Method and apparatus for heat treating of aluminium or an aluminium alloy

Abstract: A method for heat treating an aluminium or aluminium alloy part (12) is provided which includes heat treating with direct radiation from a source of infrared energy (60) until the part attains a desired state of heat treatment. The method and apparatus further include monitoring of the temperature of the part (12) and controlling the intensity of the radiation source (60) through proportional control in response to the measured temperature.

Description: After casting a part of aluminium or an aluminium alloy, it is often desirable to heat treat the part to achieve improved mechanical properties - for example, heat treatment may achieve a desired hardness to facilitate machining of the part. A common heat treatment technique involves heating the aluminium part to about 538°C (1000°F) then rapidly cooling the part. The cooling, or quenching, is followed by an ageing process to stabilize the metallurgy of the part. A typical ageing would involve heating the part to 149 to 260°C (300 to 500°F) and maintaining the part at that temperature for a period of time. By way of example, Aerospace Material Specification AMS 2771 of the Society of Automotive Engineers issued October 1, 1987, and entitled Heat Treatment of Aluminum Alloy Castings, shows heat treating aluminium alloy 356 at a temperature of 538°C (1000°F) for six hours before quenching (see AMS 2771, p. 10) and following quenching, AMS 2771 recommends soaking the cast part at 227°C (440°F) for as much as six to twelve hours (see AMS 2771, p.11). Recommended prior art procedures for wrought aluminium alloy parts are found in AMS 2770E as revised January 1, 1989. Similarly, military specification MIL-H-6088F, effective July 21, 1981, and entitled Heat Treatment of Aluminum Alloys, calls for ageing aluminium alloy 356 for one to six hours at temperatures of 149 to 160°C (300 to 320°F) (see MIL-H-6088F, p. 34). The ASM Committee on Heat Treatment of Aluminum Alloys suggests a treatment time of four to twelve hours at 538°C (1000°F) for aluminium alloy 356 followed by an ageing of three to nine hours at an ageing temperature of 154 to 246°C (310 to 475°F) (see Metals Handbook, 9th Ed., Vol. 4, p.685 American Society for Metals (1981)). As is apparent from the foregoing, the heat treatment and ageing of aluminium alloys is extremely time consuming. Furthermore, such heat treatment generally is attained in a batch process. For example, a plurality of aluminium castings are placed on a pallet or other device in a common oven and heat treated or aged as a collective group. However, there may be variations among the various castings of the batch. As a result, certain castings in the batch may not be suitably heat treated and may be subject to rejection. Metals Abstracts, vol. 22, no. 8, Aug. 1989, No. 56-1058 discloses the heat treating of an aluminium base alloy using radiant heat from infrared T-3 lamps. US-A-4229236 discloses the heat treating of steel, such as a steel band, by continuously moving the steel band through a furnace having opposed spaced-apart parallel banks of high intensity infrared radiation lamps. It is an object of the present invention, however, to provide a method for facilitating individual heat treatment. By individually heat treating a part, separate metallurgical records can be retained as to any given part. It is believed that in addition to having separate metallurgical records, the individual heat treatment will result in reduced scrap or waste associated with batch processing. According to the present invention, there is provided a method for heat treating a plurality of discrete aluminium or aluminium alloy parts, said method being characterised by heat treating said parts in a plurality of successive stations arranged in a line of travel, with one of said parts in each of said stations being heat treated with direct radiation from infrared radiation lamps until said parts attain a desired final state of heat treatment after heat treatment in said plurality of stations, and including the steps of: placing one said part in a first one of said plurality of stations, and heat treating said part in said first station with a first infrared radiation intensity independent of an infrared radiation intensity in others of said plurality of stations, said first infrared radiation intensity being selected, at least in part, in response to a measured initial temperature of said part prior to said part being heat treated in said first station; holding said part substantially stationary relative to said line of travel in said first station during said heat treatment for said part to be heated by said first infrared radiation intensity until a temperature of said part is elevated to a temperature greater than said initial temperature and for said part to at least partially attain said desired final state of heat treatment; and moving said part along said line of travel to a second one of said plurality of