The heat transfer and hence the temperature history of the workpiece is also influenced by the forging equipment. Hydraulic press has lower speed and lower strain rate during defor- mation. Mechanical press and screw press have higher speed and higher strain rate during de- formation. Hydraulic press has a longer dwell time before the deformation, which results in longer free resting heat transfer between the workpiece and the dies. Experimental work on the determination and comparison of the char- acteristics of forging presses was carried out on rings made from selected steel, titanium, and aluminum alloys [Douglas et al., 1971]. Finite element modeling was carried out to simulate the experiments and perform quantitative com- parison of press speed and contact time on heat transfer in nonisothermal ring compression tests [Im et al., 1988a] and [Im et al., 1988b].
Ti-6242 and Al6061 ring compression tests were simulated using FEM package ALPID (a parent version of DEFORM娂). The process conditions used in the computer simulation such as the dimensions of the rings, the ram speeds of the forging equipment, and the reduction in height of the rings were all identical in the ex- periments [Douglas et al., 1971]. The shear fric- tion factor used was the factor measured from the experimental ring tests. The interface heat transfer coefficient used was based on the ex- perimental work [Semiatin et al., 1987]. Table 6.2 gives the conditions used in the finite ele- ment modeling. The hydraulic press used had a capacity of 700 metric tons. The ram velocity was assumed constant during deformation. Me- chanical press used was a high-speed Erie press with scotch yoke design, rated at 500 metric tons at 0.25 in. (6.4 mm) above the bottom dead cen- ter. It had a stroke of 10 in. (250 mm) and a nominal speed of 90 strokes/min. As for the screw press, a Weingarten PSS 255 with a nom- inal rating of 400 metric tons, 2250 mkg (22⳯ 103 J) energy, was used. The starting speed of
the mechanical press and screw press is shown in Table 6.2.
Fig. 6.10 The temperature distribution at (a) the beginning and (b) the end of a Ti-6242 3:1.5:0.5 in. (76:38:12.5 mm) ring test in a hydraulic press. The section inside the rectangle is used in Fig. 6.11.
Table 6.2 Process conditions used in ring compression of Ti and Al alloys
Ram velocity
Press Reduction, % Shear friction (m) Contact time during loading, s in./s mm/s
3:1.5:0.5 in. (76:38:12.5 mm) Ti-6242 ring
Hydraulic 50 0.28 0.33 0.78 19.8
Mechanical 50 0.42 0.044 16 405
Screw 47 0.44 0.024 22 560
3:1.5:0.25 in. (76:38:6.4 mm) Ti-6242 ring
Mechanical 30.8 0.42 0.033 15 380
Screw 37.6 0.2 0.019 22 560
6:3:2 in. (152:76:50.8 mm) Al6061 ring
Hydraulic 51.2 0.65 0.83 1.23 31.2
Mechanical 51 0.53 0.079 26 660
Screw 34.9 0.49 0.051 22 560
6:3:1 in. (152:76:25.4 mm) Al6061 ring
Hydraulic 51 0.42 0.53 1 25
Mechanical 49.8 0.31 0.047 19 480
Screw 47 0.35 0.031 22 560
6:3:0.5 in. (152:76:12.5 mm) Al6061 ring
Mechanical 45.7 0.4 0.038 16 405
Screw 45.6 0.34 0.023 22 560
The interface heat transfer (h) was 0.0068 btu/s/in.2/F (20 kW/m2• K).
The other conditions used in the finite element modeling are: Ring dimensions (OD:ID:height), in (mm) 3:1.5:0.5 (76:38:12.5) for Ti-6242 3:1.5:0.25 (76:38:6.4) for Ti-6242 6:3:2 (152:76:50.8) for Al6061 6:3:1 (152:76:25.4) for Al6061 6:3:0.5 (152:76:12.5) for Al6061
Die material H-13 hot working tool steel
Billet temperature,F (C) 1750F (955 C) for Ti-6242 800F (425 C) for Al6061 Die temperature,F (C) 300F (150 C) for both Ti-6242
and Al6061 ring tests
OD, outside diameter; ID, inside diameter
The contact time during deformation obtained from simulation is shown in Table 6.2. It is seen
from Table 6.2 that the contact time during de- formation is an order of magnitude longer in hy- draulic press than in mechanical press and screw press. The temperature distribution obtained from the finite element modeling for the Ti-6242 3:1.5:0.5 in. (76:38:12.5 mm) ring compression in hydraulic press is presented in Fig. 6.10. The temperature distribution at the start of the de- formation is shown in Fig. 6.10(a) where a heat loss to the die on the bottom surface of the ring was observed. The temperature loss was due to the dwell time before the deformation started for
Fig. 6.11 The temperature distribution at the end of a Ti- 6242 3:1.5:0.5 in. (76:38:12.5 mm) ring test in (a) a hydraulic press and (b) a mechanical press
hydraulic press. Figures 6.11(a) and (b) show the temperature distribution at the end of the Ti- 6242 3:1.5:0.5 in. (76:38:12.5 mm) ring test in the hydraulic press and the mechanical press. The hydraulic press has more die chilling due to longer contact time of the workpiece to the dies before and during deformation. The ring com- pressed in the mechanical press shows a lot of heat building up during deformation. There are zones inside the ring having temperatures above 1850F (1010 C), which is around 25 F (14 C) higher than the beta transus of Ti-6242. However, the temperature gradient is less in the ring forged in the mechanical press and the de- formation is more uniform and the bulge is less pronounced in this ring. The forgers may make use of the difference in press speed and contact time on heat transfer for different forging appli- cations. For alpha-beta titanium forging, the temperature increase to above beta transus is not desired. Therefore, hydraulic press forging is beneficial. For beta titanium forging, steel forg- ing, and selected superalloy forging, tempera- ture increase during forging is not critical and high-speed forging machines such as mechani- cal press, screw press, and hammer can all be used.
