A bit of heat is always good – and it’s with this in mind that Feintool Technologie AG has joined forces with ETH Zurich to launch a development project supported by the CTI (Swiss Commission for Technology and Innovation). The primary focus here is on studies examining the temperature-dependent forming behavior of various materials, as well as the development of different process solutions for heating the pre-material.
The growing demand for lightweight, energy-efficient vehicles is leading to a global shift in the manufacturing industry. Thanks to their favorable weight to strength ratio, materials like titanium alloys and high-strength steels are increasingly representing an economically viable option, but processing these complex materials nevertheless requires further development of existing manufacturing processes, or indeed the application of innovative production methods. While fineblanking technology does permit highly economical production of a broad range of automotive parts, the current state of technology means the selection of usable materials is limited, since the process demands materials with high ductility and good flow properties.
Aim of the development project
The aim of the project is to expand the range of usable materials with the help of targeted utilization of temperature effects. Hence, for example, higher-strength steel materials or titanium alloys could potentially be processed economically by means of fineblanking, which could lead to broad expansion of the area of application for fineblanking technology.
The positive effect of increased temperature in relation to the formability of higher-strength materials is already known from a number of forming processes. Nevertheless, this correlation is not always clear and setting the optimum temperature range is often far from trivial.
When it comes to fineblanking technologies, both the strength of the material to be cut as well as the type of material and/or its microstructure represent a technological limitation. The aim of the development is to utilize thermal influences affecting the forming and fracture behavior of the material to shift the processing boundaries and tap into new areas of application. As part of the project, both the technological foundations for optimum process interpretation and pilot tools are to be developed and tested.
The planned process is also particularly challenging in relation to the plant and processing technology. Hence, for example, the tribological -system for the increased temperature range has to be reestablished and optimized.
In line with the various areas of application and the temperature effects, the project has been broken down into three different sub-areas.
Studies in the sub-area of non-alloy and alloyed carbon steels
Today the high demands placed on the fineblanked surfaces of the fineblanked component mean steels with special structural properties are often required. It’s therefore often necessary to have a carbide spheroidization level of significantly more than 90%, which can only be achieved with expensive thermal and mechanical processes (annealing and cold rolling). The effect of temperature may make it possible for fineblanked surfaces to be realized in equal or similar quality using materials that are less labor-intensive to produce and have lower carbide spheroidization levels.
Around 60% of processed steel is of a quality that requires laborious treatment with a carbide spheroidization level of more than 90%, which makes it considerably more expensive to produce than steels with lower carbide forming levels. Since the proportion of the material amounts to around 60% of the price of the part, there are big savings to be made here, and fineblanking processes could be made much more competitive.
Results from preliminary studies have shown that temperatures of around 300ºC result in both a fundamental improvement in the fineblanked surface quality and a reduction of up to ten percent of the die-roll. In addition, the cutting force, and thus the load applied on the tools, could be reduced by around one third.
Findings in the sub-area of rust free steel
When it comes to fineblanking of austenitic, stainless steel, particularly in the widespread quality of AISI 304 (V2A), forming-induced martensite may be generated in thicker sheets (from approx. 4mm). The sudden impact triggered by this effect means it can have a very negative influence on the tool operating life and the fineblanked surface quality of the part. Initial pre-trials with thermo-fineblanking have shown that even with temperatures from 120°C this effect can be reduced considerably or even suppressed completely.
Challenge in the sub-area of titanium
These days, fineblanking products are used largely in the automotive sector, but fineblanking can also be applied in the medical sphere. There are challenges here, however, since the frequently processed material titanium exhibits very poor fineblanking properties at room temperature. This means the clean-cut portion is often too low and the parts often have to be machined, which can be economically disadvantageous. The research team hoped to achieve better results through the effect of temperature here. Initial pre-trials have shown that for titanium the temperatures need to be significantly higher than 300°C.
All the studies are carried out using various different geometries. These include “trial geometry” and “real geometry”, among others.
Current results with the “trial geometry”
In order to test the positive influence of increased temperature on various materials, cutting experiments with variable clearance and different temperatures were carried out.
On the sample part (see figure below), the opposing sides have the same clearance. This means that for each experiment four different clearances (0.00mm-0.09mm) can be examined for each predetermined temperature. Greater clearances tend to have a positive effect on the tool-wear and tear, but as is well-known they reduce fineblanked surface Quality.
The first trial with 42CrMo4 hot-rolled strip with around 50% carbide spheroidization shows a clear reduction in the blanking tonnage with increasing temperature (up to -30% at 320°C). The evaluation of the die-roll also shows a significant improvement of approximately 13% at a temperature of 320°C.
In addition, there appeared to be a significant reduction in tear formation as well as tear size with increased temperatures (see figures below). This material may not be extremely critical with regard to fineblanking technology, but at high temperatures a satisfactory fineblanked surface quality could be achieved even with very large clearance of 0.05mm.
An unstable quality of the austenitic, stainless steel X5CrNi18-10 (1.4301, 304, V2A) was also examined. Here, it was possible to identify a significant reduction in the build-up of martensite, even at temperatures starting from 120°C, and with it a reduction in the sudden impact, which naturally also results in a positive influence on the fineblanked surface quality.
As expected, the high temperatures have a negative effect on the tribology. One of the most important factors in the “fineblanking tribo-system” is the lubricant. Most of the lubricants used in fineblanking are mineral-oil-based and therefore can be used only at temperatures up to approx. 200°C.
Developments with regard to temperature-resistant lubricants are ongoing with selected development partners of Feintool.
Current status of studies with “real geometry”
The fineblanking suitability of a high-strength material is being examined using the geometry of a real application. This is a hardened and tempered C55E-QT with hardness of around 42 HRC, which corresponds to tensile strength of approximately 1300 MPa. Achieving sufficient softening here and at the same time avoiding blue brittleness and a loss of hardness through the annealing effect demands a very tight process window and precise control over temperature management.
Preliminary studies in the laboratory using swage trials at various temperatures have shown that sufficient softening can occur only at temperatures from around 450°C.
By means of structural studies, it was identified that there is a change in the structure at temperatures from around 500°C. The change here is a release in the martensite, which leads to a loss of hardness. The cutting trials with the C55E-QT therefore have to take place at temperatures between 450°C and 500°C.
Initial cutting trials with real geometry C55E-QT
For the first trials, blanks were heated to around 550°C and then inserted into the tool. Calculations showed that the blanks cooled by a good 50°C to around 500°C by the time cutting began. A high-temperature lubricant developed by one of Feintool’s partners was used for lubrication.
The welcome result: The fineblanked surface shows a clean-cut portion of 100%, and the die-roll is minimal.
The initial results of the research project are promising. Further studies are being carried out and the evaluations will follow in another blog entry.