stations and holding said part substantially stationary relative to said line of travel in said second station while being heat treated with a second infrared radiation intensity independent of an infrared radiation intensity in others of said plurality of stations. Preferably, the temperature of said part is monitored during said heat treatment. In a particularly preferred arrangement, said infrared radiation lamps in each of said stations are of controllable intensity, said intensity is varied in response to said monitoring, and said intensity is controlled through a proportional controller to heat treat said part to a desired set temperature. The present invention will now be described in greater detail with reference to the accompanying drawings in which:- Figure 1 is a top plan view of apparatus in accordance with the present invention for heat treating an aluminium part; Figure 2 is an enlarged view taken along the line 2-2 of Figure 1; Figure 3 is a schematic representation of a control system for the apparatus of Figure 1; and Figure 4 is a graph showing representative readings of the control system for the apparatus of Figure 1. For the purpose of this description, the term "aluminium alloy product" means a product of aluminium, or an aluminium based alloy, which may be cast, wrought, extruded or otherwise formed. Thus, in the following examples, although the product is shown as a common automobile wheel which is cast from aluminium alloy 356, it will be appreciated by those skilled in the art that the present invention can be equally applied to a wide variety of products of aluminium or other aluminium based alloys. As best shown in Figure 1, apparatus 10 is in the form of a generally circular carousel 13, having a plurality of work stations arranged in a contiguous manner around its periphery. In fact, the carousel 13 has twelve work stations in which stations 22-26 and 28-31 are heating stations, station 21 is a load station, station 32 is an unload station, and station 27 is a transfer station, all of which will be described hereinafter. The apparatus 10 includes an indexing drive 14 centrally disposed within the carousel 13. An indexing motor (not shown) rotates drive 14 about its axis X-X. Radiating out from indexing drive 14 are a plurality of indexing arms 16 (one for each station 21-32). Arms 16 are secured to indexing drive 14 such that as drive 14 rotates about its central axis X-X, the arms 16 rotate throughout the carousel 13. Each of the arms 16 is horizontal. A terminal end of each arm 16 supports a main spindle 18 on which a part 12 is positioned (see Fig. 2). Spindle 18 is pivotally connected to arm 16 such that spindle 18 may be driven about its axis Y-Y to rotate the part 12 about an axis thereof as arms 16 rotate about axis X-X. Each of the heating stations 22-26 and 28-31 includes various heating elements. Station 24 is shown in Figure 2 in cross section. It will be appreciated that all of the stations 22-26 and 28-31 are similar in configuration to the station 24. As shown in Figure 2, station 24 includes a top refractory wall 50, a reversed L-shaped refractory inner wall 52 and an L-shaped refractory outer wall 53. The walls 50,52,53 co-operate to define an enclosed heat treating chamber 54. The chambers of each of the stations 22-26 and 28-31 are contiguous such that a part 12 passes from chamber to chamber of said contiguous stations as its associated indexing arm 16 rotates about axis X-X. As shown in phantom lines in Figure 2, L-shaped outer wall 53 may lower to expose the interior of chamber 54. A plurality of high intensity infrared heat treating lamps 60 are carried on the inner surfaces of the various walls 50,52,53. In a preferred embodiment, the infrared lamps are so-called T-3 lamps which can be heated to temperatures of about 2482°C (4,500°F) in response to current flow through the lamps. Station 21 is open to access and is a load point at which a part 12 may be loaded onto a spindle 18 with the part then moved to station 22,23, and so forth through station 26 and to station 27. Station 27 is an access point at which a part 12 may be removed from a spindle 18 and placed in a quench tank 70 and subsequently placed on a take-away conveyer 72. Optionally, the part 12 may be left on the spindle 18 and passed to station 28, from where it is then passed in turn through heat treatment stations 29-31. Station 32 is an unload station which is open to access such that an operator may remove a part 12 from spindle 18 and place the part 12 in a quench tank 77 and subsequently place the quenched part 12 on a take-away conveyor 76. Accordingly, a part 12 is loaded at station 21 and then, upon rotation of the indexing drive 14, positioned in station 22 and held in station 22 for a desired period of time. The part 12 then moves to station 23,24 and so on. In the preferred embodiment, stations 22-26 constitute heat treating stations for elevating the temperature of the part 12 to a desired heat treatment temperature, for example