Evaluation of Lubricants
The cost of lubricants is small compared to the costs of items such as raw material, equip- ment, and labor. As a result, the economic in- centive to evaluate or change lubricants is not always very significant. However, lubricant breakdown resulting in excessive die wear or die failure is one of the largest factors contributing to reduced production as a result of press down- time and part rejection. Therefore, it is essential to evaluate the lubricants in use and to compare Table 7.5 Lubrication systems for the warm
and hot forging of steels
Material Process Deformation Lubricant
Carbon steel Warm forging Severe MoS2in aqueous
solution Graphite in aqueous
solution
Hot forging Severe Graphite in aqueous
solution Stainless steel Hot forging Severe Glass in aqueous
slurry or powder
Source: [Schey, 1983]
them to alternative types of lubricants. Such an evaluation is necessary in order to utilize effec- tively the large investment required for installing a coating and lubrication line for cold forging [Shen et al., 1992].
There are many bench-type simulation tests designed to evaluate friction and lubrication in forging operations [Schey, 1983]. Here, how- ever, only two of the most common tests are presented, i.e., the ring compression test and the
Fig. 7.8 Finite element model of ring compression test. (a) Initial ring. (b) Compressed ring (50% height reduction) (shear factor m ⳱ 0.1). (Gariety et al., 2003)
Fig. 7.7 Metal flow in ring compression test. (a) Low friction. (b) High friction
Table 7.6 Lubrication systems for the warm and hot forging of aluminum, magnesium, copper, titanium, nickel, and tungsten
Material Process Deformation Lubricant
Aluminum Warm and hot forging Severe None
Graphite in mineral oil
Magnesium Warm and hot forging Severe Graphite in mineral oil
Copper Warm and hot forging Severe Graphite in aqueous solution
Titanium, nickel, and tungsten Warm forging Severe MoS2compounds
Graphite compounds
Hot forging Severe Graphite compounds
Most severe Glass in aqueous slurry or powder
double cup backward extrusion test. The ring compression test best simulates forging appli- cations with a moderate amount of deformation, where the surface expansion induced is on the order of only 100%, while the double cup back- ward extrusion test best simulates more severe forging applications, where the surface expan- sion and the interface pressure induced are over 500% and 290 ksi (2000 MPa), respectively.
In determining the friction factor, f, or the shear factor, m, for hot forming, in addition to lubrication effects, the effects of die chilling or heat transfer from the hot material to colder dies must be considered. Therefore, the lubrication tests used for determining friction factors must include both lubrication and die-chilling effects. Consequently, in hot forming, a good test must satisfy the following requirements [Altan et al., 1983]:
● The specimen and die temperatures must be approximately the same as those encoun- tered in the actual hot forming operation.
● The contact time between specimen and tools under pressure must be approximately the same as in the forming operation of in- terest.
● The ratio of the new generated deformed sur- face area to the original surface area of the undeformed specimen (i.e., surface expan- sion) must be approximately the same as in the process investigated.
● The relative velocity between deforming metal and dies should have approximately the same magnitude and direction as in the forming process.
7.8.1 Ring Compression Test
Lubricity, as defined by the friction factor, f, or the shear factor, m, is commonly measured by using the ring test [Male et al., 1970] [Doug- las et al., 1975]. In the ring test, a flat ring-shape specimen is compressed to a known reduction (Fig. 7.7). The change in internal and external diameters of the forged ring is very much de-
Fig. 7.9 Theoretical calibration curves for ring compression test having indicated OD: ID:thickness ratios. (a) 6:3:2 ratio. (b) 6:3:1 ratio. (c) 6:3:0.5 ratio. [Altan et al., 1983]
pendent on the friction at the die/ring interface. If friction were equal to zero, the ring would deform in the same way as a solid disk, with each element flowing radially outward at a rate proportional to its distance from the center. With increasing deformation, the internal diameter of the ring is reduced if friction is high and is in- creased if friction is low. Thus, the change in the internal diameter represents a simple method for evaluating interface friction.
