Patent Application
20200316663
The present invention relates to a method for producing aluminum alloy parts through plastic working of heat-treatable aluminum alloy extrusions, and to a production system suitable for carrying out the method.
Automobile parts such as collision protectors (e.g., bumper reinforcements) and body frames require higher strengths due to recent enhancement of automobile crash safety standards and pedestrian protection standards. On the other hand, the automobile parts also require lower weights due to demands for higher maneuverability and higher fuel efficiency. To have higher strengths and lower weights, some of these automobile parts are made from heat-treatable aluminum alloy extrusions. The extrusions may undergo a plastic working such as bending or crushing for higher design freedom and smaller number of parts.
An exemplary conventional method and system for producing an aluminum alloy member (a bumper reinforcement, herein) will be illustrated step by step with reference to a schematic view of
(1) Hot Extrusion
An aluminum alloy billet 2 which has been heated to a temperature at which the billet is extrudable is placed in a container 3 in an extruding press 1 and is extruded forward through a die 5 by moving a stem 4 forward. The automobile aluminum alloy parts (such as collision protectors and body frames) are often made from 6xxx-series or 7xxx-series heat-treatable aluminum alloys.
(2) Cooling
The extrusion 6 extruded from the extruding press 1 moves on a table 7, during which the extrusion 6 is cooled and quenched by a cooler 8 (fan air cooler or water cooler). The cooling of the extrusion 6 starts immediately after extruding (at a position about 0.5 to 1.5 m from the outlet of the die 5). The extrusion 6 requires rapid cooling, because such heat-treatable aluminum alloys generally contain large amounts of alloy contents, and have increasing quench sensitivity with an increasing strength.
The cooled extrusion 6 is stocked on the table 7. The extrusion 6 may move from the table 7 to another storage area.
(3) Stretching
The extrusion 6 is generally long and has a length of 30 to 50 m. The extrusion 6 is stretched and straightened by a stretcher (not shown) before cutting.
(4) Cutting
The continuous extrusion 6 is fed to a fixed cutting machine (only a circular saw 9, which is a cutting tool, is shown) and cut to a predetermined length (length corresponding to one aluminum alloy part). The cut extrusions are fed lot by lot to a plastic working machine (including a bending press and a crushing press).
(5) Bending
Bumper reinforcements are often bent at both ends. When the work undergoes stretch bending, the stretching can be omitted.
(6) Softening Heat Treatment
When the extrusion receives large plastic strain at ends typically through crushing, the extrusion undergoes solution heat treatment or reversion treatment (see Japanese Patent No. 5671422) as needed, to cancel natural age hardening to thereby eliminate or minimize cracking during working.
(7) Crushing
The extrusion in a partial region in the lengthwise direction (machine direction) undergoes crushing to change the cross sectional shape of the region, for design requirements or for prevention of interference with another part. Instead of the softening heat treatment, warm pressing or hot pressing may be performed in the crushing.
(8) Artificial Aging
The extrusion (aluminum alloy part) after the plastic working undergoes artificial aging to have higher material strength. When higher strength is oriented, a T5 treatment (general temper aging) is appropriately selected, whereas, when stress corrosion cracking prevention is oriented, a T7 treatment (over-aging) is appropriately selected.
The aluminum alloy parts (such as collision protectors and body frames) are generally made from 6xxx-series or 7xxx-series aluminum alloy extrusions. These heat-treatable aluminum alloys undergo natural aging, have higher strengths, and have lower elongations with the elapse of time after extruding. In a conventional production process, a continuous extrusion having a length of about 30 to about 50 m is stocked on the table or in another storage area, and after the elapse of some significant period of time, cut to a predetermined length (length corresponding to one aluminum alloy part), and the cut extrusions are fed lot by lot to the plastic working machine and undergo plastic working. The extrusions as materials may undergo plastic working such as bending, crushing, shearing (e.g., piercing), burring, swaging, or another press forming. Unfortunately, the extrusions have lower elongations due to progress of natural aging and suffer from rupture and/or cracking when undergoing plastic working with large deformation. In addition, the extrusions differ from one another in springback amount due to difference in increase in material yield strength, where the difference in increase is caused by difference in progress of natural aging. This disadvantageously causes lower dimensional accuracy. The aluminum alloys suffer from these disadvantages more with an increasing strength of the materials (alloys).
