Conform Product Thermomechanical Treatment

The present invention relates generally to extrusion processes and apparatus, and more particularly to the type of extrusion apparatus generally known as conform machines designed to permit continuous extrusion of a feedstock material into various shapes and sizes.

In the typical conform extrusion machine, solid feedstock such as an aluminum rod or other solid or powdered material to be extruded is fed in an unheated state into the machine along a rotating wheel. The wheel has an endless groove at its periphery to receive the feedstock. A portion of the circumference of the wheel, typically about one-quarter of the length thereof, is maintained in close contact with a fixed heavy metal block known as an extrusion shoe. At the end of the contacting portion, a blocking abutment that enters the groove obstructs the path of the feedstock, preventing it from being carried farther along the groove in the rotating wheel. As the extrusion material is pushed against the abutment by the frictional force exerted by the continuously rotating wheel, sufficient force is produced to extrude the material through a die retained at the end of a chamber in the shoe adjacent to the blocking abutment.

The advantages of the conform extrusion machine over heretofore conventional extrusion apparatus include the provision of a theoretically continuous extruding process, with attendant simplification of subsequent handling techniques and elimination of billet discards, and the use of cold solid or powdered feedstock with avoidance of any need to preheat the material prior to extrusion thereof. Examples of prior art conform extrusion apparatus of the aforementioned type are described in U.S. Pat. Nos. 3,765,216 to Green and 4,055,979 to Hunter et al.

Considerable heat is generated by the enormous frictional resistance and resulting axial stress encountered by the feedstock as it is fed along the groove by the rotating wheel as a consequence of the close contact of the latter with the extrusion shoe. The frictional force and attendant heat cause the feedstock to yield and flow through the die. In a typical process, the extruded product may be fed into a water quench tank located some five to ten feet from the exit die. It has been found that such prior art conform machines produce extruded products having non-uniform grain size and large surface grains which cause "orange peel" of the product when it is subjected to mechanical bending or other similar high stress working operations. Furthermore, products of the conventional conform process have been found to exhibit occasional blisters on the surface and relatively soft material of non-uniform hardness.

Accordingly, it is a principal object of the present invention to provide a conform extrusion machine for producing extruded products with uniform small grain size and improved mechanical properties.

In the copending U.S. patent application of U.K. Sinha et al. entitled "Improved Conform Extrusion Process and Apparatus" Ser. No. 140,165 filed 12-31-87, assigned the same assignee as the present invention and hereinafter referred to as the copending Sinha et al. application, it is observed that an expansion chamber may be provided in the extrusion shoe, located adjacent to the blocking abutment and upstream of the die, to allow extrusion of product of larger cross-section than the feed material in the conform extrusion process. The frictional forces on the feed material are higher along the extrusion shoe, which is fixed relative to the moving material, than along the grooved rotating wheel against which the material is moved. As a result, the temperature of the feed material is higher in the region adjacent to the shoe than in the region adjacent to the rotating wheel. In the conformed product (i.e., the extruded product), the portion subjected to the higher temperature during the extrusion process has a larger grain size. As a result of the unusual orientation of the conform machine, the lower region of the feed material experiences the higher temperature and, thus, as it leaves the die, the lower portion of the conform product has larger grains than the upper portion of the product.

The surface of the conform product recrystallizes more rapidly than the product interior because of the hardening process. Additionally, because of the high exit temperature of the conform product as it leaves the die, it undergoes a spontaneous secondary recrystallization along the edges of its surface, with consequent further grain growth. The resulting product suffers seriously inconsistent grain size and attendant structural deficiencies.