Simulation of Cold Forging Conditions. The ring test has an advantage when applied to the study of friction under cold forging condi- tions. In order to measure friction with this test, the force necessary to deform the ring and the flow stress of the specimen do not have to be known. Thus, evaluation of test results is greatly simplified. To obtain the magnitude of the fric- tion factor, the internal diameter of the com- pressed ring must be compared with the values predicted by using various friction factors, f, or shear factors, m. Today, these values are most often predicted by the finite element method (FEM). Figure 7.8 shows an example of an FEM model used for this purpose. The results are plot- ted in the form of “theoretical calibration curves,” as can be seen in Fig. 7.9, for rings having OD:ID:thickness ratios of 6:3:2, 6:3:1, and 6:3:0.5. The internal diameters used in this figure are the diameters at the internal bulge. Un- der cold forging conditions, these calibration curves may be considered as “universal” be- cause changes in material properties (i.e., strain hardening) have little effect on the curves. In determining the value of the shear factor, m, for a given experimental condition, the measured di- mensions (reduction in height and variation in internal diameter) are plotted on the appropriate calibration figure. From the position of that point with respect to theoretical curves given for vari- ous values of “m,” the value of the shear factor, m, which existed in the experiment is obtained.
Some results obtained from ring compression tests conducted in a 160-ton hydraulic press with a ram velocity of 15 mm/s and a ring height reduction of 50% are shown in Table 7.7.
Simulation of Hot Forging Conditions. In contrast to the simulation of cold forging con- ditions, the simulation of hot forging conditions do not provide for a “universal” set of calibra- tion curves. The friction calibration curves must be generated for the specific ring material under the specified ring and die temperatures and the ram speed conditions. Hence, knowledge of the flow stress of the material is required [Lee et al., 1972].
The results from some ring compression tests conducted under hot forging conditions have been compiled. The results from ring compres- sion tests conducted for various materials in a 500 ton mechanical press with a nominal speed of 90 strokes/min and a total stroke of 10 in. (255 mm) are shown in Table 7.8.
Fig. 7.10 Metal flow in double cup backward extrusion test
Table 7.7 Values of frictional shear factor, m, obtained from ring compression tests conducted in a hydraulic press
Specimen/die
temperatures Ring size OD:ID:h(a)
Frictional shear
Material ⬚F ⬚C in. mm factor (m) Lubrication system
AISI 1018 200/200 95/95 1.75:1.13:0.5 44.5:28.7:12.7 0.040 Zinc phosphate coatingⳭ soap
0.045 Metallic compoundsⳭ sulfur compounds(b)
0.060 Mineral oilⳭ EP additives
Copper 75/75 24/24 2:1:0.67 50.8:25.4:16.9 0.30 Emulsion
0.27 Water-based synthetic
(a) OD, ring outside diameter; ID, ring inside diameter; h, ring height. (b) Environmentally friendly lubrication system developed to replace zinc phosphate coating based systems. Source: [Gariety et al., 2003] [Hannan et al., 2000]
7.8.2 Double Cup
Backward Extrusion Test
Lubricity, as defined by the friction factor, f, or the shear factor, m, is also measured by using the double cup backward extrusion test. As
shown in Fig. 7.10, the test is a combination of the single cup forward and single cup backward extrusion processes. The ratio of the cup heights, H1/H2, is very dependent on the friction at the
billet/die and billet/punch interfaces [Buschhau- sen et al., 1992] [Forcellese et al., 1994]. In par-
Fig. 7.11 Double cup backward extrusion test tooling at the ERC/NSM
Table 7.8 Values of frictional shear factor, m, obtained from ring compression tests conducted in a mechanical press (die temperatures⬇ 300 ⬚F, or 150 ⬚C)
Specimen
temperature Ring ratio OD:ID:t
Frictional shear
Material ⬚F ⬚C in. mm factor (m) Contact time, s Lubrication system
