North American Forging is Advanced Manufacturing
Part 2: Deformation Control
At the direction of Alfred Krupp, the words “Fritz Let Fly” were inscribed on a 50-ton steam hammer in Essen, Germany, to commemorate the auspicious visit by Emperor William (Wilhelm) in 1877.
As re-told seven years later in THE RECORD, No. 178, Valparaiso, Sept. 12, 1884, Volume 13, “When in 1877 the Emperor William visited the works at Essen, this steam-hammer attracted his attention. Alfred Krupp … presented to the Emperor the machinist Fritz, who he said handled the hammer with such nicety and precision as not to injure or even touch an object placed on the center of the block. The Emperor at once put his diamond studded watch on the spot indicated, and beckoned to the machinist to set the hammer in motion. Master Fritz hesitated out of consideration for the precious object; but Mr. Krupp urged him on by saying ‘Fritz let fly!’ Down came the hammer, and the watch remained untouched. The Emperor gave it to the machinist as a souvenir. Mr. Krupp added one thousand marks to the handsome present and caused the above words to be inscribed on the hammer.”
The “Feel” of Forging
This story clearly illustrates the care, precision and control of metalworkers throughout the ages. Today, although the forging industry is replete with skilled forgers, they are aided by advanced technologies, especially computer controls that enable the controlled application of forging forces to tame the microstructure and frame the shape of a forging. As one tours a forge shop today, one might not see such colorful and intriguing quotes on a hammer or a press. If you look closely, however, to one side there will be computer controls either assisting the operator of the deformation unit or even running it without an operator.
This, the second installment of our series, will illustrate how modern forging equipment is both controlled and advanced, harnessing the forces of a hammer or a press.
In an earlier series on forging equipment (see FORGE, January 2009, p. 10-12), we indicated that programmable hammer control can be used to control the energy imparted to the workpiece. The impact velocity (energy) is controlled through the timing of control valves. The deceleration during forging is a function of the workpiece, material, temperature, die design and process conditions.
A common cause of variation in forgings has been the operator, resulting from a culture developed over the centuries. A forging operator had the latitude to make adjustments to the process based on judgment and experience. This has been the case in hammer shops where operators controlled the energy going into each “blow” and other process variations. An experienced hammer operator developed an intuition in the production of forgings that varied from company to company or even shift to shift in the same plant. The variations between “hit it soft at the start, then apply full blows until it fills” and “add a few degrees to the furnace, soak it longer and hit it as hard as possible” are more than philosophical.
Differences in the part can range from mechanical properties to microstructure and in-service performance. The risk of intuition-based forging is that it lacks science as a basis for decision making. It is compounded by undocumented variation and deviation from procedures. The industry has moved toward controlled processes that have been developed with a mix of trial runs, process simulation, mechanical testing and microstructural evaluation.
Today, computerized energy controls are available on hammers to help the operator control the forging process in a reproducible manner. There is no longer an expectation of shift-to-shift variation in hammer-forged parts. Programmable hammers are repeatable to within about 1% of the requested energy.
New Press at Rock Island Arsenal
In 2013, a $3.3 million contract to supply a forging hammer was awarded to Lasco Engineering Services by the U.S. Army. The hammer was installed in the Joint Manufacturing and Technology Center of Rock Island Arsenal in Illinois (Fig. 1).
This hammer includes a programmable controller with various settings and has a liquid-crystal display (LCD) as the operator’s interface. The controller includes various modes of operation, including “Setting,” “Continuous Blow” and “Program” modes. The setting mode allows the ram to be moved up and down for service and tooling changes and allows the accumulator to be charged and discharged. During continuous-blow mode, the machine is operated via the dual foot switch to deliver blows of a prescribed energy level. In program mode, the hammer will run a sequence of blows with up to 20 individual steps.
Each program step can contain unique blow energy, as well as a pause between each blow, delays for material to be received or an air-blast impulse for die-cleaning purposes. The controller also has the capability to control hydraulic-fluid temperature. Up to 250 different programs can be stored away and recalled when ready to run that set of dies again.
Near-Net Shapes and Quality Levels
Automotive forgings have evolved at an astonishing rate during the last three to four decades in terms of process control, design sophistication and automation. In 1985, it was common to hot- or warm-forge drivetrain components with ample stock to machine the majority of the forged surface after a heat treatment. The forging process combined a mix of operators and pick-and-place (assembly-line) automation. Quality levels were maintained through machining, sorting, inspection and rework to very impressive dimensional controls in the 100-1,000 ppm defect range.