The copending Sinha et al. application discloses an improved conform machine employing special cooling systems to enhance the structural properties of the final product, and more specifically, which allow the extrusion process to be carried out at a preselected desired temperature and which maintain the material in the extrusion chamber at a uniform temperature. In addition, the conform apparatus described therein is provided with plural cooling systems for maintaining a preset extruding temperature and for inhibiting secondary recrystallization of the product. To that end, the conform extrusion apparatus disclosed in the copending Sinha et al. application employs a first cooling system for maintaining a desired temperature in the extrusion chamber of the apparatus. The first cooling system provides means at both sides of the extrusion chamber for sensing the temperature thereat, a coolant supply stream to both sides of the chamber, and control means responsive to changes in the temperature at either side relative to a predetermined extrusion chamber temperature for varying the flow of coolant at each side respectively. In this manner, the temperature of the material is maintained substantially uniform throughout the extrusion chamber, to produce a conform product having substantially uniform small grain structure and consequent improved mechanical properties.

According to another aspect of the invention disclosed in the copending Sinha et al. application, a second cooling system is provided to cool the conform product as it exits the die, and thereby to inhibit secondary recrystallization and grain growth at the surface of the product. That application further observes that in conventional extrusion processes which do not use the conform technique, it is customary to provide cooling within the die. However, the heating problem in the conform process is different from that encountered in the conventional extrusion process and requires a vastly different solution which takes into account the presence of localized hot spots contributing to the different grain sizes in the final product. Prior art proposals suggest the use of various types of cooling systems in the type of conform extrusion apparatus which uses entry feed material in molten rather than solid or powdered form, but in those instances the proposed cooling has been for purposes of solidifying the molten material. Examples of this may be found in U.S. Pat. No. 4,393,917 to Fuchs, Jr., and European Patent Application Publication No. EP 0110653.

In contrast to the prior art proposals, the invention disclosed in the copending Sinha et al. application provides a cooling system for conform extrusion apparatus by which the extrudable material is maintained at uniform temperature at both sides of the extrusion chamber, and the conform product is subjected to cooling immediately as it is extruded from the die. Although the latter fast cooling at the die exit--by spray cooling the conformed product as it exits the die, for example--serves to restrict the grain growth, it has been found to increase the hardness of the product. In some instances, this increase in product hardness may exceed product specifications.

It is another object of the present invention to provide a process and apparatus for control of both surface grain size and hardness of the conform product.

According to the present invention, the conform product is completely quenched in water contained in an in-line quench tank as the product is extruded from the die. In particular, the quench tank is controlled such that the product exiting the tank is maintained at a predetermined fixed elevated temperature, by way of example, approximately 850.degree. F., in the preferred embodiment. The maintenance of the temperature of the product leaving the quench tank is achieved by controlling the flow of water into the tank. According to the preferred embodiment, the flow rate of water into and out of the tank is microprocessor-controlled in accordance with several factors including temperature of the product entering the tank, temperature of the product leaving the tank (desired to be maintained), production rate of the product, temperature change (.DELTA.t) of the water flowing through the tank, and product material involved.

According to another aspect of the invention, following the exit of the product from the quench tank, it is subjected to a second cooling phase after a delay period which is selected to be sufficient to provide self-annealing to reduce the hardness of the product to a desired level. In the preferred embodiment, the second cooling phase is provided by another quench tank into which the product is fed after the selected delay period. The point of entry for the product into this second quench tank is at a distance from the exit point of the first quench tank which depends upon the production rate of the product (and, thus, the rate at which it is fed from the die), the material of which the product is composed, and the time required for the selfannealing. In the specific case of conform product of aluminum, hardness achieved using a process and apparatus conforming to the preferred embodiment of the invention was HRH-27 (Rockwell "H" hardness), well within the specified maximum limit for hardness for the product; and well below the hardness experienced for product produced by a process which subjected it to spray cooling upon exiting the die, wherein the hardness exceeded HRH-30.

It will be observed, therefore, that the present invention provides conform product having hardness within desired limits, together with the further advantages of improved surface grain structure, fine grain size which resists "orange peel" during cold processing, and suppression of the tendency toward blister formation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features, and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a partial sectional side elevation of a conform extrusion apparatus including a portion of the temperature regulating system for the extrusion chamber according to the copending Sinha et al. application; and

FIG. 2 is a side elevation of a preferred embodiment of a cooling system for the extruded conform product according to the present invention, for use with conform extrusion apparatus of the type such as that shown in FIG. 1, and schematically illustrating the control system for the cooling system.

