Carsten Siemers

A finite element model of high speed metal cutting with adiabatic shearing

A finite element model of a two-dimensional orthogonal cutting process is developed. The simulation uses standard finite element software together with a special mesh generator that is able to mesh the chip completely with regular quadrilateral elements and a strong mesh refinement in the shear zone for continuous and segmented chips. The techniques of remeshing and to ensure convergence of the implicit calculation is described. Results for the formation of segmented chips are presented and the segmentation process is studied. Of special interest is the occurrence of split shear bands. The influence of the elastic properties and of the cutting speed is also discussed.

The influence of thermal conductivity on segmented chip formation

A two-dimensional finite element model of the machining process is presented. After a short discussion of the modelling technique and the remeshing algorithm used, the influence of thermal conductivity on the chip segmentation process is studied. Increasing thermal conductivity leads to a decreasing degree of segmentation and to an increase in the cutting force. The influence of the thermal conductivity on the width of the shear bands and on maximum temperatures is also discussed.

Cytocompatibility of a free machining titanium alloy containing lanthanum

Titanium alloys like Ti6Al4V are widely used in medical engineering. However, the mechanical and chemical properties of titanium alloys lead to poor machinability, resulting in high production costs of medical products. To improve the machinability of Ti6Al4V, 0.9% of the rare earth element lanthanum (La) was added. The microstructure, the mechanical, and the corrosion properties were determined. Lanthanum containing alloys exhibited discrete particles of cubic lanthanum. The mechanical properties and corrosion resistance were slightly decreased but are still sufficient for many applications in the field of medical engineering. In vitro experiments with mouse macrophages (RAW 264.7) and human bone-derived cells (MG-63, HBDC) were performed and revealed that macrophages showed a dose response below and above a LaCl3 concentration of 200 microM, while MG-63 and HBDC tolerated three times higher concentrations without reduction of viability. The viability of cells cultured on disks of the materials showed no differences between the reference and the lanthanum containing alloy. We therefore propose that lanthanum containing alloy appears to be a good alternative for biomedical applications, where machining of parts is necessary.

Shear melting and high temperature embrittlement: theory and application to machining titanium

We describe a dynamical phase transition occurring within a shear band at high temperature and under extremely high shear rates. With increasing temperature, dislocation deformation and grain boundary sliding are supplanted by amorphization in a highly localized nanoscale band, which allows for massive strain and fracture. The mechanism is similar to shear melting and leads to liquid metal embrittlement at high temperature. From simulation, we find that the necessary conditions are lack of dislocation slip systems, low thermal conduction, and temperature near the melting point. The first two are exhibited by bcc titanium alloys, and we show that the final one can be achieved experimentally by adding low-melting-point elements: specifically, we use insoluble rare earth metals (REMs). Under high shear, the REM becomes mixed with the titanium, lowering the melting point within the shear band and triggering the shear-melting transition. This in turn generates heat which remains localized in the shear band due to poor heat conduction. The material fractures along the shear band. We show how to utilize this transition in the creation of new titanium-based alloys with improved machinability.

The measurement of internal strain in core–shell Ni3Si(Al)–SiOx nanoparticles

Internal defects and strain in nanoparticles can influence their properties and therefore measuring these values is relevant. Powder diffraction techniques (neutron and synchrotron) are successfully used to characterize internal strain in the core-shell Ni(3)Si(Al)-SiO(x) nanoparticles having mean diameters of approximately 80 nm. The nanoparticles, which are strain-free after extraction from the bulk alloys, develop internal strain on heating. Both micro- and macro-strains can be measured from the analysis of Bragg peak shift and broadening. It is identified that differences in thermal expansion coefficient of the metallic core and the amorphous shell of the nanoparticles, as well as partial disordering of the L1(2) ordered core phase, are the main causes of strain evolution. Synchrotron measurements also detected partial crystallization of the amorphous silica shell.

