TECHNICAL FIELD
This invention relates to the production of forged preforms in the manufacture of aluminum compressor wheels for turbocharger applications, and more particularly to an improved forging method and apparatus that minimizes variability in grain orientation in the forged wheel preforms.
BACKGROUND OF THE INVENTION
In the manufacture of forged milled aluminum compressor wheels, an ingot cut from cast or extruded aluminum bar stock is heated and forged in a press to produce a preform or forging, which is then milled to final shape and finish. The aluminum grain orientation in the preform is known to have a significant influence on the wheel's resistance to cyclic fatigue failure, and ideally, the grain orientation should be aligned with the loading the wheel will experience in operation. While the initial grain orientation in an ingot is primarily axial (that is, primarily transverse to the expected loading), the forging process upsets the ingot material to re-direct the grain orientation in a more radial direction that parallels the contour of the die for optimal cyclic fatigue life.
In a typical forging operation, the aluminum ingots are placed in an oven or infrared (IR) heater, and pre-heated to an established temperature for optimizing material flow during forging; and the forging die is also pre-heated to minimize ingot heat loss into the die. Die pre-heating is typically accomplished by placing a portable IR heater on the bottom die, and removing it when the dies reach a set temperature. The die temperature during forging following removal of the heater will vary somewhat due to heat gain from the ingots and heat loss to the forging press and the environment.
Our analysis of aluminum wheel preforms manufactured according to conventional forging practices as outlined above revealed a considerable part-to-part variation in the grain orientation, even with maximum pre-heating of the ingots. Accordingly, what is needed is a way of improving the conventional forging process so as to produce forged wheel preforms that consistently exhibit the desired primarily radial grain orientation.
SUMMARY OF THE INVENTION
The present invention is directed to an improved method and apparatus for producing forged aluminum wheel preforms that consistently exhibit a desired primarily radial grain orientation. Key to the invention were the twin recognitions that the part-to-part grain orientation variability observed in conventionally produced forged preforms was due almost entirely to temperature variation in the forging dies during forging; and that closely controlling the temperature of the forging dies during forging virtually eliminates the part-to-part variability and deviations from the desired primarily radial grain orientation in the forgings.
The improved forging process of the present invention includes the novel steps of pre-heating both top and bottom dies of the forging press to a specified temperature such as 200 degrees C., and maintaining both dies at the specified temperature during subsequent forging operation with closed-loop temperature control systems. In a preferred embodiment, the top and bottom forging dies are each coupled to a dedicated thermic fluid heater, the temperature of each die is individually measured and compared to a respective target temperature (which may be the same temperature), and the respective thermic fluid heaters are controlled in relation to detected deviations of the measured temperatures from the target temperatures. Wheel preforms produced according to this improved method consistently exhibit primarily radial grain orientations that exactly match the expected values.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a conventional process flow diagram for manufacturing a forged milled aluminum compressor wheel.
FIG. 2 is an illustration of a forged compressor wheel preform with grain flow analysis overlays.
FIG. 3 is a process flow diagram for hot forging wheel preforms according to this invention.
FIG. 4 is a diagram of a representative plant layout for producing forged compressor wheel preforms according to the forging process outlined in the flow diagram of FIG. 3.
FIG. 5 is a diagram of top and bottom forging press dies and a pair of thermic fluid heaters for regulating the temperature of the dies during forging.
FIG. 6 is a photograph depicting a grain flow analysis of a compressor wheel preform manufactured according to this invention.
FIG. 7 is a graph depicting the results of bending fatigue testing of various forged compressor wheel preforms, including forged compressor wheel preforms produced according to this invention.