Referring now to FIG. 1, an apparatus for continuously extruding material into a desired conform product includes a wheel 10 mounted for rotation on a shaft 12. Wheel 10 has an endless channel or groove 15 suitably formed in its circumferential edge 17. The wheel 10 rotates in close proximity to an extrusion shoe 20 which remains stationary relative to the wheel. A channel blocking abutment 22 is affixed to shoe 22 and enters the channel 15 in close proximity to the walls thereof, so that the wheel is free to rotate but a barrier is formed by abutment 22 to anything that may be carried in that passageway. The extrusion shoe 20 includes an extrusion chamber 25 disposed adjacent to the blocking abutment 22. A die block 28 at the end of the extrusion chamber forms a wall of the chamber and retains a die 29 therein to permit material to be extruded therethrough into a desired shape.

The apparatus thus far described is completely conventional structure in conform extrusion machines of the prior art. In operation, a solid feedstock or feed material 30, which may be an aluminum rod, for example, of a size adequate to be received within the channel 15, is fed into the apparatus with the assistance of a coining roll 33. The material is fed under pressure in any conventional manner (not shown) such that it frictionally engages the shoe 20 and the walls of the channel 15 as the wheel 10 rotates in a counterclockwise direction, as viewed in FIG. 1. The material eventually encounters the blocking abutment 22 in channel 15, typically located about one-quarter of the circumference of the wheel from the entry point for the material 30. Under the frictional forces and the pressure exerted on the feed material, and the accompanying axial stress set up in the material, it will begin to yield at a point which depends on the heat generated by the process and the yielding strength of the particular material used.

The yielding material commences to flow and is thereby forced into the extrusion chamber 25 and ultimately extruded through the die 29, to produce the desired conform product 37. If desired, an expansion chamber may be used with an appropriately larger die, to permit extrusion of product having a cross-section larger than the cross-section of the feed material. In any event, considerable heat is generated in the process of continuous extrusion by the conform apparatus. As observed in the copending Sinha et al. application, the typical orientation for conform machines of the prior art results in the feed material experiencing greater frictional force along the stationary member side of the machine; that is, on the side toward the extrusion shoe 20. The portion of the material encountering this higher frictional force is heated to a higher temperature than that portion of the material subjected to lower frictional force. Accordingly, the flowable material in the lower part of the extrusion chamber is typically at a substantially higher temperature than that in the upper part of the chamber. As a result, the grain size of the extruded product is irregular, with the larger grains at the lower part of the product relative to the size of the grains in the upper part, and with a concomitant deleterious effect on the mechanical properties of the final extruded product.

According to the invention disclosed in the copending Sinha et al. application, the conform extrusion apparatus is provided with a system for controlling the temperature of the flowable material in the extrusion chamber to remove the undesired hot spots. That system will be described briefly herein for the sake of convenience, and while the inclusion of that system in the conform apparatus is desirable it should be noted that its use is not essential to the present invention. In any event, the disclosure of the copending Sinha et al. application is hereby incorporated by reference into this specification. The extrusion chamber 25 may be integrally formed in the extrusion shoe 20 by machining the latter, or it may be provided in a separate component fitted into or on the shoe, to maintain the above-described relationship of the chamber to the blocking abutment 22 in the channel 15.

As shown in FIG. 1, such a component is an expansion chamber member 40 having an arcuate surface 43 conforming to an arc at the circumference of rotatable wheel 20 and confronting the channeled edge of the wheel when member 40 is fastened to the shoe 20. The extrusion chamber (expansion chamber) 25 is formed in part in chamber member 40 and in further part in a pair of feeder blocks 45,46 which are also fitted and secured in the shoe. That portion of chamber 25 provided by the openings in the feed blocks 45,46 may be larger than the chamber portion in member 40, and the portion in feeder block 46 may be tapered down toward the die block 28 forming the end wall of the extrusion chamber.

