to 350 of the system 300, together with the light metal castings 100 arranged therein, in the direction indicated by arrows. The system 300 is operated in particular fully automatically. The transfer of the charging frame 200 can be accomplished with the aid of at least one robot (not shown).

[0027] In a first process step, the light metal castings 100 arranged on the charging frame 200 are preheated in the first fluidized-bed heating chamber 310 and then, in a second step, solution-annealed in the solution-annealing furnace 320. The transfer of the preheated light metal castings 100 from the first fluidized-bed heating chamber 310 to the solution-annealing furnace 320 takes place as quickly as possible so that the preheated light metal castings 100 do not cool down during transfer and are introduced into the solution-annealing furnace 320 largely without temperature loss. Therefore, the first fluidized-bed heating chamber 310 is preferably arranged in close proximity to the solution-annealing furnace 320 to enable a short transfer time (handling time). A solution-annealing temperature of 500°C is specified here as an example. (Solution annealing of aluminum castings is usually carried out at temperatures between 450°C and 545°C.) According to the invention, the temperature reached during preheating in the first fluidized-bed heating chamber 310 is below the solution annealing temperature, as explained above. A preheating temperature of 450°C is specified here as an example.

[0028] In a third process step, the light metal castings 100 solution-annealed in the solution annealing furnace 320 are quenched together with the charging frame 200 in the fluidized bed cooling chamber 330, preferably to room temperature (RT) or even to a temperature below room temperature.

[0029] In a fourth process step, the previously quench-cooled light metal castings 100, still arranged on the charging frame 200, are preheated in the second fluidized-bed heating chamber 340 and then, in a fifth step, artificially aged in the aging furnace 350. Here, too, the transfer of the preheated light metal castings 100 from the second fluidized-bed heating chamber 340 to the aging furnace 350 takes place as quickly as possible, so that the preheated light metal castings 100 do not cool down during the transfer and are introduced into the aging furnace 350 largely without temperature loss. Therefore, the second fluidized-bed heating chamber 340 is preferably arranged in close proximity to the aging furnace 350 to enable a short transfer time (handling time). An artificial ageing temperature of 200°C is specified here as an example. (Aluminum castings are typically aged at temperatures between 150°C and 250°C.) It is preferably provided that the temperature reached during preheating in the second fluidized-bed heating chamber 340 is below the artificial aging temperature, as explained above. (This results in particular in time savings due to a shortened process time, as well as energy savings.) By way of example, a preheating temperature of 150°C is specified here.

[0030] After artificial aging and cooling, the heat-treated light metal castings 100 are removed from the charging frame 200. The charging frame 200 can then be reused.

Claims

[1] Method for the heat treatment of flat and thin-walled light metal castings (100), in particular for the heat treatment of flat and thin-walled aluminum castings, comprising the following steps: - arranging several light metal castings (100) in a charging frame (200); - preheating the light metal castings (100) arranged in the charging frame (200) in a fluidized bed heating chamber (310), wherein the charging frame (200) is immersed in the heating fluidized bed together with the light metal castings (100) arranged therein; - subsequent solution annealing of the light metal castings (100) arranged in the charging frame (200) in a solution annealing furnace (320) operated as a convection furnace, wherein the temperature reached during preheating in the fluidized bed heating chamber (310) is between 50 K and 100 K below the solution annealing temperature in the solution annealing furnace (320); - subsequent quenching of the solution-annealed light metal castings (100) arranged in the charging frame (200). [2] The method of claim 1, further comprising the following steps: - preheating the quench-cooled light metal castings (100) arranged in the charging frame (200) in another fluidized bed heating chamber (340); - subsequent artificial aging of the light metal castings (100) arranged in the charging frame (200) in an aging furnace (350). [3] Method according to claim 2, characterized by that the aging furnace (350) is also operated as a convection furnace. [4] Method according to claim 2 or 3, characterized by that the temperature reached during preheating in the other fluidized bed heating chamber (340) is below the artificial ageing temperature. [5] Method according to one of the preceding claims, characterized bythat the quenching of the solution-annealed light metal castings (100) arranged in the charging frame (200) takes place in a fluidized bed cooling chamber (330). [6] Method according to one of the preceding claims, characterized by that quartz sand or aluminum oxide corundum is used as heating or cooling medium in the fluidized bed heating chamber (310, 340) and / or in the fluidized bed cooling chamber (330). [7] Method according to one of the preceding claims, characterized by that this method is carried out fully automatically, wherein at least one robot is used for the transfer of the charging frame (200) together with the light metal castings (100) arranged therein.

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