FIG. 8 is a bar graph of illustrating the durability improvement achieved by forgings produced according to the method of this invention, as compared with baseline forgings produced with conventional methods.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the flow diagram blocks 10-19 generally outline a conventional process for manufacturing forged milled aluminum compressor wheels (also known as billet wheels). As noted at blocks 11-12, a bar of cast or extruded aluminum is cut to form an ingot, and the ingot is subjected to shot blasting to provide a consistent surface texture for infra-red heating. Then the ingot and bottom die of the forging press are pre-heated in preparation for the forging process, as indicated at block 13. The ingots are typically pre-heated in an oven or an IR heater, and the dies are typically pre-heated by placing a portable IR heater between the top and bottom dies until the dies reach a set temperature, such as 200 degrees C. Then the heated ingot is placed in the bottom die, and the press is activated to carry out the forging operation, as indicated at block 14. As noted at blocks 15-16, the wheel preform is then removed from the press and placed on a cooling conveyor and allowed to cool before excess material (flash) is trimmed from the perimeter of the preform. The trimmed wheel preforms are then inspected and subjected to a conventional heat treatment process, as indicated at blocks 17-18. And following heat treatment, the wheel preform is milled to final shape and finish, as indicated at block 19.
Quality control analyses of forged aluminum wheel preforms manufactured as outlined in the flow diagram of FIG. 1 exhibit a considerable part-to-part variation in the grain orientation that cannot be corrected by the usual tweaking of process control parameters such as pre-heat temperature. And as noted above, the grain orientation in aluminum wheel preforms is known to have a significant influence on the wheel's resistance to cyclic fatigue failure. Consequently, compressor wheels produced using the conventional forging process exhibit inconsistent part-to-part durability in use.
Further testing and analysis revealed that the part-to-part grain orientation variability observed in conventionally produced forged preforms is due almost entirely to temperature variation in the forging dies during forging. This is illustrated in the grain orientation diagram of FIG. 2, where the reference numeral 20 designates theoretical prediction of grain flow (and therefore, orientation) due to material upset during forging. Modeling of the forging process predicts a significant deviation of grain orientation when the die temperature is too low; this is illustrated by the trace 22, which designates the predicted grain orientation with a 50 degree C. drop in die temperature from the initial (pre-heat) temperature. Such temperature drops are not uncommon in conventional forging operations, and in fact, the reference numeral 24 designates an even worse deviation in grain orientation that actually occurred in a conventionally produced compressor wheel preform.
This testing and analysis led us to the discovery that the part-to-part variability and deviations from the expected grain orientation in aluminum compressor wheel preforms could be virtually eliminated by maintaining the temperature of the forging dies constant during forging. We accomplished this by coupling a pair of thermic fluid heaters directly to the top and bottom dies of the forging press. The die temperatures were monitored during the forging process, and the set/desired temperatures were maintained during forging though a closed-loop control of the thermic fluid heaters to eliminate detected deviations of the monitored temperatures from the set/desired temperatures. This process is outlined by the flow diagram blocks 30-37 of FIG. 3. The step of pre-heating the ingots in an IR heater (block 31) is a conventional step, as described above. Concurrent with the ingot pre-heating step, the thermic fluid heaters coupled to the top and bottom dies of the forging press are activated as indicated at block 32. The die temperatures are monitored, and when they reach the target temperatures, a pre-heated ingot is placed in the lower die, as indicated by the blocks 32-34. Then, as indicated by blocks 35-36, the forging press is activated, and the closed-loop die temperature controls are continued. That is, the monitored die temperatures during forging are compared to the target temperatures to create temperature error signals in relation to the deviation of the monitored temperatures from the target temperatures, and the error signals are used to control the heating of the respective thermic fluid heaters. While this is a classic closed-loop control, it is the first time that a continuous temperature control has been applied to forging press dies to maintain the temperatures of the top and bottom dies at target temperatures during the forging process. Finally, as indicated at block 37, the forged wheel preforms are transferred from the forging press to the cooling conveyor.
FIG. 4 depicts a plant layout for carrying out the manufacturing process outlined in the flow diagram of FIG. 3. The various process steps correspond to a series of work stations, including a storage bin 40 for the cast or extruded aluminum bar stock, a bar cutting station 42 for cutting an ingot from the bar stock, a shot blasting station 44 where the ingots are shot blasted to provide a consistent surface texture for infra-red heating, an IR heater 46 into which the shot-blasted ingots are placed for pre-heating, a forging press 48, a cooling conveyor 50, a trimming station 52, an inspection station 54, and a heat treat station 56. As implied in the illustration, the IR heater 46 will typically include an internal conveyor for moving the ingots through the heater. The transfer of parts from one station to another may be performed manually or automatically.
