Dr.-Ing. Malte Stonis

Doctoral thesis

Defects like folds can arise using forging for the production of long flat pieces made of aluminium. A special defect is the formation of inner folds. These can be seen in the grain flow. Inner folds have a negative effect on the dynamic properties of the forged part. As a production process, forging can be divided into single-directional and multi-directional forging. The formation of inner folds was observed at the single-directional forging. By using the multi-directional forging, a forming operation working from different directions, the forming can be set variably. Thus the development of folds can be prevented. A newly developed method can help in the selection of the forming process and in determining an appropriate tool geometry. Here especially the area is adapted, where the development of inner folds occur. Therefore a calculation model was developed. It integrates a computer-aided identification of the inner folds. Using this model, a correction of the parametrically constructed forging tool is possible.

multidirectional-forging, long flat pieces, aluminium, fibre orientation

Publications

The most common bulk forming process is closed die forging with flash. One goal of the industry is to reduce flash. For geometrically difficult parts like crankshafts flash reduction can be achieved by flashless preforming and flash-reduced final forging. The corresponding process design is challenging and defects like an insufficient cavity filling often occur in final forging. A controlled, moveable flash gap enables the alteration of the material flow, increasing the filling of the cavity again. In this paper, the flashless preforming for crankshafts and the influence of a controlled flash gap on cavity filling are described.

Forging, die design, material flow, FEA, flash land

In common forging processes for geometrically complicated parts such as crankshafts, an excess on material (flash) is technically needed to produce a good part, which results often in a material utilization between 60 % and 80 %. But the material costs in forging represent up to 50 % of the total production costs. By decreasing the flash ratio, the material usage and production costs in forging operations can be reduced significantly. For a crankshaft, the development of a new forging sequence was necessary, to achieve the reduction of flash. This development was performed for an industrial two-cylinder crankshaft, based on finite element analysis (FEA) simulations. The new forging sequence consists of three flashless preforming operations, an induction reheating followed by a multidirectional forging and the final forging. By use of this forging sequence the flash ratio was reduced from about 54 % to less than 10 %. Due to the huge reduction of the flash ratio, material as well as energy can be saved from now on, thus increasing the competitiveness of the company.

hot forging, FEA simulation, reduction of flash, multidirectional forging, resource efficiency

The more complicated a forging geometry is, the more flash is necessary to achieve a form filling and a part free of defects. Most small and medium sized enterprises (SME) forge many different parts in small and medium batch sizes and cannot afford the high effort to design more efficient forging processes. In the paper the development of a resource efficient forging process chain for crankshafts is summarized. The forging sequence consists of flashless preforming steps and a flash reduced final forging. The tools were designed to work on industrially used fast moving mechanical presses. The last of the four flashless preforming steps is a multidirectional forming of the crank webs and a pin offset. To keep the forging forces on a low level and enable a stable forging process, an induction reheating of the preform before multidirectional forging was designed. The crankshaft was successfully forged with a reduced flash ratio of less than 10 %.

forging, flash, induction heating, preform, crankshaft

Pistons for combustion engines are usually made of aluminum. But increasing requirements on efficiency and performance can be met by use of steel pistons that will probably spread in the automotive industry in the next years. The pistons are forged and an expensive machining process is necessary to finish them. In the usually unidirectional forging process it is not possible to pre-forge some areas, such as the pin bores as they represent an undercut. By the help of a multidirectional forging operation it is possible to forge undercuts. This process is distinguished by a pre-forging of the pin bores and an improved material usage. Furthermore, the following machining operation will be simpler due to an easier positioning of the part. Currently, the forging tools are under development. Once they are finished they will be tested in an industrial environment on an eccentric press. The tools and parts will be analyzed concerning the quality of the parts, the die wear and the economic efficiency of this new process.

piston, steel, forging, pin bore, undercut, multidirectional

In this paper the comparison of simulations of cross wedge rolling processes with real trials using flat cross wedge tools is presented. The investigated materials are titanium and bainitic grade steel. First simulations were used to find the suitable parameter combinations for the investigated materials. Afterwards tools were manufactured with these parameters and additionally with some variations to investigate a field of parameters around this range of parameter values. The purpose of these tests is to find geometrical and process parameters with which a stable cross wedge rolling process for bainitic grade steel and titanium is possible.