about 538°C (1000°F). Stations 28-31 collectively are ageing stations for soaking the heat treated part 12 at a temperature of about 204°C (400°F). As will become more apparent, it is desirable to monitor the temperature of the part 12 in each station 21-26 and 28-31. In the preferred embodiment, a plurality of optical pyrometers 80,82,89 are provided to monitor the temperature of the part 12. For example, an initial pyrometer 80 is provided in station 21 positioned to be directed at a part 12 carried on a spindle 18 at rest in station 21. A plurality of first and second optical pyrometers 82, 89 are provided in each of stations 22-26 and 28-31. Upper pyrometers 82 are directed towards the location of a part 12 at rest within the station. Pyrometers 89 are directed to the chamber 54 to measure background temperature within the chamber. The use of optical pyrometers is attributed to the difficulty of placing a thermocouple on the part 12 since the part is moving throughout the carousel 13 and is rotating on a spindle 18. Accordingly, optical pyrometers are utilized to measure the temperature of the part 12. However, the use of optical pyrometers in measuring the temperature of aluminium presents significant problems. For example, the aluminium is highly reflective. Also, the background temperature (i.e., the temperature of the lamps and reflection and emission from the refractory material within each of the stations) is high. These factors co-operate in providing a read-out from the optical pyrometers which is inaccurate. Thus both of the pyrometers 82 and 89 should be used as well as empirically derived evidence to compensate for known errors to provide a true temperature of a part 12 within the given chamber. Specifically, through empirical studies, it has been noted that the true temperature of the part 12 during the heat-up phase within a station varies from a temperature reading of optical pyrometer 82 alone (i.e., the apparent temperature). The amount of variation is found to vary with both the reading of the background optical pyrometer 89, the part optical pyrometer 82, a thermocouple 94 placed within the refractory insulation of each station and the current and voltage applied to the lamps 60 in the station. Figure 4 is a graph showing the relation between the true temperature of the part 12 and the readings of the part optical pyrometer 82. As shown, the true temperature (line A) of the part 12 (measured from a thermocouple in a test application) during the heat-up phase of the lamps 60 increases but lags behind the temperature read off the background optical pyrometer 89 (line B). Also, the apparent temperature as measured by part pyrometer 82 similarly lags (as shown in line D). When the desired temperature (line C) of the part 12 is attained and the lamps 60 are turned off or phased back, the optical pyrometers 82,89 will note and sense the loss of energy to the lamps. Accordingly, the optical pyrometers would falsely read a decrease in temperature of the part. The decay in intensity of lamps 60 as measured by background pyrometer 89 is shown in Fig. 4 as line B'. The part pyrometer 82 also senses the loss in energy and, if uncorrected, would report a false delay in the temperature of the part 12. The false decay is shown as line D'. Therefore, during the decay phase of the lamps 60, the amount of the decay (for example distance B 1 ) is added back to the apparent temperature of the part 12 (illustrated as distance D 1 ) to give an adjusted reading (line D") indicative of the true temperature (line A') of the part. It will be appreciated that the foregoing procedure for compensating for optical pyrometers results from the use of optical pyrometers with highly reflective aluminium alloys. Placement of a thermocouple on the part directly measures the temperature of each part which would result in avoiding the need for compensating for inaccuracies in the optical pyrometer readings. While such a temperature sensing is not utilized in a preferred embodiment (due to the difficulty of attaching a thermocouple to moving and rotating parts 12) it will be appreciated that such a measurement technique is contemplated to be within the scope of the present invention. Figure 3 shows a control system 200 for controlling the intensity of the lamps 60 in each of the stations. As shown, the controller 90 includes software 91 for calculating a true temperature which is sent as an output 92 to a proportional controller 93 for controlling the intensity of the infrared lamps 60. The input to software 91 includes the measurement from the insulation thermocouple 94, the background optical pyrometer 89 and the part optical pyrometer 82. Also, a voltage and current meter 96 measures the voltage and current to the lamps 60 and provides the measured voltage and current as input to the software 91. Finally, the software 91 uses memory 98 which includes the empirical data for converting apparent temperatures measured from the optical pyrometers to the true temperature