To resolve the disadvantages, the extrusions have to undergo canceling of natural age hardening to have lower yield strengths and higher elongations before plastic working. To cancel the natural age hardening of the extrusions, the solution treatment and restoration treatment as described above are considered as a possible solution. Disadvantageously, however, these treatments require reheating and cooling of the extrusions immediately before plastic working, and this causes increased spending on heat treatment facilities and additional steps and causes increased cost.
Aluminum alloys, when having high quench sensitivity, have to be rapidly cooled after extruding, to undergo sufficient hardening by artificial aging. Assume that the extrusions have a hollow section and include one or more inner ribs inside thereof. Disadvantageously, however, the inner ribs, which are not directly cooled from the outer periphery, undergo insufficient quenching due to delayed temperature fall, and the extrusions fail to have a predetermined strength after artificial aging. In addition, the extrusions after cooling disadvantageously have large deformation due to difference in thermal shrinkage, because of uneven temperature distribution in the cross section during cooling.
The present invention has an object to eliminate or minimize cracking during plastic working at low cost as compared with a technique including solution treatment or restoration treatment, in production of aluminum alloy members through plastic working of heat-treatable aluminum alloy extrusions.
The present invention has another object to allow extrusions to less deform after cooling and to allow inner ribs to have higher strengths after artificial aging, when the heat-treatable aluminum alloy extrusions have a hollow section and include one or more inner ribs.
The present invention provides, in an embodiment, a method for producing aluminum alloy parts. The method includes hot-extruding a heat-treatable aluminum alloy using an extruding press to give an extrusion. The extrusion extruded from a die of the extruding press and moving forward is cooled and cut to a predetermined length. The extrusion after cutting is conveyed to a plastic working machine. The conveyed extrusion receives a plastic working before the extrusion has a yield strength exceeding 120 MPa, where the yield strength increases due to natural aging. The extrusion after the plastic working then receives an artificial aging.
In the production method, the extrusion extruded from the die is cooled by natural cooling or forced cooling (such as fan air cooling or water cooling) during moving forward. Assume that the extrusion has a hollow section and includes one or more inner ribs in the cross section. In this case, the extrusion is preferably cooled also from the inside by inserting a nozzle from front into the cross section of the extrusion extruded from the die and moving forward, and injecting a coolant from the nozzle into the inside of the extrusion.
The present invention also provides, in another embodiment, a system for producing aluminum alloy parts. The production system is advantageously used for carrying out the production method. The production system includes an extruding press, a cutting device, a conveyer, and a plastic working machine. The extruding press hot-extrudes a heat-treatable aluminum alloy to give an extrusion. The cutting device is disposed downstream from the extruding press, cuts the extrusion to a predetermined length, and thereby isolates the extrusion from the extruding press. The conveyer and the plastic working machine are disposed in parallel with the extruding press. The cutting device includes a cutting tool operable to move forward at a speed equal to the extruding speed of the extrusion. The conveyer conveys the extrusion after cutting to the plastic working machine, where the extrusion has been cut to a predetermined length by the cutting device. The plastic working machine imparts a plastic working to the extrusion conveyed by the conveyer and thereby forms the extrusion into an aluminum alloy part.
The production system preferably further includes a cooler downstream from the extruding press. The cooler forcedly cools, typically by fan air cooling or water cooling, the extrusion extruded from the die and moving forward. Assume that the extrusion has a hollow section and includes one or more inner ribs in the cross section. In this case, the cooler preferably includes a nozzle that injects a coolant. The nozzle is movable back and forth along the extruding direction and is insertable from front into the cross section of the extrusion, to cool the extrusion from inside.
With the aluminum alloy part production method according to the embodiment of the present invention, the extrusion extruded from the extruding press and moving forward is cut in situ to a predetermined length, conveyed to the plastic working machine, and undergoes a plastic working before progress of natural aging (before the yield strength exceeds 120 MPa), without being stocked on a table or in another storage area. This configuration allows the extrusion to undergo a variety of plastic working while maintaining a low yield strength and a high elongation and to resist cracking during the plastic working, without solution treatment and restoration treatment performed before the plastic working. The production method eliminates the need for solution treatment and restoration treatment, provides a low yield strength upon plastic working, and can produce aluminum alloy parts having low springback and high precision with low cost. In addition, the production method can reduce residual stress accompanied with pressing and allows the resulting aluminum alloy parts, even when being high-strength 7xxx-series aluminum alloy members, to have better stress corrosion cracking resistance.
