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The abrasivewear of dies influences dimensional accuracy and the surfacefinish of products during hot forging processes [12,13].In this study, in order to predict the wear profile of a diein metal forming processes, Archard wear model is appliedas shown in Eq. (5) [14]:V = kPl3h(5)where V is the wear depth, k is the wear coefficient, P is thenormal pressure on die surface, l is the sliding distance andh is the surface hardness of the die.To estimate the die service life based on abrasive wear, itis needed to consider the hardness change at high tempera-ture of a die and the wear amount increase of surface layerwith regard to the contact time and temperature. A numericalmodel of abrasive wear as shown in Eq. (6), is developed byconsidering the hardness change of a die toward the directionof wear depth. The normal pressure (σn), the sliding velocity (vs), andthe temperature distributions on die surface are calculatedfrom the rigid-plastic FE analysis, and the permitted amountof abrasive wear and the critical value of surface hardnesswere obtained from wear test and thermal softening experi-ments.The amount of abrasive wear at each point on the die sur-face for one forging cycle was calculated through the wearanalysis of Eq. (6), and then compared with the permittedvalue. Also, the hardness at the worn surface that resultedfrom this amount of abrasive wear was compared with thecritical value. If the amount of abrasive wear is smaller thanthe permitted value, and the hardness at worn die surface isstill greater than the critical value, then abrasive wear anal-ysis will repeat until the integrated amount of abrasive wearreaches the permitted value. Finally, the production quantitywhich expresses die service life was determined from thetotal number of wear analysis. The flowchart of a methodfor estimating the die service life based on abrasive wear isshown in Fig. 3.3. Analyses and resultFig. 4 shows a hot forging product to be analyzed basedon plastic deformation and abrasive wear. One of auto-mobile components, spindle part, is manufactured in threestages composed of upsetting and two forward/backward hot- forging operations. Fig. 5 shows the process design result forthe hot forming of spindle part.This product has the height of 320mm, maximum diam-eter of 131mm and a long extruded part. This discrete partrequires a minimum machining and high dimensional accu-racy. Unfortunately, abrasive wear or plastic deformation ofthe die occurred at the stepped corners as shown as point 1, 2in Fig.
4, the die service life of this part depends on the change of the initial shape and dimension of these stepped cornersduring hot forging. The forming analysis conditions and thevariations of process variables for estimating die service lifeare listed in Tables 1 and 2, respectively. The distributions ofdamage value at final stage obtained from the FE analysis isshown in Fig. 6, these values appeared highly at two steppedcorners. The damage factor can be used to predict fracture informing operations [15,16].Therefore, the damage degree of these corners may di-rectly relate to die service life. When the initial die tempera-ture is low, it may influence product quality.When the initialdie temperature is high, die hardness decreases. When theforming velocity becomes faster, the contact time betweenthe hot deforming material and the dies is shortened and theequivalent temperatures become low. The initial die temper- 3.1. Influence of the initial die temperatureIn metal forming process, both plastic deformation andfriction contribute to the heat generation. The temperaturesdeveloped in the process influence lubrication conditions,tool life, the properties of the final product, and the rate ofproduction [4]. Above all, when the initial die temperature ishigh, the temperature difference between inside and outsideof a billet becomes small, and this small temperature differ-ence assists the sound metal flow. On the other hand, a highsurface temperature may reduce die service life. But the lowtemperature of die surface can disturb metal flow and causethe surface defects.As can be seen in Fig. 7, the temperature on die surface attwo stepped corners (point 1, 2) increase differently, due to initial die temperature effect, for the same forging process.For the initial die temperature 400 ◦C at point 1, the die tem-perature is initially higher, but the maximum temperature islower than for either 200 or 300 ◦C.Also, these results clearlyindicate that the temperature gradient for the initial die tem-perature 400 ◦C is very large at point 2. The distributions ofnodal force and velocity are shown in Fig. 8. It can be seenthat nodal force acting on die surface decreases as the initialdie temperature increases, whereas velocity of the workpieceat the vicinity of the die/material interface increases as theinitial die temperature increases.