The effects of using coated tools in the deep drawing of aluminium

The effects of using coated tools in the deep drawing of aluminium

Journal o[ Materials Processing Technology, 29 (1992) 235-244 235 Elsevier The effects of using coated tools in the deep drawing of aluminium K.S. ...

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Journal o[ Materials Processing Technology, 29 (1992) 235-244

235

Elsevier

The effects of using coated tools in the deep drawing of aluminium K.S. Lee, K.H.W. Seah and C.Y.H. Lim Department of Mechanical and Production Engineering, National University of Singapore, Kent Ridge, Singapore 0511 (Received June 6, 1991; accepted July 7, 1991 )

Industrial Summary The performance of titanium nitride coated punches and dies in deep drawing is compared with that of uncoated tools, in terms of two parameters: the maximum punch force, and the limiting draw ratio. The effectiveness of titanium nitride coatings on tools with different corner radii is studied also. Deep-drawing experiments were conducted on aluminium using all possible combinations from two similar sets of tools: one uncoated, the other coated with titanium nitride. Both sets consisted of punches and dies with different comer radii. It was found that the titanium nitride coated dies reduced the maximum punch forces by 7.45 to 13.40%. The coated dies also improved the limiting draw ratios by about 4.59%, with the combination of uncoated punch and coated die yielding the largest limiting draw ratios. The punch forces were also reduced when the punch and die corner radii increased. The limiting draw ratios were improved with the increase of die corner radius, while punches of profile radii 6 to 7 times the blank thickness gave the highest limiting draw ratios. The experimental results agreed reasonably well with the theoretical predictions obtained by an approximate solution proposed by Duncan and Johnson.

1. Introduction

The use of cutting tools with hard, wear-resistant coatings of carbides, nitrides, and oxides is widespread today. These coatings are known to prolong tool life, increase productivity, improve workpiece quality, and reduce machining forces. However, relatively little research has been done on the effects of coatings on drawing tools although it has been shown that coatings can improve the performance of many forming and cutting tools. Two parameters of major industrial interest in deep drawing are the maxim u m forming force required and the deep-drawability of the material, generally expressed by its limiting draw ratio (LDR). In the case of an axisymmetric cup, the LDR is defined as the ratio of the blank to the punch diameter where the blank diameter is the largest that can be drawn successfully in one operation. One of the main factors affecting these two parameters is the friction between the blank and the drawing tools. Kumpulainen et al. [1 ] showed that 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

236 two different friction conditions dominated in deep drawing: stretch friction at the punch/blank interface where only a little sliding occurs, and sliding friction at the die/blank interface. Younger et al. [2 ] observed that failure in the drawing of flat-bottomed cups occurs usually by splitting around the punch profile radius, due to the stretching and thinning of the material as it is wrapped around the punch nose. By increasing the friction between the punch and the blank, the sliding of the material over the punch is inhibited, thus restricting the stretching that leads to tensile failure. From deep-drawing experiments conducted with polished and knurled punches, they found that LDRs were improved by using the knurled punches (i.e. by increasing the roughness of the drawing punch radius ). These results are consistent with a similar study by Spurgeon and Younger [3]. On the other hand, reduction of friction is desirable at the blank/die interface. Pearce [4] noted that where the blank periphery is flowing into the die cavity, good lubrication (low friction) is required to assist the process. In practice, methods such as using lubricants and having better surface finish or harder tools, are employed to reduce friction. Studies with coated cutting tools indicate that a contributory factor to the success of coatings is the low coefficient of friction between the tool and the workpiece, due to the high lubricity of the coatings. The use of coatings on deep-drawing tools may therefore be an alternative method for reducing friction. Wick [ 5 ] has identified the major advantages of depositing hard coatings on cutting tools. These included longer tool life (2 to 10 times that of uncoated tools), increased productivity, improved surface finishes and closer tolerances, and reduced machining forces; all of which result in cost savings. He ventured that these benefits arise from the combination of high wear resistance in the coatings with good toughness and high strength in their substrates. The coatings also provide a chemical barrier and good shock resistance during machining. The coefficient of friction between the tool and the workpiece is reduced by the high lubricity of most coatings, with titanium nitride (TIN) coatings having been found to have the lowest coefficient of friction (highest level of lubricity). Similar findings were presented also by Korhonen et al. [6]. They found that ion-plated TiN coatings showed excellent wear resistance, and that TiNcoated deep-drawing tools had a tool life 10 times that of uncoated tools. The results of friction and wear tests showed that TiN ion-plating on tool steel significantly lowered the coefficient of friction. They predicted that these coatings can be applied most successfully to cases where scratching, adhesive or abrasive wear are the dominant mechanisms. This paper compares the performance of titanium nitride coated punches and dies with that of uncoated tools in the deep drawing of axisymmetric aluminium cups. The parameters investigated are the maximum punch force (i.e. the maximum force the punch delivers during a draw), and the limiting draw