The heat-treatable aluminum alloy extrusion extruded from the die is quenched by natural cooling or forced cooling (such as fan air cooling or water cooling). Assume that the extrusion has a hollow section and includes inner ribs in the cross section. In this case, the method may also employ cooling by a nozzle that injects a coolant employed as a cooling mechanism. This configuration allows the extrusion to cool not only from the outside of the cross section, but also from the inside thereof and to have lower temperature difference in the entire cross section in the cooling process. This allows the extrusion to less deform by thermal shrinkage during cooling, to have lower temperature history difference within the cross section, and, after temper aging, to have material properties that are uniform within the cross section. The combination use of the nozzle allows the extrusion to cool at a higher cooling rate and to be quenched even when the material alloy is an aluminum alloy having high quench sensitivity, such as a high-strength 7xxx-series aluminum alloy. This promises still higher strength of the extrusion after temper aging.
A system and method according to the embodiment of the present invention for producing aluminum alloy parts will be illustrated below, with reference to the schematic view of
The production system illustrated in
The extruding press 11 is as with conventional one and hot-extrudes a heat-treatable aluminum alloy. Non-limiting examples of the heat-treatable aluminum alloy include 2xxx-series, 6xxx-series, and 7xxx-series aluminum alloys prescribed in Japanese Industrial Standards (JIS) or registered with Aluminum Association (AA).
The cooler 12 includes at least one of a fan air cooler or a water cooler and forcedly cools and quenches an extrusion 17 which is extruded from the die of the extruding press 11 and moves forward on a table 16. The cooler 12 also includes a nozzle 18 that injects a coolant (such as air or a coolant liquid). The nozzle 18 is supported by a support mechanism 19 movably back and forth along the extruding direction. The rear end (right end in
The cooler 12 is used when the extrusion 17 is unquenchable by natural cooling alone. The nozzle 18 is used as needed in combination with the fan air cooler or water cooler when the heat-treatable aluminum alloy has high quench sensitivity, and is advantageously used particularly when the extrusion 17 has a hollow section and includes one or more inner ribs in the cross section. The nozzle 18 goes into the hollow section of the extrusion 17 which is extruded through the die of the extruding press 11 and moves forward. The nozzle 18 injects the coolant from orifices into the hollow section to cool the extrusion 17 from the inside and gets out of the hollow section after the completion of cooling. To uniformly cool the extrusion 17 from the inside of the hollow section, the nozzle 18 may be configured to go into and get out of the hollow section while injecting the coolant.
The fan air cooler or water cooler may work in combination with the nozzle 18 in the cooler 12, to cool the extrusion 17 both from the inside and the outside. The combination use can reduce the temperature difference and temperature history difference of the entire cross section of the extrusion 17 in cooling process. This configuration allows the extrusion 17 to less deform and to have more uniform material properties in the cross section after temper aging, where the deformation will be caused by thermal shrinkage during cooling. The configuration also allows the extrusion 17 to cool at a higher cooling rate and to be quenched in the entire cross section including the inner ribs, even when the material aluminum alloy is one having high quench sensitivity, such as a high-strength 7xxx-series aluminum alloy. In addition, the configuration can realize a high cooling rate as compared with conventional equivalents, thereby enlarges the controllable range of cooling rates, and enlarges control ranges of size and distribution of precipitates in artificial aging. This promises higher material strength and higher stress corrosion cracking resistance of the extrusion after temper aging.
The cutting device 13 includes a cutting tool 21 (a circular saw in this example) and a pair of clamps 22, 22. The cutting device 13 also includes a drive mechanism (drive motor) that drives (rotates) the cutting tool 21; a drive mechanism that drives the clamps 22, 22; and a traveling mechanism that moves the cutting tool 21 and the damps 22, 22 back and forth along the extruding direction, where all these mechanisms are not shown. The cutting tool 21, the damps 22, 22, and the mechanisms are disposed typically above the table 16.