REFERENCES
[Altan et al., 1970]: Altan, T., Gerds, A.F., “Temperature Effects in Closed-Die Forging,” ASM Technical Report No. C70-30.1, Oct 1970.
[Altan et al., 1973]: Altan, T., et al., “Forging Equipment, Materials and Practices,” MCIC Handbook HB-03, Battelle, Columbus, OH, 1973.
[Burte et al., 1989]: Burte, P., Semiatin, S.L., Altan, T., “Measurement and Analysis of Heat Transfer and Friction During Hot Forging,” Report No. ERC/NSM-B-89-20, ERC for Net Shape Manufacturing, Ohio State University, June 1989.
[Douglas et al., 1971]: Douglas, J.R., Altan, T., “Characteristics of Forging Presses: Deter- mination and Comparison,” Proceedings of the 13th M.T.D.R. Conference, Birmingham, England, September, 1971.
[Farren et al., 1925]: Farren, W.S., Taylor, G.I., “The Heat Developed During Plastic Extru- sion of Metals,” Proc. R. Soc., Ser. A, Vol 107, 1925, p 422–451.
[Im et al., 1988a]: Im, Y.T., Shen, G., “A Study of the Influence of Press Speed, Contact Time and Heat Transfer in Nonisothermal Upset Forging of Ti and Al Rings,” 16th North America Manufacturing Research Conference Proceedings, 1988, p 91–98.
[Im et al., 1988b]: Im, Y.T., Vardan, O., Shen, G., Altan, T., “Investigation of Non-Isother- mal Forging Using Ring and Spike Tests,” Ann. CIRP, Vol 37/1, 1988, p 225–230.
[Lahoti et al., 1975]: Lahoti, G.D., Altan, T., “Prediction of Temperature Distribution in Axisymmetric Compression and Torsion,” ASME, J. Eng. Mater. Technol., Vol 97, 1975.
[Lahoti et al., 1978]: Lahoti, G.D., Altan, T., “Prediction of Metal Flow and Temperatures in Axisymmetric Deformation Process,” Ad- vances in Deformation Processing, J.J. Burke and V. Weiss, Ed., Plenum Publishing, 1978, p 113–120.
[Semiatin et al., 1987]: Semiatin, S.L., Coll- ings, E.W., Wood, V.E., Altan, T., “Determi- nation of the Interface Heat Transfer Coeffi-
cient for Non-Isothermal Bulk-Forming
Processes,” Trans. ASME, J. Eng. Ind., Vol 109A, Aug 1987, p 49–57.
[SFTC, 2002]: Scientific Forming Technologies Corporation, DEFORM 7.2 User Manual, Co- lumbus, OH, 2002.
[Vigor et al., 1961]: Vigor, C.W., Hornaday, J.W., “A Thermocouple for Measurement of Temperature Transients in Forging Dies,” in Temperature, Its Measurement and Control, Vol 3, Part 2, Reinhold, 1961, p 625.
CHAPTER 7
Friction and Lubrication
Mark GarietyGracious Ngaile
7.1 Introduction
In forging, the flow of metal is caused by the pressure transmitted from the dies to the deform- ing workpiece. Therefore, the frictional condi- tions at the die/workpiece interface greatly influ- ence metal flow, formation of surface and internal defects, stresses acting on the dies, and load and energy requirements [Altan et al., 1983]. Figure 7.1 illustrates this fundamental phenomenon as it applies to the upsetting of a cylindrical work-
piece. As Fig. 7.1(a) shows, under frictionless conditions, the workpiece deforms uniformly and the resulting normal stress,rn, is constant across
the diameter. However, Fig. 7.1(b) shows that un- der actual conditions, where some level of fric- tional stress,s, is present, the deformation of the workpiece is not uniform (i.e., barreling). As a result, the normal stress,rn, increases from the
outer diameter to the center of the workpiece and the total upsetting force is greater than for the frictionless conditions.
Fig. 7.2 Stribeck curve showing onset of various lubrication mechanisms. [Schey, 1983]