237 ratio (LDR). Also studied is the effectiveness of titanium nitride coatings on tools with different corner radii.

2. Apparatus The deep-drawing experiments were conducted on a single-action 1.5 MN capacity hydraulic press with a die space of 457 mm, a stroke length of 229 mm, and a maximum press speed of 15 mm/s. A hydraulic blank-holding system using four hydraulic cylinders was incorporated to hold the blanks in place. Circular blanks were cut from cold-rolled pure aluminium sheets of average thickness 0.88 mm with diameters ranging from 90 to 122 mm in increments of 1 mm. The aluminium had an average ultimate tensile strength of 85.11 MPa and a yield strength of 73.79 MPa. Two sets of punches and dies were used: one uncoated, the other coated with titanium nitride (TiN) to a thickness of about 2-3/~m by the physical vapour deposition (PVD) process. The punches were of 60 mm diameter with profile radii of 2 mm, 4 mm, 6 mm, 9 mm, and 12 mm. The dies had internal diameters of 61.99 mm with throat corner radii of 2 mm, 4 mm, 6 mm, 9 mm, and 12 mm. This gave a fixed punch-to-die clearance of about 0.995 mm or about 13% more than the blank thickness. All punches and dies were of ASSAB DF-2 tool steel and were hardened to 61 RC by heat treatment. However, the coated tools suffered a slight drop in hardness to 59 RC due to tempering during the coating process. The base tool material contains 0.90% C, 0.30% Si, 1.2% Mn, 0.50% W, and 0.10% V. Two blank-holders were employed: an uncoated one for use with the uncoated dies, and a TiN-coated one for the coated dies. Both were made of ASSAB DF-2 steel. The uncoated blank-holder had not been hardened by heat treatment. Its coated counterpart was actually a coated die mounted on a holder. This die was processed in the same way as the punches and dies described above.

3. Experimental procedure Cups were drawn from the aluminium blanks using all possible punch and die combinations. For each combination, cups were drawn with increasing draw ratios until the bottom of the cup ruptured. The criterion for a successful draw was based on the formed cup being free of tearing or severe wall wrinkling. Slight wrinkling near the top edge of the cup was allowed. The forming force (i.e. the punch force) was monitored by a load cell sandwiched between the punch and the top of the press. The vertical punch motion during the draw (i.e. the punch travel) was measured using a linear variable differential transducer (LVDT). The zero punch travel position was taken to be the position at which the punch was just in contact with the blank before

238

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drawing began. Plots of the punch force against the punch travel were generated on an X-Y chart recorder as the cups were being drawn. A schematic diagram of the experimental set-up is shown in Fig. 1. The experiments were conducted at room temperature without the use of lubricants. The press speed was kept constant at 7.0 m m / s and the blank-holder pressure was set at 2.75 MPa. A number of experiments were performed twice to check for repeatability. 4. R e s u l t s and d i s c u s s i o n

