Temperature measurement with thin film sensors during warm forging of steel

Warm forged components have better surface properties and higher dimensional accuracy than hot forged components. Diamond-like-carbon (DLC) coatings can be used as wear protection coatings, which are anti-adhesive and extremely hard (up to 3500 HV), to increase tool service life. In the first funding period of the research project at the IPH – Institut für Integrierte Produktion Hannover gGmbH and the Institute for Surface Technology (IOT) of the Technical University of Braunschweig in cooperation with the Fraunhofer Institute for Surface Engineering and Thin Films (IST), the influence of different coating types and process temperatures on tool wear was investigated. The result is, that DLC coatings can reduce tool wear in some cases significantly, but that their service life is strongly dependent on the temperature. Coating-integrated temperature measurement could not be realised at that point, due to adhesion challenges. During the second funding period, the effect of multilayer DLC coatings on tool wear was investigated. Also, an additional method of the temperature measurement on the engraving surface using thin film sensors was developed in order to correlate the local process temperature and local layer wear. In this work, the development of and the results gathered by the thin film temperature sensors are presented, which enable for more accurate temperature measurements than commonly used thermocouples. Their functionality and durability under high loads were investigated and showed to be promising.

Analysis of the Modification of Tool Surfaces by Abrasive Blasting and Laser Polishing

The surface treatment of tools plays an important role for the operational behaviour of forming processes. Up to now, industrial standard is the manual finishing of tool surfaces, which can lead to a varying quality of the surface finish and therefore influence the tool service life and the forming results. One method to perform the polishing operation automatically is to remelt the top layer of materials by laser polishing. This is accompanied by a considerable change in the material properties in this zone. Therefore, the effect of laser polishing with respect to the local modification of the tool surface is investigated in this study. The results of the investigations reveal that a precise adjustment of the laser parameters is required in order to reduce the roughness of the surface. The heat input also leads to a significant influence on the microstructure of the material. In this study laser polishing remelts the material up to a depth of approximately 20 µm. Furthermore, it can be observed that the heat input during the process results in a heat affected zone of up to a depth of 30 µm. As a contrast to laser polishing, abrasive blasting is investigated as a roughness increasing surface modification.

Effect of the Forming Zone Length on Helical Rolling Processes for Manufacturing Steel Balls

This paper begins with a brief overview of the methods for producing balls. It then discusses the rolling processes for producing balls in helical passes. Next, a method for designing tools for helical rolling (HR) is described. Six different cases of rolling using tools with helical passes of different lengths are modeled by the finite element method (FEM). The simulations are performed with the use of Simufact Forming version 13.3. Based on the 3D simulations, the distributions of effective strain, damage criterion, and temperature, as well as the variations in loads and torques, are determined. This study also predicts the rate and manner of wear of the helical tools, depending on the tool design. As a result, it has been found that an increased length of the helical forming passes is advantageous in terms of tool service life. It has also been found that excessive elongation of the forming zone is not cost-effective.

Experimental Study of Cutting Performance for Inconel 718 Milling by Various Assisted Machining Techniques

The five stage experiments including without assistance, single and hybrid assisted machining systems on Inconel 718 milling were conducted in this study. First of all, the milling experiment without assistance was performed to investigate the variations of cutting performance and the results were used for a suitable process parameter planning in the subsequent stage experiments. Next, a laser assisted system was introduced in the second stage where the spacing distance between the laser spot and cutting-tool along the cutting direction was modified to test whether laser preheating may effectively reduce the cutting force. A biaxial ultrasonically assisted system with only one-axis oscillation (x or y direction) and with two-axis simultaneous oscillations (x and y directions) were subsequently introduced at the third to fourth stage experiments, respectively. While a biaxial ultrasonically and the laser assisted systems were integrated together to construct a hybrid assisted cutting system at the last stage experiment. Under these assistances, milling experiments of Inconel 718 by cutting-tool of tungsten carbide with nanoSi® coating were conducted. And the full-factorial experiments of process parameter combinations such as spindle speed, radial cutting depth and feed rate were planned. The results indicated that the laser-preheating assisted system could effectively reduce the cutting force as well as enhance the cutting performance. The effect of the biaxial ultrasonic oscillation on tool service life could greatly be promoted. Furthermore, the cutting performance exhibited in the integrated hybrid assisted milling prevails over that in milling without assistance as well as with each single assisted system. Under this hybrid assisted milling, the better surface roughness of 0.216μm was obtained under a combination of spindle speed of 6000 rpm, radial cutting depth of 0.01 mm, and feed rate of 300mm/min, accompanied by a maximum cutting-tool wear of 13.849μm.

Generation of Tool-Life-Prolonging and Chatter-Free Efficient Toolpath for Five-Axis Milling of Freeform Surfaces

Short tool service life is always a major concern when milling hard materials, such as Ni-based superalloy. In the current research of tool life optimization in multi-axis machining of freeform surfaces, the focus is mostly on choosing suitable cutting parameters and better application of coolant. In this paper, aiming at averaging the tool wear on the entire cutting edge and hence prolonging the tool service life, we report a study on how to generate a multilayer toolpath with a varying tool lead angle for multi-axis milling of an arbitrary freeform surface from an initial raw stock. The generated toolpath is guaranteed to be free of chatter, which is well known for its detrimental effect on the cutting edge. In this study, we first experimentally construct the chatter stability lobe diagram, which reveals the relationship between the lead angle and the cutting depth. With the chatter stability lobe diagram as the major constraint, we then generate the machining toolpath by selecting a proper pair of the best lead angle and cutting depth along the toolpath. While the proposed algorithm currently is restricted to the iso-planar type of toolpath, it can be adapted to other types of milling. The physical cutting experiments performed by us have convincingly confirmed the advantage of the proposed machining strategy as compared to the conventional constant lead angle and constant cutting depth strategy—in our tests the maximum wear on the cutting edge is reduced by as much as 39%.

Die Failures in Copper Hot Extrusion

Low life service of steel dies used in copper extrusion brought us to inspect several discarded copper hot extrusion dies in order to identify failures and failure mechanisms. The investigations performed are mainly hardness tests and microscopic investigations. The profiles extruded are circular, rectangular and hexagonal. It was observed that die design is not at fault; rather the die material is to blame for low life service of the tool. Discarded extrusion dies after copper hot extrusions were investigated. The failure of the dies was first observed by naked eye and the observations recorded. From the dies the ones with most plastic deformations and obvious signs of failure were selected. An improvement in die design could expand the service life, but our main concern is the material: a balance between toughness and hot strength is critical, yet this balance is hard to achieve. Other thermal processing of the die is still in question and investigations are currently being performed. At least an increase of tool service life could be achieved by thermal processing of the material to a state prior nanometric carbide precipitation between martensitic plates.