6061 Al 800 425 6:3:0.5 150:75:13 0.40 0.038 (a) 6:3:1 150:75:25 0.31 0.047 (a) 6:3:2 150:75:51 0.53 0.079 (a) Ti-7Al-4Mo 1750 955 3:1.5:0.25 75:38:6.5 0.42 0.033 (b) 3:1.5:0.5 75:38:13 0.42 0.044 (b) 3:1.5:1 75:38:25 0.42 0.056 (b) 403 SS 1800 980 3:1.5:0.25 75:38:6.5 0.23 0.029 (b) 3:1.5:0.5 75:38:13 0.24 0.039 (b) 3:1.5:1 75:38:25 0.34 0.047 (b) 1950 1065 3:1.5:1 75:38:25 0.28 0.06 (b) 2050 1120 3:1.5:1 75:38:25 0.35 0.06 (b) Waspaloy 2100 1150 3:1.5:1 75:38:25 0.18 0.06 (b) 17-7PH SS 1950 1065 3:1.5:1 75:38:25 0.28 0.06 (b) 2100 1150 3:1.5:1 75:38:25 0.35 0.06 (b) Ti-6Al-4V 1700 925 3:1.5:1 75:38:25 0.30 0.06 (b) 1750 955 3:1.5:1 75:38:25 0.46 0.06 (b) Inconel 718 2000 1095 3:1.5:1 75:38:25 0.18 0.06 (b) 2100 1150 3:1.5:1 75:38:25 0.33 0.06 (b) Ti-8Al-1Mo-1V 1750 955 3:1.5:1 75:38:25 0.27 0.06 (b) 1800 980 3:1.5:1 75:38:25 0.27 0.06 (b) Udimet 2050 1120 3:1.5:1 75:38:25 0.40 0.06 (b) 7075 Al 700 370 5:3:1 125:75:25 0.37 0.06 (a) 800 425 5:3:1 125:75:25 0.31 0.06 (a)
SS, stainless steel. (a) Caustic precoatⳭ graphite coating Dag 137 (Acheson) on the specimens and graphite spray Deltaforge 43 (Acheson) on the dies. (b) Glass- based coating Deltaforge 347 (Acheson) on the specimens and graphite spray Deltaforge 43 (Acheson) on the dies. Source: [Douglas et al., 1975]
ticular, it has been found that the ratio of the cup heights increases as the friction factor, f, or the shear factor, m, increases. In other words, if there is no friction, the cup heights will be the same and the cup height ratio, H1/H2, will be
equal to one. Thus, the ratio of the cup heights represents a simple method for evaluating inter- face friction.
Figure 7.11 shows the actual tooling used for the double cup backward extrusion test. It should be noted that the lower punch was raised out of the container for illustration purposes only. Dur- ing the test, the container and lower punch are fixed on the bed of the press and held stationary with the lower punch located completely inside the container. In addition, the upper punch is fixed on the ram of the press and moves down- ward. Thus, there is a relative velocity between the container and the upper punch, but not be- tween the container and the lower punch. There- fore, the material flow to the lower punch is more restricted in the presence of friction. This ex- plains why the height of the upper cup is larger than the height of the lower cup.
To obtain the magnitude of the friction factor, f, or the shear factor, m, the ratio of the cup heights, H1/H2, must be compared with the val-
ues predicted by using various friction factors, f, or shear factors, m. Today, these values are most often predicted by the finite element method (FEM). Figure 7.12 shows an example of an FEM model used for this purpose. The results are plotted in the form of “theoretical cal- ibration curves,” as can be seen in Fig. 7.13. In determining the value of the shear factor, m, for a given experimental condition, the measured di- mensions (cup height ratio and stroke) are plot- ted on the calibration figure. Figure 7.14 illus-
trates how the cup height ratio and stroke are measured. From the position of that point with respect to the theoretical curves given for vari- ous values of “m,” the value of the shear factor, m, which existed in the experiment, is obtained. It should be noted that in addition to interface friction, the metal flow in this test is dependent on the billet material, the billet diameter, and the
Fig. 7.13 Theoretical calibration curves for double cup backward extrusion test with experimental data point (shear factor m⬇ 0.065). [Ngaile et al., 2002]
Fig. 7.12 FEM model of double cup backward extrusion test (shear factor m⳱ 0.1) (dimensions in millimeters). (a) Initial. (b) Final. (Ngaile et al., 2002)
Fig. 7.14 Measurement of cup height ratio and stroke. Cup height ratio: Rch⳱ H1/H2. Stroke: S⳱ initial
punch diameter. Thus, the flow stress of the ma- terial must be known and the appropriate theo- retical curves should be used to quantify the in- terface friction. In other words, there is no “universal” set of calibration curves for this test. Several double cup backward extrusion tests have been conducted for various materials in a 160 ton hydraulic press with a ram velocity of 15 mm/s and a punch stroke of 21 mm. The re- sults of these tests are summarized in Table 7.9.
REFERENCES
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[Bay, 1995]: Bay, N., Aspects of Lubrication in Cold Forging of Aluminum and Steel, Pro- ceedings of the 9th International Cold Forg- ing Congress, Solihull, UK, May 1995, p 135–146.