A typical experimental plot of punch force versus punch travel is shown in Fig. 2. In this example using a coated 2 mm punch and a coated 4 mm die, the cup with a draw ratio of 1.78 ruptured at the bottom as the punch force peaked. The limiting draw ratio for this particular tool combination was therefore 1.77 and the corresponding maximum punch force was 9.04 kN. When other tool combinations were used, the plots of punch force against punch travel followed the same characteristic trend. On all curves there exists a second peak, which in some instances, is higher than the first. This phenomenon is due to ironing. Ironing occurs because compressive hoop stresses in the flanges cause the material to thicken as it approaches the die cavity. When the thickness exceeds the punch-to-die clearance, extra force is required to squeeze the material through, which is manifested by the presence of the second maximum. It was observed that ironing was more severe when the blank diameters were large, due to the increasing thickening effect at the edges. The severity of ironing appeared to be unaffected by the use of coated tools. In fact, the use of a

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coated die in some instances, led to greater ironing. One explanation could be that the extra layer of coating actually decreased the clearance between the punch and the die. Another possibility could be that because of the lower coefficient of friction of the coatings, flow over the die corner was improved, so less stretching and thinning occurred, resulting in thicker cup walls.

4.1 Limiting draw ratio In Fig. 3, a comparison is made of the formability of the process when different tool combinations were used. The experiments were repeatable to within 1% of the limiting draw ratio (LDR). Each data point represents the LDR for a certain combination. The punch profile radius for the combinations represented here is 9 mm, but all the different die corner radii and coating combinations {i.e. uncoated punch with uncoated die, uncoated punch with coated die, coated punch with uncoated die, and coated punch with coated die) are presented. Graphs drawn for punches of other profile radii also share the same trend. It can be observed that the uncoated punch with coated die combination yields the best LDRs. This can be attributed to the higher coefficient of friction of the uncoated punches preventing the stretching of the material around the punch profile that leads to failure, while the lower friction of the coated dies improving flow into the die cavity, resulting in lower tensile stresses in the material as it is deformed over the die corner. These results are in line with the

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results of studies by Younger et al. [ 2 ], Spurgeon and Younger [ 3 ], Pearce [4 ], and the authors' observations on mild steel [ 7 ]. The second best coating combination is the coated punch-coated die, followed by the uncoated punch-uncoated die. The worst combination is the coated punch-uncoated die, where conditions are contrary to all those required to suppress premature failure of the partly drawn cup. A different representation of the above data is shown in Fig. 4(a), where comparisons are made between the LDRs obtained with 9 mm punches and 4 mm dies in the four different coating combinations. Fig. 4 (b) shows the corresponding percentage differences in LDRs between the various combinations. From similar graphs for other punch and die corner radii combinations, it was found that the replacement of uncoated dies with coated ones increased the LDRs by an average of 4.59%, while a similar replacement with the punches decreased the LDRs by about 1.15 %. Replacing both punches and dies brought an increase of 3.38%. It was also found that the improvements in the LDRs were greatest when coated dies were used with 2 m m punches (5.52%) and smallest with 12 m m punches (3.77%), while the rest had almost the same increases (4.41-4.68%). This may be due to the drawing with punches with sharp corners involving higher stresses at the punch nose because the material is bent more severely. Under such circumstances, improved flow over the die throat corner may be highly significant. However, when punch profile radii are large, the stresses are lower and the benefits of better flow are relatively smaller. It is clear from Fig. 3 that the increase in the LDR is highest when the die

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radius is 4 mm. This is because metal flow at the die corner is poor when the radius is small, so the improved flow due to the coating is highly significant. With dies of larger corner radii, flow is relatively good even when the dies are uncoated, so the contribution of the coatings is less pronounced.