cross wedge rolling, bainitic grade steel, titanium, finite element simulations

Reducing die wear is an effective way to decrease costs within bulk forming processes. Therefore, specific tool materials and heat treatments as well as special coatings are used to prolong the lifetime of the tools. Diamond-like carbon (DLC) coatings show high hardness and superior frictional behavior. However, these coatings seem to be inappropriate for hot forming due to degradation processes at elevated temperatures. But for warm forming, due to the lower temperature input into the cavity DLC might be an appropriate coating. Friction influences the shear stresses on the cavity surface and is therefore an important factor for reducing die wear. Hence, the frictional behavior of DLC coatings within warm forming will be analyzed within this paper by using the ring compression test. An amorphous hydrogenated carbon coating and six metallic doped amorphous hydrogenated carbon coatings (Cr, V and W each in two variants) are compared to CrN and no coating. Firstly, nomograms are graphed by the use of Finite-Element-Analysis. Thereafter two test series are carried out varying forming temperature and lubrication. The results show that DLC coatings with and without metallic doping are able to reduce friction in warm forming. Within the investigations an amorphous hydrogenated carbon doped with 15 % chromium shows the lowest friction factor and is able to reduce the friction factor compared to no coating by up to 64 % within warm forming.

diamond-like carbon (DLC), friction, warm forming, bulk forming, ring compression test

The challenge in developing a hydroforming process for Tailored Hybrid Tubes is to provide feasible forming conditions for both the steel and the aluminum part of the semi-finished hybrid material. The aim of these investigations is to develop one-stage tube hydroforming processes for steel-aluminum-joints. Therefore, the effects on the materials’ forming behavior, especially the flow stress, of the parameters alloy, thickness, forming rate and seam design are being investigated. Forming tests were carried out on sheet metal specimens of joined steel and aluminum to investigate the impact of the varied parameters and to analyze the joint area’s forming behavior. Based on the results of the forming tests an FEA-simulation will be set up in order to design a hydroforming process for Tailored Hybrid Tubes of laser joined steel and aluminum. Eventually the hydroforming process of Tailored Hybrid Tubes shall be executed for a sample part.

hydroforming, automotive, laser brazing, laser soldering, steel-aluminum, car body, engine downsizin

Hot forging dies are subjected to high mechanical, thermal and chemical loads. These loads can lead to die failure and therefore are one of the main cost factors in forging industry. The billet temperature is the process parameter mainly influencing the loads in forging processes. In warm forging the billet temperature is much lower than in hot forging. Warm forging becomes more and more important due to the advantages against hot and cold forging. This paper describes an analysis of FE-simulations of the change in loads on forging dies and the effect on the die life time by changing from hot to warm forging processes. Furthermore a method is presented which enables a calculation of thermal stresses in forging processes.

warm forging, hot forging, thermal stresses, tool wear

Using hybrid forging techniques, both bulk or sheet metal can be formed and joined in a single step, rendering subsequent joining processes unnecessary. Moreover, using sheet metal rather than bulk material means that material-savings can be made and/or more economical forming units can be used.

hybrid, forging, sheet metal forming, joining technology, steel, adhesive bond

Intensive research work on flashless precision forging of steel materials has been made at the IPH in the past. In aluminum forging processes a thin flash often occurs in the gap areas between the forming elements. This thin flash adversely affects the component quality. For this reason flashless aluminum forging previously could not be realized. In this publication, the approach for the investigation of the thin flash formation is explained. A FEM model of a one-stage upsetting process in a closed die has been created. With this model the influence of the parameters workpiece temperature, forming velocity, the wideness of the flash gap and the pressing forces on the thin flash formation can be examined. Owing to these research activities a possibility for aluminum forging without flash and thin flash shall be provided.

flashless, forging, thin flash, aluminum, FEA

Preforming is an essential step in flashless forging processes. This paper describes the development of a four stage process chain for flashless forging of a crankshaft with pin and flange. The process consists of cross wedge rolling, lateral extrusion, bi-directional forging and final forming. The finite-element-analysis (FEA) performed with the software Forge 3 and experimental tests are executed with different process parameters, like billet and tool temperature, rolling velocity and steel. To reduce process steps, like lateral extrusion, a direct combination of cross wedge rolling and bidirectional forging is analysed with FEA-software Forge 3 for a one cylinder crankshaft without pin and flange. The results of the FEA give suitable forming angles alpha for cross wedge rolling and several geometric parameters for a modification of the bi-directional tool.

cross wedge rolling, bi-directional forging, crankshaft forging without flash, preforming

The production of titanium parts in small batches using a flexible machine and a fast layout method as well as a temperature and defect measurement were not considered in investigations yet. However the industry using valuable materials showed interest in this technology and the CWR module in order to realize the advantages of the CWR technology for small and medium batch sizes. Therefore a CWR machine for flat wedge tools will be developed in the EU project CoVaForm. The wedge tools used in this project will be designed for rolling titanium and micro-alloyed steel. To help SMEs a software tool will be developed for designing CWR processes. Especially for rolling valuable materials like titanium, the boundary conditions are mostly unknown; therefore pre-tests will be done to get process parameters which will enable a working CWR process. To ensure a defect-free production, a quality control will be developed based on thermography which detects defective parts directly during the rolling operation.