[Bhushan, 2001]: Bhushan, B., Modern Tribol-
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[Buschhausen et al., 1992]: Buschhausen, A., Lee, J.Y., Weinmann, K., Altan, T., “Evalua- tion of Lubrication and Friction in Cold Forg- ing Using Double Backward Extrusion Pro- cess,” J. Mater. Process. Technol., Vol 33, 1992, p 95–108.
[Doege et al., 1996]: Doege, E., Seidel, R., Ro- manowski, C., “Increasing Tool Life Quantity in Die Forging: Chances and Limits of Tri- bological Measures,” Technical Papers of the North American Manufacturing Research In- stitution of SME, 1996, p 89–94.
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Ngaile, G., Altan, T., “Identification of Lu- bricants and Enhancement of Lubricant Per- formance for Cold Heading—Progress Report 2—Preliminary Lubrication Tests for Cold Heading,” Report No. PF/ERC/NSM-02-R- 32B, 2003.
[Hannan et al., 2000]: Hannan, D., Ngaile, G., Altan, T., “Development of Forming Pro- cesses for Copper Components for Stanford Linear Accelerator,” Report No. PF/ERC/ NSM-B-00-20, 2000.
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[Lee et al., 1972]: Lee, C.H., Altan, T., “Influ- ence of Flow Stress and Friction Upon Metal Flow in Upset Forging of Rings and Cylin- ders,” Trans. ASME, J. Eng. Ind., Aug 1972, p 775.
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Table 7.9 Values of frictional shear factor, m, obtained from double cup backward extrusion tests conducted in a hydraulic press (punch/die temperatures⬇ 75 ⬚F, or 24 ⬚C) Specimen temperatures Frictional shear factor
Material ⬚F ⬚C (m) Lubrication system
AISI 8610 75 24 0.065 Zinc phosphate coating
Ⳮ soap
0.035 Metallic compounds
Ⳮ sulfur compounds(a) 0.075 Zinc-based dry film(b)
AISI 1038 75 24 0.050 Zinc phosphate coating
Ⳮ soap
(a) Billet size⳱ 1.25 in. diam ⳯ 1.25 in. height (31.75 mm diameter ⳯ 31.75 mm height). (b) Environmentally friendly lubrication system developed for re- placement of zinc phosphate coating based systems. Source: [Ngaile et al., 2002]
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Forging, Spring 1994, p 39–44.
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[Ngaile et al., 2002]: Ngaile, G., Schumacher, R., Gariety, M., Altan, T., Kolodziej, J., “De- velopment of Replacements for Phoscoating Used in Forging Extrusion and Metal Form- ing Processes,” 2002, Report No. PF/ERC/ NSM-02-R-85.
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A., Kropp, E., Altan, T., “A Method for Eval- uation of Friction Using a Backward Extru- sion Type Forging,” J. Mater. Process. Tech- nol., Vol 33, 1992, p 109–123.
CHAPTER 8
Inverse Analysis for
Simultaneous Determination of
Flow Stress and Friction
Hyunjoong Cho
8.1 Introduction
The finite element analysis (FEA) based simulation of metal forming processes has been widely used to predict metal flow and to opti- mize the manufacturing operations. In using user-friendly commercial FEA software, it is necessary to assign input parameters for the simulation. Among those inputs, the parameters in the flow stress equation, friction factor, and anisotropy coefficients of a material are usually obtained from the appropriate tests. The results of process simulation are extremely sensitive to the accuracy of flow stress and interface friction that are input to FEM programs. Therefore, it is essential that these input values are determined using (a) reliable material tests and (b) accurate evaluation methods. A test used to determine material properties should replicate processing conditions that exist in practical applications.
A common method for the determination of the flow stress data for forging simulation is the cylinder upset test as discussed in Chapter 4 be- cause (a) during the test the deformation is done in a state of compressive stress, which represents well the true stress state of most forging pro- cesses and (b) the test can be done for a large strain. However, even in the simplest cylinder upset test, interface friction leads to an inevitable bulging of the sample and thereby to an inac- curate flow stress determination. Thus, the eval- uation of the test results should be able to over- come difficulties introduced by friction and
inhomogeneous deformation. It is desirable to consider the unavoidable friction at the tool/ workpiece interface in the test and to identify the friction together with flow stress using an appropriate evaluation method. In this chapter, an inverse analysis technique for the accurate determination of the input data for FEM simu- lation is introduced to determine material pa- rameters in the flow stress model and the friction at the tool/workpiece interface.