of the parts. The proportional controller 93 accepts as inputs the true temperature 92 as well as a set point 100 or desired temperature of the part 12 and part identification 102 which would include such identifying factors as the mass of the part and its emissivity. The proportional controller 93 may also be fed a proportional band or proportional band may be preset within the controller 93. The proportional controller then controls the intensity of the lamps based on the inputs. As is known in proportional control, if the true temperature 92 of the part is below the proportional band, the lamps 60 are at full intensity. If the true temperature 92 is above the proportional band, the lamps 60 are at full off. If the true temperature is within the proportional band, the intensity of the lamps 60 is varied. It will be appreciated by those skilled in the art that proportional control as thus described performs no part of this invention per se. Proportional control is more fully described in our US patent 5,050,232. With the apparatus as thus described, each part 12 may be separately heat treated. A part 12 is placed in the heat treating station. In the heat treating station (stations 22-26), the part 12 is heated to 538°C (1000°F) and maintained at that temperature for about 2 to 2.5 minutes. The heat treated part can then be removed at station 27 and quenched. Following quenching the part 12 may be either placed on conveyor 72 or submitted to the ageing station (stations 28-31) where it is heated to about 204 to 232°C (400 to 450°F) and held at that temperature (soak phase) for about 2 to 2.5 minutes. The aged part 12 is then removed at station 32 and quenched in tank 77 and placed on take-away conveyor 76. The stations 22-23 co-operate. Namely, the station 23 accepts station 22's output temperature and inputs the temperature for station 23. Stations 28-31 are closed-loop controlled with each station, comprising an independent heat treating station. Having described the structure and operation of the present invention, benefits of the present invention in comparison to prior art heat treatment techniques can be appreciated. In a typical heat treating system, a part that is to be heat treated arrives at the heat treat facility directly from a casting operation. Such a part may have a wide variety of temperatures. For example, the temperature of such a part may be anywhere from 315 to 399°C (600 to 750°F). This is particularly true in the present invention where the part arises from a casting operation. For example, if the part handler misses one of the indexing steps, the part may be in ambient temperature for 4 to 5 minutes which affects the temperature at which it enters the first station. Accordingly, the first station is primarily designed to stabilize the temperature of the part to be within a definable and controllable range of temperatures. A secondary function of the first station is to start the part in the heat treating process of the present invention. With the teachings of the present invention, one skilled in the art will recognize the importance of a plurality of heat treating stations. As described, a part moves from one station to another in an indexing fashion with the part permitted to dwell in a station for a requisite period of time. As a result, at each station, the part enters with a known temperature (or actual temperature which varies from a known temperature by a predescribed minimum tolerance). Within the station, the part is heated over a relatively narrow range of temperatures. With a narrow range of heat treating within a station and with a narrow range of tolerance for admission to a station, accurate closed-loop control of temperature within a station is more readily attainable. Accordingly, the succession of indexed, multiple, closed-loop controlled stations are very important to the present invention because they permit the part to be examined and treated in a closed-loop fashion within a fairly narrow range of temperatures. It has been found that the use of proportional control permits heat treatment of aluminium parts through direct contact with infrared energy. A heat treating and ageing process can be achieved that consumes a total of about 4 to 5 minutes of hold time and a total cycle time (which includes hold time and heat-up time) of about 10 minutes. This can be compared with prior art heat treatment which required up to 6 hours for heat treating and up to 12 hours for ageing. Also, each part is separately heat treated to uniform temperatures. This results in reduced rejections of parts. Also, a metallurgical history can be made of each part. The foregoing description has shown an embodiment which includes a heat treating station followed by an ageing station. It will be understood and appreciated by those skilled in the art that the present invention can be practised without use of the ageing station and simply use a plurality of stations to heat treat a part according to the teachings of the present invention.