Of particular note to the present invention are the two thermic fluid heaters 58 and 60, and the chiller 62. There are two continuous fluid paths: a first path through the first thermic fluid heater 58, the bottom die of the forging press 48, and the chiller 62; and a second path through the second thermic fluid heater 60, the top die of the forging press 48, and the chiller 62. These fluid paths are signified by the arrows 63a, 63b and 63c, 63d and 63e. The thermic fluid heaters 58 and 60 add heat to the fluid supplied to the forging press dies, while the chiller 62 is capable of cooling the fluid if the respective die temperatures rise above the target temperatures. The block 64 designates a computer-based controller for carrying out the closed-loop control of the die temperatures; controller 64 is responsive to input signals representing the continuously measured die temperatures, and produces output control signals for the thermic fluid heaters 58 and 60, and the chiller 62 if needed, for eliminating detected deviations of the measured temperatures from the target temperatures.
The diagram of FIG. 5 illustrates the top and bottom dies 65 and 66 of the forging press 48 during the manufacture of a compressor wheel forging F, and the fluid paths coupling the dies 66 and 65 to the thermic fluid heaters 58 and 60. The chiller 62 is omitted from this diagram. The top die 65, also referred to as a punch, has a peripheral cavity 65a through which the fluid from thermic fluid heater 60 circulates. The cavity 65a is sealed by a pair of O-rings 65b, 65c seated in peripheral grooves above and below the cavity 65a, and held in place by the seal ring 65d. The bottom die 66 is radially captured in a seal ring 67, and the fluid from thermic fluid heater 58 circulates through inlet and outlet fluid passages 67a, 67b in the seal ring 67 and a spiral peripheral cavity 66a formed in the bottom die 66. The top and bottom dies 65 and 66 are supported on a pedestal 68.
The photograph of FIG. 6 illustrates the results of grain orientation analyses performed on forged aluminum compressor wheel preforms manufactured according to this invention. The grain orientation is numerically measured at two specific locations. Ideally, the grain orientation is aligned at a fixed angle with respect to the centerline C of the wheel preform. Variation between forgings produced according to the method of the invention has been found to be minimal and symmetric about the centerline C, as indicated by the traces 70 and 72 in FIG. 6. This is in contrast to conventional forgings that typically exhibit an order of magnitude greater variation.
The fatigue lifetime benefit of forged aluminum compressor wheel preforms produced according to this invention has been assessed using disk specimens in diaphragm bending tests. The disk specimens are cut from the back of the forged wheel preforms so that all variation in material structure can be evaluated. The results of the bending tests, depicted in the graph of FIG. 7, show that forgings produced according to this invention exhibit superior fatigue life, and much reduced variability compared to a conventional benchmark (baseline) forgings. For the baseline forgings, the data points are signified by diamonds, and the broken line 74 represents a minimum expected lifetime (in cycles to failure) based on the data. For the forgings produced according to this invention, the data points are signified by squares, and the minimum expected lifetime is represented by the broken line 76. As indicated by the arrow 78, which links the lines 74 and 76, the minimum expected lifetime of forgings produced according to this invention exceeds that of the baseline forgings by a factor of five, which is both surprising and unexpected.
The superior fatigue life of forgings produced according to this invention was similarly confirmed by compressor wheel cyclic spin testing, the results of which are depicted by the bar chart of FIG. 8. The bar heights represent the compressor wheel fatigue life in spin cycles to failure. Whereas the baseline compressor wheels (represented by the bars 80 and 82) exhibited a fatigue life of approximately 70,000 cycles, the compressor wheels produced according to this invention (represented by the bar 84) exhibited a fatigue life of over 120,000 cycles, a mean life improvement of approximately 72%, which is also both surprising and unexpected.
In view of the foregoing, it will be understood that the forging process of the present invention is practical and easily implemented in the manufacturing environment, and that it affords a significant improvements in average fatigue life of forged compressor wheels with greatly reduced part-to-part variability. It will also be understood that while the process has been described in reference to the illustrated embodiments and diagrams, numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art, and still fall within the intended scope of the invention.
Patent Application
20180221937