4.2 Maximum punch forces Figure 5 shows the graphs of the maximum punch force versus draw ratio for the four coating combinations using a 2 mm punch and a 6 mm die. The experiments were repeatable to within 5% of the punch force. From this and

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similar graphs for other punch and die corner radii combinations, it was found that in most cases, the use of a coated die led to a reduction in the punch forces. This occurred with both the uncoated and coated punches. An alternative representation is given in Fig. 6 (a). Here, comparisons of the maximum punch forces are made between different coating combinations. The punch force is given as a range, the lower limit corresponding to the maximum punch force at the smallest draw ratio, and the upper limit at the LDR. The corresponding percentage differences are shown in Fig. 6 (b). W h e n coated dies were used in place of uncoated ones, most of the cases showed a reduction in forces averaging 7.45 to 13.40%. This is hardly surprising since frictional forces due to the sliding of the blank over the die surface would be lowered by the lubricating effect of titanium nitride (TIN). In addition, the lower friction improves metal flow at the die corner, so that less force is required to deform the material over the corner. The effects of using coated punches are less certain. It was observed that when the punch profile radius was 2 mm or 9 mm, there was a definite reduction of punch forces associated with the coated punches. On the other hand, there was just as significant an increase due to the coating for the 4 mm punches, and to a lesser extent, for the 6 mm punches as well. The results for the 12 mm punches show no clear trend. Generally, the experimental curves of Fig. 5 lie within + 2.5 k N of the theoretical predictions by Duncan and J o h n s o n [8]. W h e n the punches and dies are of small corner radii, the experimental lines lie above the theoretical but

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they fall progressively to below the theoretical as the corner radii increase: this is because the theory assumed the deep drawing operation to be frictionless and neglected both the punch and die corner radii. It also made the assumption that the punch force versus punch travel diagram corresponds to the first half of a sine wave. These are only approximations to reality, and Duncan and Johnson had shown from experiments that the equation underestimated the punch force when dies of small corner radii were used but gave an overestimate in the case of larger dies, which were the results observed here.

244 5. C o n c l u s i o n s

T h e conclusions drawn from the study into the effects of T i N - c o a t e d tools in the deep drawing of aluminium cups are as follows: (1) T h e combination of uncoat ed punches with coated dies resulted in the largest limiting draw ratios ( L D R s ) , while coated punches with uncoat ed dies yielded the poorest LDRs. (2) T h e t i t a n i u m nitride ( T I N ) coatings on the dies reduced the m a x i m u m p u n c h forces quite significantly. (3) T h e experimental m a x i m u m p u n c h forces agreed fairly well with theoretically predicted values. (4) T h e recommended tool combination which requires moderate punch forces and results in the best L D R is an u n c o a t e d p u n c h with profile radius 6 to 7 times the blank thickness and a coated die of corner radius 6 to 10 times the blank thickness.

References

1 J. Kumpulainen, A. Ranta-Eskola and R. Rintamaa, ASME J. Eng. Mater. Technol., 105 (1983) 119. 2 A. Younger, D. Spurgeon and D.E. Moore, Effect of material variables on the deep drawing of aluminium using smooth and roughenedpunches, Proc. Int. Conf. Met. Soc., London, 1983, pp. 90-94. 3 D. Spurgeon and A. Younger, The effect of punch surface roughness on the deep drawability of metals, Proc. 12th Biennial Congr. Int. Deep Drawing Research Group, Italy, 1982, pp. 141-157. 4 R. Pearce, Some effects of friction in punch-stretching, Proc. Int. Conf. Met. Soc., London, 1983, pp. 249-254. 5 C. Wick, Coatings improve tool life, increases productivity, Manuf. Eng., (December 1986) 26-31. 6 A. Korhonen, E. Sirvio and H. Sundquist, New ways of improving the surface properties of metal forming tools, Proc. 12th Biennial Congr. Int. Deep Drawing Research Group, Italy, 1982, pp. 175-182. 7 K.H.W.Seah and K.S. Lee, The effects of titanium nitride coatings on punches and dies in the deep drawing of cold-rolled mild steel, Int. J. Mach. Tools Manuf., 28(4) (1988) 339407. 8 J.L. Duncan and W. Johnson, Approximate analysis of loads in axisymmetric deep drawing, Proc. 9th Int. Machine Tool Design and Research Conf., Birmingham, September 1968, Pergamon, Oxford, 1969, pp. 308-318.