Claims: A method for heat treating a plurality of discrete aluminium or aluminium alloy parts (12), said method being characterised by heat treating said parts (12) in a plurality of successive stations (22-26) arranged in a line of travel, with one of said parts in each of said stations being heat treated with direct radiation from infrared radiation lamps (60) until said parts attain a desired final state of heat treatment after heat treatment in said plurality of stations, and including the steps of: placing one said part in a first one of said plurality of stations, and heat treating said part in said first station with a first infrared radiation intensity independent of an infrared radiation intensity in others of said plurality of stations, said first infrared radiation intensity being selected, at least in part, in response to a measured initial temperature of said part prior to said part being heat treated in said first station; holding said part substantially stationary relative to said line of travel in said first station during said heat treatment for said part to be heated by said first infrared radiation intensity until a temperature of said part is elevated to a temperature greater than said initial temperature and for said part to at least partially attain said desired final state of heat treatment; and moving said part along said line of travel to a second one of said plurality of stations and holding said part substantially stationary relative to said line of travel in said second station while being heat treated with a second infrared radiation intensity independent of an infrared radiation intensity in others of said plurality of stations. A method according to claim 1, characterised in that the temperature of said part is monitored during said heat treatment. A method according to claim 2, characterised in that said infrared radiation lamps (60) in each of said stations (22-26) are of controllable intensity, and said intensity is varied in response to said monitoring. A method according to claim 3, characterised in that said intensity is controlled through a proportional controller (93) to heat treat said part to a desired set temperature. A method according to claim 4, characterised in that said proportional controller (93) includes means for defining a proportional band surrounding said set temperature with said intensity of said lamps being at substantially full intensity when said measured temperature is lower than said band and with said intensity of said lamps being at a variable reduced intensity when said measured temperature is within said band. A method according to any one of claims 2 to 5, characterised in that said monitoring is provided utilising an optical pyrometer (80, 82, 89). A method according to claim 6, characterised in that an output of said optical pyrometer is modified in response to a predetermined algorithm to derive an approximate true temperature of said part. A method according to any preceding claim, characterised in that each said lamp (60) is a T-3 lamp. A method according to any preceding claim, characterised in that said part is moved in a path of travel which is at least an arc of a circle as said part moves between said plurality of successive stations. A method according to any preceding claim, characterised in that said part is moved relative to said infrared radiation lamps while maintaining said part stationary relative to said line of travel for uniform heat treatment of said part. A method according to any preceding claim, characterised in that at least one of said parts is rotated whilst remaining in at least one of said plurality of stations. A method according to any preceding claim, characterised in that said part is quenched (27) subsequent to a heating group of said stations (22-26) and prior to an ageing group of said stations (28-31).

Inventors: EPPELAND JOHN ROY, MANNERUD JACK EYLORD, EPPELAND, JOHN ROY, MANNERUD, JACK EYLORD
Applicants: BGK FINISHING SYSTEMS INC, BGK FINISHING SYSTEMS, INC.
Application year: 1992
Main classification: C21D1
IPC codes: C22F1, F27B9, C21D1, C21D9, C21D11