In 2015, automated forging lines are designed to forge net- or near-net-shape components with significantly less forging stock and minimal machining. Mechanical properties are achieved through controlled cooling of microalloyed steels that were not available in the 1980s. The press structure and forge tools are designed using FEM analysis to analyze and control stiffness and deflection, resulting in stock and tolerances at less than 10% of those in the past. Robots are supplemented by vision systems to feed the press, transfer parts between stations and place them in the post-forge cooling line without surface defects.
Automotive forging lines are technically sophisticated, fast and reproducible. Technology has been applied to this process over many decades to transform an impressive process into one that consistently produces near-net-shape steel parts faster and more cost effectively. The days of sorting and rework are rare to nonexistent. Part quality is astonishing, with nonconformance rates in the 1-10 ppm range.
In spite of impressive advances, every current limit is being challenged, with the ultimate goal of “forging net-shape parts with holes, splines and threads and a 0-ppm nonconformance rate.” Many years ago, there was a saying that a forge shop was not a chocolate factory. Today, that distinction has diminished significantly.
Forgings in Critical-Service Applications
Forged components in aircraft represent the epitome of a critical-service application requiring total process control, including deformation process control. The rotating parts in turbine engines illustrate this point. Titanium and nickel superalloy disks and shafts rotate at thousands of revolutions per minute with temperatures well over 1000°F in the hot section. Failure can be catastrophic. In the 1970s and 1980s, aircraft-engine manufacturers tested every heat and heat-treated lot of forgings with a complete cut-up for microstructural evaluation and mechanical testing.
Mechanical properties for nickel superalloy disks are the result of controlling the microstructure, primarily grain size. Grain refinement requires tight thermal controls to minimize grain growth. Refinement from the original billet grains are the result of recrystallization, which is dominated by dynamic recrystallization during forging. Tight temperature and strain-rate controls are required, with strain levels controlled by increasingly sophisticated forging designs and forging operations.
Titanium alloy forgings used on the fan and compressor stages require similar levels of control, which are compounded in complexity by alpha and beta phases that vary with chemistry. Process variations were correlated to failures in mechanical tests, nonconforming microstructures and a range of premature maintenance issues. The forging industry worked diligently on controlling processes to the extent that forgings are more reliable than ever.
Process controls include computer-controlled forging presses. In advanced hydraulic presses, for example, the velocity is controlled as a function of die stroke (Fig. 2) and is repeated forging after forging. With tight thermal controls in furnaces and on forging dies, engine manufacturers use statistical testing to qualify these parts, with results far superior to the days of testing and sorting.
It is common for a set of tools to have a limited load capacity. In the past, operators tried to “back off” the forging force at the end of the stroke to mitigate this limitation, especially when a small part was forged on a large press. Spectacular, sudden tool failures have occurred, with injury and loss of life being an unfortunate outcome. One of the authors has first-hand experience with catastrophic die failures prior to the ability to control tonnage, and the dangers are not overstated.
Over the years, engineering analysis has provided a better understanding of the root cause of tool failures. This leads to a question of why a tonnage limit can’t be applied to a forging press. In many cases, the rate of load increase is faster than an operator or control system can respond. In the 1980s, the first controls were placed on hydraulic presses using computer-control systems written in very efficient assembly code with custom high-speed control valves designed to prevent overloading of tools and equipment. At that time, many control-program techniques were applied to minimize the risk of overloading the tools.
As control systems became faster and high-speed control valves were developed, more robust speed and load-control capabilities were introduced to a range of forging presses. Hydraulic presses have the most sophisticated control-system options because they are extensively used in aerospace forging. Load limits and energy/stopping controls are available on screw presses. Even servo-controlled mechanical presses have the ability to control a speed profile where a press behaves like a mechanical press for a family of parts. With control changes, it behaves like a knuckle press for other parts.
As the sophistication of forging increases, so will the demands on the workforce designing, building, installing, operating and maintaining the equipment. These positions are technically challenging and rewarding. Today’s Fritz is assisted by HAL, a droid or Siri. May we borrow your Rolex to illustrate this point?
This series of articles is being co-authored by Jon Tirpak of SCRA Applied R&D and John Walters of SFTC. Contributions include a teamed investment being made by the Defense Logistics Agency under the cost-shared, manufacturing technology program known as PRO-FAST, which includes reach across forging supply chains throughout the North American forging industry. Ken Berger of GKN, Mike Gill of Lasco Engineering Services, Bill Goodwin of Erie Press Systems and Rock Island Arsenal have all contributed to this series. The next article will focus on furnace and induction heating raw material for a forging hammer or press.
Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or firstname.lastname@example.org. Co-author Jon D. Tirpak is the executive director of FDMC and FAST program manager. He is also vice president of ASM International. He may be reached at 843-760-4346; or email@example.com