Thermocouples 49 and 50 are housed at or near the longitudinal surface of chamber 25 in feeder block 46, preferably close to the die block 28. Each of the thermocouples is formed in a conventional manner from a pair of dissimilar thermoelectric materials, and each generates an electrical signal representative of the temperature at the junction of the dissimilar materials. The thermocouples are electrically insulated from the feeder block and from each other, and have their respective junctions positioned as close as practicable to the surface of chamber 25 to detect the temperature of feed material in the chamber or of that portion of the feeder block immediately adjacent to the chamber. The location of the thermocouples next to the die block assures that the temperature of the material in the extrusion chamber is sensed at a point or points reasonably close to the point from which the material is extruded from the chamber to form the desired conform product 37. Thermocouple 49 is positioned at the upper side of the extrusion chamber 25 and thermocouple 50 is positioned at the lower side of the chamber to sense the temperature of regions of the material which are typically at the lowest and highest temperatures, respectively, in the selected portion of the chamber.

Feeder blocks 45,46 are provided with ducts or passageways 52 and 53 therethrough, respectively running adjacent to the upper and lower sides of chamber 25 so as to be in heat exchange relationship principally with those portions of the chamber. Each of the ducts is adapted to carry a coolant fluid therethrough, such as water or liquid nitrogen. Ducts 52 are joined together at a single inlet having an electrically controlled valve 56, such as a solenoid valve (not shown) to regulate the flow of coolant fluid therethrough. A corresponding but completely separate cooling system arrangement is provided for lower ducts 53 which re joined at a single inlet having an electrically controlled flow regulating valve (not shown). At the opposite ends of the upper and lower ducts, suitable conventional means are provided for recirculating the coolant fluid back to the source thereof.

Each of the thermocouples 49,50 is electrically connected to control circuitry not shown herein, but an embodiment of which is shown and described in detail in the copending Sinha et al. application. The circuitry may include sampling and digitizing circuitry to condition the electrical signal outputs of the thermocouples, which are representative of the temperature values at the respective thermocouple junctions, for control purposes. For example, a microprocessor may be used to compare the sensed temperature signal value from thermocouple 50 to the signal value derived from thermocouple 49 and to null the difference by generating an output which is converted to an analog signal for that purpose. Since the temperature of the material in the lower region of the extrusion chamber attributable to the conform extrusion process is almost invariably higher than the temperature of the material in the upper region of the chamber, the analog control signal derived from the microprocessor may be used to control the lower valve to allow flow of the coolant fluid through ducts 53 until the temperature sensed by thermocouple 50 is reduced to the temperature sensed by thermocouple 49.

The extruded product 37 resulting from the provision of a substantially uniform temperature of material at the point of extrusion has uniformity of grain size throughout and consequent improved mechanical properties. However, the extruded product undergoes secondary recrystallization at and near its surface, attributable to the high exit temperature of the product, causing some grain growth in the affected region near the product surface. According to a feature of the invention disclosed in the copending Sinha et al. application, a second system is provided for spray cooling the product as it is extruded from the die, to inhibit the secondary recrystallization. That purpose is well served, but the extruded product has been found to undergo some increase in hardness, and in certain instances, may be at an undesired level of hardness.

Referring now to FIG. 2, there is shown a preferred embodiment of a cooling or temperature control system for the extruded conform product according to the present invention. An in-line quench tank 60 is positioned at the outlet of die 29 in abutting relationship to the extrusion shoe 20 and/or die block 28, to receive the extruded conform product 37 as it exits from the die. The conform product passes through quench tank 60 as the product moves along its flow path or feed path. The quench tank has an inlet conduit 62 and an outlet conduit 63 for water flow therethrough. The purpose of this quenching, which is a first phase of cooling the product, is to maintain the temperature of the product at a predetermined fixed temperature level at the point 65 of egress of the product from the tank.