The cutting tool 21 may be another tool such as a chain saw. The damps 22, 22 are disposed immediately downstream and upstream from the cutting tool 21 in the longitudinal direction, grasps the extrusion 17, which is extruded from the die and moves forward, at the rear and front positions (grasping positions), and locates the extrusion 17 with respect to the cutting tool 21. The cutting tool 21 and the damps 22, 22 grasping the extrusion 17 move forward at a speed approximately equal to the extruding speed of the extruding press 11 (traveling speed of the extrusion 17). During this process, the cutting tool 21 works to cut the extrusion 16. The cutting position is determined to allow the extrusion 17 after cutting (extrusion 17a) to have a predetermined length (length corresponding to one aluminum alloy part). The predetermined length is set to be identical to the length of the final product aluminum alloy part along the extruding direction, or to be somewhat larger in consideration typically of a grasp margin in stretching.
The damps 22, 22 are arranged to grasp the extrusion 17 at a position immediately after extruding (at a position of about 0.5 to about 1.5 m downstream from the die outlet of the extruding press 11). The cutting position and the grasp positions are highly possibly at high temperatures at the time point when the clamps 22, 22 grasp the extrusion 17. To eliminate or minimize the deformation during cutting of the extrusion 17, which will soften at such a high temperature, at least one of the damps 22, 22 and the cutting tool 21 preferably includes a cooling device or mechanism that cools the cutting position and grasp positions of the extrusion 17 (by air cooling or water cooling).
The cutting device 13 cuts the extrusion 17 while the cooler 12 cools the extrusion 17. However, cooling by the cooler 12 and cutting by the cutting device 13 do not always have to start and end simultaneously.
Where necessary, the cutting device 13 may function as a stretcher for the extrusion 17a after cutting and cooling, by the working of the pair of damps 22, 22, the drive mechanism that drives the damps 22, 22, and the traveling mechanism that moves the damps 22, 22 back and forth along the extruding direction. The extrusion 17a after cutting can be stretched and straightened by allowing the damps 22, 22 to grasp both ends of the extrusion 17a and widening the distance between the damps 22, 22.
Instead of allowing the cutting device 13 to function as a stretcher, it is also acceptable to provide a purpose-built stretcher 23 adjacent to the cutting device 13 and to allow the stretcher 23 to stretch and straighten the extrusion 17a after cutting. When the extrusion 17a undergoes tensile bending in the below-mentioned plastic working machine 15, the extrusion 17a also undergoes stretch straightening during process of the tensile bending, and this eliminates the need for preliminary stretch straightening by the damps 22, 22 or the stretcher 23.
As described above, cutting of the extrusion 17 immediately after extruding to a predetermined length, and cooling concurrently with or in combination with the cutting eliminates the need for such a huge table 7 (see
The conveyer 14 grasps the cut extrusion 17a and feeds the same to the plastic working machine 15. As described above, the extrusion 17a generally has a short length, and this allows the system to employ, for example, a robot arm equipped with a gripper as illustrated in
The plastic working machine 15 imparts, to the extrusion 17a, at least one plastic working such as bending, crushing, shearing (e.g., piercing), burring, swaging, or another press forming in the cold. The plastic working machine 15 includes a necessary apparatus such as a press, according to the type of the aluminum alloy part (the type of plastic working to be imparted to the extrusion 17a). For example, assume that the aluminum alloy part is a bumper reinforcement; and that bending (bend forming) is imparted to the both ends of the extrusion 17a, and then crushing is imparted to a portion in the longitudinal direction. In this case, the plastic working machine 15 includes a bending press and a crushing press.
The extrusion 17 (17a), which is formed from the heat-treatable aluminum alloy, undergoes natural aging from immediately after cooling and increases in yield strength with time. The plastic working is completed before the yield strength (0.2% yield strength) of the extrusion 17a exceeds 120 MPa. The production method and production system illustrated in
The extrusion 17a, which has undergone natural aging not so much, has a low yield strength and a high elongation upon plastic working. This allows the extrusion 17a to less suffer from rupture and cracking; to have a small springback, and to give high-precision aluminum alloy parts, even when the extrusion 17a undergoes tough plastic working (such as crushing). The extrusion 17a (aluminum alloy part), as having such a low yield strength in plastic working, can have smaller residual stress imparted by plastic working and have better stress corrosion cracking resistance. The yield stress of 120 MPa in plastic working is a numerical value that serves as an index for the advantageous effects (see Japanese Patent No. 5671422).
