Tensile stresses in fine blanking tools and their relevance to tool fracture behavior

https://doi.org/10.1016/j.ijmachtools.2017.12.005Get rights and content

Highlights

  • Validated finite element model for fine blanking including cutting and tool retraction.

  • Localized tensile tool load components present during tool retraction.

  • Quantitative correlation between local loading and critical local defect size proposed.

  • Location of tensile loads and experimentally found origins of tool fracture coincide.

  • Surface quality requirement estimated as a function of friction coefficient.

Abstract

In order to enhance the productivity of the fine blanking process there is a tendency to apply high strength tool materials with superior resistance to wear and plastic deformation such as WC-Co hard metals. Due to the mean stress sensitivity of these high strength materials under fatigue loading conditions, there is a need to describe the tool load situation including previously unconsidered tensile tool load components and mean stress. To this end a finite element-based tool load analysis was performed that describes the cutting process and the tool's retraction from the remaining sheet material. The tool load was revealed to contain significant tensile components that occur during tool retraction. Their location on the lateral punch surface coincides with origins of tool fracture in two thirds of investigated fine blanking punches that failed close to the cutting edge in an industrial production scale fine blanking environment. The influence of friction between punch and sheet on the magnitude of the cyclic stress range and mean stress that act locally at the tool surface was investigated. As one of the main results, a first approximation for a surface quality requirement for fine blanking tools made of WC-Co hard metal was quantitatively estimated as a function of the friction coefficient based on linear elastic fracture mechanics.

Introduction

Fine blanking is a production process that provides high precision parts from metallic sheet material with smooth cut surfaces and excellent part flatness [1]. It differs from conventional blanking with punch and die plate by the application of a blank holder, a counterpunch and in some cases a v-ring indenter, see Fig. 1 [1].

Sheet material bending is minimal and sheet material deformation is highly localized in the die clearance [2] of about 0.5 % of the sheet thickness [1]. The small die clearance and the relatively high cut sheet thickness result in elevated levels of cyclic mechanical tool loads and in many cases in early tool failure.

Localized buildup of tensile residual stresses triggered by cyclic tool loads can be spatially correlated with the origins of tool failure close to cutting edges as demonstrated for blanking tools [3] and for milling tools in substrate [4] and hard coating [5]. It is therefore beneficial for tool life and consequently the economic competitiveness to avoid local plastic deformation in tools by application of high strength tool materials such as WC-Co hard metals. They exhibit roughly twice the yield strength of high-speed steels [6] and a higher wear resistance [7]. The failure behavior of high strength materials such as WC-Co hard metals is very sensitive to tensile load components under monotonous [8] and cyclic loading conditions [6]. Under cyclic loading conditions, the fatigue strength of WC-Co hard metals rises with increasing compressive mean stress, i.e. decreasing stress ratio R = σminmax [6].

In order to improve the lifetime of highly loaded fine blanking punches, WC-Co hard metals are under investigation as punch materials. In order to safely apply hard metals as tool materials, it is important to understand the complex tool load situation in the fine blanking process, e.g. by advanced finite element (FE) simulation-based tool load analysis [9]. In the past, many FE simulation studies were conducted to investigate the role of various process parameters such as die clearance [10], punch cutting edge radius [11] or coefficient of friction [12] on the peak cutting stress Smax. Smax is obtained by normalizing the punch force by the punch circumference and the thickness of the sheet. However, Smax always occurs during the cutting stage of the entire process when the punch is exposed to high levels of hydrostatic pressure and only compressive stresses occur in the whole punch. As previously suggested [12], Smax depends linearly on the material strength, cutting radius and thickness of the cut sheet with a constant frictional force component. Once the cut is completed, only the frictional component between punch and sheet remains. A virgin fine blanking punch shows a smooth surface, which provides a low friction coefficient and therefore low frictional forces. While the punch is in service, iron adhesions form on the lateral punch surface, as seen in Fig. 2. The kinetics of material transfer to the tool, also referred to as galling, depend on e.g. surface roughness [13], number of strokes, applied load or hard coating type [14]. The iron adhesions stem from the cut sheet, get ripped out of the sheet and stick on the punch because of the high contact pressure and shear stress in the contact region during cutting. Galling does increase the friction coefficient acting between tool and workpiece, which typically ranges from 0.15 to 0.8 for coated surfaces [15].

In the current work, a complete tool loading cycle of fine blanking punches including the unloading cycle with punch pull-out of the cut sheet was investigated for the first time. Special attention was paid to tensile tool load components and local stress ratio. A minimal size of defects able to grow under fatigue loading conditions was estimated quantitatively for the tool surface via implementing a threshold for fatigue crack propagation for WC-Co hard metals. The locations of the minima of this defect size were compared to the locations of the origins of fracture in tools from fine blanking experiments in an industrial scale environment.

Section snippets

Simulation setup and experimental FE model validation

The local loading in an axisymmetric fine blanking tool was investigated in this work. The commercial FE package ABAQUS version 6.12–1 [16] and the element type CAX4R were used for a static implicit 2D simulation. Elastic-plastic material behavior with kinematic hardening was assigned based on experimentally obtained stress-strain curves. Those curves were obtained from uniaxial tensile tests with ferritic S550 steel for the sheet material and from uniaxial compression tests with HSS steel for

Local tool loading and critical defect radius for cyclic crack propagation acc

Fig. 6a) shows the maximum and minimum principal stresses for the entire fine blanking cycle including cutting and tool retraction from the cut steel sheet over the path defined in Fig. 5.

The highest absolute values for maximum and minimum principal stress that occur during the entire duration of the cutting and retraction operation are observed close to the cutting edge in zone three, which corresponds to the lateral punch surface close to the cutting edge. Minimum principal stress values

Conclusion

An experimentally validated FE simulation model of the production process of fine blanking based on experimentally obtained input data revealed a region of tensile tool load components present during tool retraction from the cut steel sheet. Tensile tool load maxima occurred on the lateral side of the punch close to the cutting edge with their magnitudes rising with increasing friction coefficient between punch and workpiece. A fracture mechanically based procedure was proposed to

Acknowledgement

Financial support by the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie and Bundesministerium für Wissenschaft, Forschung und Wirtschaft) represented by Österreichische Forschungsförderungsgesellschaft mbH (837900) and the Styrian and the Tyrolean Provincial Government (1000032317), represented by Steirische Wirtschaftsförderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Programme is gratefully

References (25)

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