In Focus

Mould construction for the automotive sector has undergone profound changes in recent decades.

The combination of the automotive industry that demanded high quality printers, improved productivity, economic efficiency and reduced delivery times, resulted in an immediate search for innovative processing techniques, materials and production processes.

New materials have been made available by steel mills, new machine tools have appeared on the chip removal market, new tools have allowed the application of cutting speeds unthinkable even in the recent past.

All this has allowed printers to successfully respond to the appeal of the automobile industry, but as an old popular saying goes, the solution to a problem is always another problem.

We will briefly discuss the problems arising from the increase in the performance of high-speed mechanical processes, inspired by in-depth studies (2012-2013) in association with the Fraunhofer- Gesellschaft institute, the largest applied research organisation in Europe, and Materials Center Leoben Forschung GmbH (MCL).

CHIP FORMATION IN HIGH-SPEED MILLING

Sollecitazioni del materiale in diverse zone durante il taglio

Fig. 1: Material stresses in different areas during cutting

The mechanical machining processes for chip removal inevitably introduce structural changes to the surface of a piece: this phenomenon is strongly influenced and amplified by the synergistic and progressive variation of the cutting parameters.

This “superficial alteration” occurs due to the rapid development of a high thermal gradient and strong cortical plastic deformations, with consequent metallurgical transformation and possible chemical interactions between the cortical area of the piece and the surrounding environment.

The worked surface can have a very different structure compared to that of most of the material: this problem has been, and still is, the object of research and/or discussion in the field of so-called “superficial integrity”, a term that includes all aspects surface finish, white layers, metallurgical changes and residual stresses.

The white layers and the cortical plastic deformations, formed during the process, have strongly negative effects on the surface condition and especially on the fatigue resistance of the products, making the surface to become fragile, brittle/hardened, causing the permeation of micro-cracks and sometimes the “failure” of the product.

Fig. 1 illustrates the interaction between the various “regions” of the material being processed and the white layer formation mechanisms.

Region (I) represents the material in front of the cutting tool and below the machined surface. In this region the material is subject to plastic compression and heat propagated by the cutting areas. When the material enters region (II), where the friction of the side of the tool against the piece is significant, the heat (which can reach peak values higher than the melting point of the processed material) is generated and then carried into the piece and tool. In addition, the material is subjected to strong stresses of compression and tearing in this region.

When the cutting edge of the tool leaves the contact area and moves, several events occur in region (III):

In first place, there is the discharge of stresses due to the fact that there are no more forces acting on the surface; secondly there is the rapid cooling of the heated mass both by dispersion of heat in the environment and by dissipation by the cooling liquid, if used; finally there is a possible chemical reaction with the environment and/or the cooling lubricant.

Therefore, three general contributory mechanisms that are associated or responsible for the formation of white layers can be identified, these are:

  • The plastic deformation flow mechanism, that produces a cortical layer with a very fine grain structure,
  • The fast heating and tempering mechanism that translates into transformation products,
  • The mechanism of surface reaction with the environment such as nitriding and/or cementation and/or oxidation.

The term “white layer” originates from the fact that these surfaces appear white, “devoid of structure” at optical microscope observation or without details in a scanning electron microscope (SEM).

Thus, in literature, the term “white layer” is used as a generic phrase referring to very hard surface layers (in some cases micro-hardnesses higher than 1200 HV 0.001 have been detected) formed of ferrous materials in a variety of conditions, which appear white under a microscope.

RELATIONSHIP BETWEEN WHITE LAYER FORMATION AND TOOL WEAR

In the interactive dynamics of the process, it is inevitable that the cutting conditions progressively worsen, greatly amplifying the phenomena described in Fig. 1.

Industry studies compare the white layer and the wear on the side of the tool for various cutting speeds.

The thickness of the white layer increases progressively with the cutting speed and with the wear on the side of the tool.

As for the trend of the curves, the depth of the white level reaches an asymptotic maximum at a particular depth of cut (p), as shown in Fig. 2.

Fig. 2:  Thickness of the white layer as a function of side wear and cutting speed

The phenomenon described above is a very critical element even during heat treatment.

The “altered surfaces”, much harder than the base material, are fertile ground for the propagation of cracks and/or micro-cracks, with the possibility of spreading to the core of the material.

It is always best to provide post-finish stress-relieving heat treatment in order to attenuate the phenomena described above.

Fig. 3 shows, by way of example, the variation of residual stresses in the sub-cortical area for different types of retail steel, under different cutting conditions.

                            Fig. 3: Residual tensions in the sub-cortical area

Fig. 4 shows, by way of example, the variation of the micro-hardness in section in the sub-cortical area, for steel 1.2343, under different cutting conditions.

Fig. 4: Micro-hardness gradient in section in the sub-cortical area under different cutting conditions

SURFACE DAMAGES

Some metallography, with different magnification, showing the superficial damages as a consequence of mechanical machining (SEM scanning electron microscope images).

SURFACE DAMAGES SEEN IN SECTION

Some metallography in section, with different magnification, showing the superficial damages produced by mechanical machining (SEM scanning electron microscope images).

IMAGES IN OPTICAL MICROSCOPY

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