To that end, thermocouples 67,68 are positioned at the inlet and outlet conduits 62,63 at respective points at or close to the actual entry and discharge points for the water to and from the quench tank 60. These thermocouples serve to detect the temperature of the water entering and leaving the tank. Third and fourth thermocouples or other suitable temperature sensors 70,71 are positioned at the conform product points of entry into and egress from the quench tank, respectively, to detect the temperature of the product at those points. A conventional flow meter 73 may be positioned at the inlet conduit 62 to measure the rate of flow of the water therethrough. Of course, under normal conditions the flow rate of the water discharged from the quench tank will be equal to the flow rate of the water discharged from the quench tank.

Preferably, the electrical signals generated by the thermocouples as representative of the respective temperatures at the sensing points, are sampled at a suitable clock rate and converted to digital data in a conventional manner, as by use of sampling and A/D converter circuitry 75, for entry into a microprocessor 80. In addition, digital information indicative of the production rate of the conform product is fed into the microprocessor for use in the first cooling phase, to maintain the desired fixed temperature of the extruded product at the point of egress 65 from quench tank 60.

The rate of water flow through the quench tank for the latter purpose is determined by the microprocessor according to the following expression: ##EQU1## where W is the flow rate of the water through the quench tank in gallons per minute, M is the production on rate of the conform product in pounds per minute, C.sub.p is the specific heat of the material of which the conform product is composed (i.e., the feed material to the conform extrusion apparatus), T.sub.2 is the temperature of the conform product at the point of entry to the quench tank, T.sub.1 is the temperature of the conform product at the point of egress from the quench tank, .DELTA.t is the rise in temperature of the water between the inlet and the outlet of the quench tank. And 8.345 is the weight of water in pounds per gallon.

In an exemplary process performed using the preferred embodiment as thus far described, it was found that under steady state conditions, in which the temperature (T.sub.1) of the conform product leaving the quench tank at point 65 was maintained at a desired fixed level of 850.degree. F., T.sub.2 was 950.degree. F., .DELTA.t was 10.degree. F., was 16.7 lb/min., and the conform product was aluminum, the required flow rate of water through quench tank 60 was about five gallons per minute.

To regulate the temperature of the emerging product at the desired level, the microprocessor produces an output indicative of the water flow rate in accordance with the above mathematical expression. This digital output is converted to an analog signal, as by a D/A converter 83, amplified if necessary, and finally delivered to an electrically operated flow valve 85, such as an adjustable orifice solenoid valve, in the inlet conduit 62 for the quench tank to control the flow rate of the water therethrough. It is clear from the mathematical expression used to control the exit temperature of the product, that the maintenance of the desired predetermined temperature at point 65 requires a faster rate of flow as the temperature difference between the tank entry point and egress point for the product becomes larger. In particular, the water flow rate required varies directly with that temperature difference and with the production rate of the product, and varies inversely with the temperature change (.DELTA.t) for the incoming and outgoing water of the tank.

A second phase of cooling of the product is provided by a second quench tank 90 in the flow path of the extruded product. The point of entry 92 to the second quench tank is located downstream of the point of egress 65 of the product from the first tank, which itself is downstream of the die 29 in the product flow path. The distance between these two points is selected to be sufficient to allow the extruded product to self-anneal as it moves through that distance. For example, aluminum conform product at the above-mentioned production rate and egress temperature at point 65 was found to complete a self-annealing process in about a 30 second time interval, which reduced the product hardness to approximately HRH-27. This was also found to provide more uniform and reproducible hardness of the conform product. In the present example of the process, the distance between point 65 and point 92 is sufficient to allow 30 seconds of travel time therebetween for a given point on the product, to allow the desired self-annealing.

In addition to the improvements in product hardness, including uniformity and reproducibility, the foregoing process provides cooling of the product sufficient to inhibit grain growth. Therefore, a fine grained product is produced which resists "orange peel" during further processing.

Although a preferred embodiment has been described herein, it will be apparent to those of ordinary skill in the art to which the invention pertains that variations and modifications of the described embodiments may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention be limited only to the extent required by the appended claims and applicable rules of law.