cross wedge rolling, CWR machine, titanium, bainitic grade steel, common rail, hip implant

The production of titanium parts in small batches using a flexible machine and a fast layout method as well as a temperature and defect measurement were not considered in investigations yet. However the industry using valuable materials showed interest in this technology and the CWR module in order to realize the advantages of the CWR technology for small and medium batch sizes. Therefore a CWR machine for flat wedge tools will be developed in the EU project CoVaForm. The wedge tools used in this project will be designed for rolling titanium and micro-alloyed steel. To help SMEs a software tool will be developed for designing CWR processes. Especially for rolling valuable materials like titanium, the boundary conditions are mostly unknown; therefore pre-tests will be done to get process parameters which will enable a working CWR process. To ensure a fault-free production, a quality control will be developed based on thermography which detects faulty parts directly during the rolling operation.

cross wedge rolling, CWR machine, titanium, bainitic grade steel, common rail, hip implant

By increasing the height of wind turbine generators the usable wind energy increases exponentially. However, tower weight increases accordingly. Current tower designs limit the tower height due to the explained correlation. The application of lightweight design concepts can lead to a significant weight reduction at comparable tower stiffness. As part of a research project, lightweight design concepts have been developed. These design concepts allow a mass reduction for onshore wind turbine towers up to 20 %. In this paper, the development and results regarding these lightweight concepts are presented.

xxl-product, large-scale, xxl, wind turbine, wind power, lightweight construction, tower constructio

Schedule variances in the cycle of field assembly often cause high unplanned costs. This is particularly true for capital-intensive, large-scale products. Currently no model exists which supports cost determination already during planning, implementation or after assembly. Thus, comprehensive logistics controlling is not possible which prevents high schedule adherence and low logistics costs even on the occurrence of disruptions. In this paper, a research project is presented, which has the goal of developing a method by which a deadline- and cost controlling of the material availability for the assembly of wind turbines can be secured.

material provisioning, site assembly, cost controlling, schedule deviation, early warning system, fo

Hydoroforming of titanium tubes was investigated by the Institut für Integrierte Produktion Hannover (IPH) and the Institute of Metal Forming Siegen. The basic research for designing a multi-stage process chain of hollow profiles made of titanium grade 2 was divided into three sub-items, the material model, the annealing treatment and friction value determination. The results were brought together in a simulation model, which enabled developing a multistage process chain.

hydroforming, FEA, titanium

Different billet dimensions or progressive die wear increase the geometric fluctuations of forged parts. This can lead to waste. Mostly this geometric fluctuations are compensated by increased billet masses which leads to lower efficiency in the manufacturing process. In a research project a controlled, moveable flash gap was examined, that enables the alteration of the material flow to increase the part quality.

forging, die design, material flow, FEA, flash land

In this study two approaches in automotive lightweight construction have been combined: weight reduction based on material and based on structural design. Steel and aluminum are used for the first approach, and hydroforming is the method for design optimization by shaping. Steel and aluminum show different forming behaviors due to their characteristic yield stresses. The challenge in developing a hydroforming process for THTs is to provide feasible forming conditions for both the steel and the aluminum part. Forming tests were carried out on sheet metal specimens of laser brazed and laser soldered steel and aluminum to investigate the impact of specific parameters and to analyze the joint area’s forming behavior. An FEA simulation was set up in order to design a tube hydroforming process for THTs of laser joined steel and aluminum. Eventually, the hydroforming process for THTs will be designed and executed.

hydroforming, automotive, laser brazing, laser soldering, steel-aluminum, car body, engine downsizin

The project REFoCh is about resource efficient forging of complex high duty parts, e.g. crankshafts. Within this project founded by the European Union, a new process chain for a crankshaft will be developed with the aim of reducing the flash quota significantly. In the conventional forging sequence, 54 % flash occur, the aimed flash quota is 15 %. To reach this objective, a multidirectional forging step will be implemented in the new process chain in order to reduce the material significantly.

flash reduced, multidirerectional forging, cranskshafts

The material costs represent up to 50 % of the total production costs. By decreasing the flash ratio the material usage and production costs in forging operations can be reduced. For complicated forging parts the development of a new forging sequence is necessary to achieve a flash reduced forging sequence. This development was performed for a two-cylinder crankshaft. The new flash reduced forging sequence is using flashless preforming operations, an induction reheating of the preformed complicated work piece and a flash reduced final forging. With the reduction of the flash ratio from 54 % down to 10 % the total energy consumption was reduced and the competitive capacity of forging SME’s is increasing.

flash reduced, process chain, crankshaft, induction heating

Job offers

  • Industry 4.0, Artificial Intelligence

    Student job, Master thesis, Bachelor thesis, Project thesis, Internship

    Industry Dating Platform