To eliminate or minimize rupture and cracking during the plastic working more surely, the system and method may perform warm pressing or hot pressing typically in the temperature range of 150° C. to 300° C. using a press forming tool equipped with a heater. In this case, reheating of the extrusion 17a can be omitted by controlling the extruding temperature of the extrusion 17 and the temperature fall of the extrusion 17 (17a) in a time period from immediately after extruding to cutting and conveying, and thereby maintaining the temperature of the extrusion 17a during the plastic working within the temperature range. The warm pressing or hot pressing is performed before the yield strength of the extrusion 17a exceeds 120 MPa at the pressing temperature.
The extrusion 17a after the plastic working receives artificial aging to be an aluminum alloy part. The artificial aging can be performed lot by lot using a heating furnace. As with conventional equivalents, when higher strength is oriented, a T5 treatment (general temper aging) is appropriately selected, whereas, when stress corrosion cracking prevention is oriented, a T7 treatment (over-aging) is appropriately selected. The heating furnace may be disposed in the same floor with the production system as a portion of the system, or may be disposed at another appropriate location.
The aluminum alloy extrusion as a material for aluminum alloy parts obtained according to the embodiment of the present invention is advantageously, but not limitingly, selected from high-strength 7xxx-series aluminum alloy extrusions, which are susceptible to stress corrosion cracking. Such a 7xxx-series aluminum alloy preferably has a chemical composition typically including Zn in a content of 3 to 8 mass percent, Mg in a content of 0.4 to 2.5 mass percent, Cu in a content of 0.05 to 2.0 mass percent, Ti in a content of 0.005 to 0.2 mass percent, and at least one element selected from the group consisting of Mn in a content of 0.01 to 0.5 mass percent, Cr in a content of 0.01 to 0.3 mass percent, and Zr in a content of 0.01 to 0.3 mass percent, with the remainder being Al and impurities.
The aluminum alloy extrusion may also be selected from 6xxx-series aluminum alloy extrusions. Such a 6xxx-series aluminum alloy preferably has a chemical composition typically including Mg in a content of 0.35 to 1.1 mass percent, Si in a content of 0.2 to 1.3 mass percent, Ti in a content of 0.005 to 0.2 mass percent, Cu in a content of 0.15 to 0.7 mass percent, and at least one element selected from the group consisting of Zr in a content of 0.06 to 0.2 mass percent, Mn in a content of 0.05 to 0.5 mass percent, and Cr in a content of 0.05 to 0.15 mass percent, with the remainder being Al and unavoidable impurities.
The aluminum alloy extrusion may also be selected from 2xxx-series aluminum alloy extrusions. Such a 2xxx-series aluminum alloy preferably has a chemical composition typically including Si in a content of 1.3 mass percent or less, Fe in a content of 1.5 mass percent or less, Cu in a content of 1.5 to 6.8 mass percent, Mn in a content of 1.2 mass percent or less, Mg in a content of 1.8 mass percent or less, Cr in a content of 0.10 mass percent or less, Zn in a content of 0.50 mass percent or less, and Ti in a content of 0.20 mass percent or less, with the remainder being Al and unavoidable impurities.
The present invention is advantageously usable for the production of aluminum alloy parts for collision protectors (energy absorbing members) and body frames of automobiles such as passenger automobiles, light automobiles, and trucks. Non-limiting examples of the parts for collision protectors include bumper reinforcements, door beams, crush boxes (bumper stays), bumper reinforcements with integrated stays, pedestrian leg protection parts, and under-run protectors. Non-limiting examples of the body frame parts include rear and front side members, radiator supports, front upper members, roof rails, rear and front headers, rockers (rocker panels), and floor cross members.
In addition, the present invention is also usable for the production of body frame parts of motor-bicycles and bicycles, and any other aluminum alloy parts.
This application claims the benefits of priority to Japanese Patent Application No. 2019-070883, filed Apr. 2, 2019. The entire contents of the above application are herein incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-070883 | Apr 2019 | JP | national |