Development of a Free-Machining (α + β) Titanium Alloy Based on Ti-6Al-2Sn-4Zr-6Mo

Titanium alloys are widely used in the aerospace and power generation industries due to their high specific strength and corrosion resistance. Titanium alloys like the modern (α +β) titanium alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) are difficult to machine due to the formation of long chips which hinders automated manufacturing. In the present study, lanthanum has been added to Ti-6246 alloy to improve its machinability, i.e., to reduce the chips’ length. As lanthanum and Tin form La5Sn3 intermetallic phase, the 2% of Tin had to be replaced by 3% of zirconium. The matrix of Ti-6Al-7Zr-6Mo with lanthanum contains pure metallic lanthanum precipitates which have a relatively low melting point compared to titanium. Besides the standard alloy Ti-6246 two modified alloys, namely, Ti-6Al-7Zr-6Mo-0.9La and Ti-6Al-7Zr-6Mo-0.5La were investigated. During machining of these new free-machining alloys, short and strongly segmented chips are observed enabling automation of machining operations. This can be explained by softening of lanthanum particles during segmented chip formation. The microstructure, phase composition, and deformability of the new free-machining alloys were analyzed after different thermomechanical treatments. In addition, the mechanical properties of the modified alloys are investigated which are similar compared to the standard alloy.

Development of Advanced and Free-Machining Titanium Alloys by Micrometer-Size Particle Distribution

The chip formation process of four different titanium alloys has been studied in several cutting experiments. Alloys containing more than 50% of a-phase at room temperature and aged metastable b-alloys form segmented chips independent of the cutting conditions. Solution treated metastable b-alloys show a cutting parameter dependent change from continuous to segmented chip formation. Lanthanum has been added to all four alloys. The microstructure of these alloys consists of a titanium matrix and micrometer-size particles. The presence of grain boundary particles leads to enhanced grain stability at elevated temperatures. In addition, short chips are observed during metal cutting only in case pure metallic rare-earth metal particles are present.

Chip Formation and Machinability of Nickel-Base Superalloys

Nickel-base superalloys like Alloy 625 are widely used in power generation applications and in the oil and gas industry due to their unique properties especially at elevated temperatures. The chip formation process of Alloy 625 is not yet well understood. Therefore, the cutting process of this alloy has been studied in detail by means of orthogonal cutting experiments at conventional cutting speeds and in the high-speed cutting regime. Alloy 625 shows a cutting parameter dependent change in the chip formation process from continuous to segmented chips. Silver has been added to Alloy 625 to improve the machinability. During machining of these modified alloys short breaking chips develop so that cutting processes are eased and can be automated.

Structure and Anelasticity of Fe-Ge Alloys

Several ternary Fe – Ge - C alloys with Ge contents ranging between 3 and 27 at. % have been studied. The structure, anelastic, thermodynamic and kinetic phenomena in Fe - 3, - 12, - 19/21 and – 27 Ge have been examined by X-ray diffraction (XRD), heat flow (DSC), vibrating sample magnetometry (VSM), optical-light and scanning electron microscopy, and internal friction (IF) methods. The Fe - 3Ge and Fe - 12Ge alloys form b.c.c. solid solutions. A Snoek-type internal friction (P1) peak is recorded in the Fe - 3Ge alloy with parameters similar to those for α-Fe: Н = 0.86 eV, Δ = 0.015, β = 0.72 and τ0 = 2 × 10-15 s, showing that Ge atoms have little influence on the diffusivity of carbon in iron. The Fe - 12Ge alloy, with a Curie point around 1008 K, has several IF peaks: a broad Snoek-type (P1 and P2), the P3 peak caused by structural changes in as quenched specimens during annealing, and a P4 (Zener) peak at higher temperature (Tm ≈ 773 K at f = 2 Hz, β ≈ 0.7). The Fe - 21Ge alloy has bcc or bcc plus hexagonal structure depending on heat treatment. The structure of the Fe3Ge-type alloy (Fe - 27Ge) consists mainly of hexagonal phases, i.e. hexagonal ε (D019), β (B81), and cubic ε′ (L12), and exhibits corresponding magnetic ordering transitions below 873 K which are not well-reflected in the common Fe - Ge phase diagrams. In particular a high stability of the hexagonal ε phase at room temperature is noted. A broad internal friction relaxation peak with Δ = 0.0036, H ≈ 1.8 eV and τ 0 = 2 ⋅ 10-17 s is found in Fe – 27 Ge and is classified as a double Zener peak in the ε and β two-phase mixture.

Tool Wear Mechanisms during Machining of Alloy 625

Nickel-base superalloys like Alloy 625 are widely used in power generation applications due to their unique properties especially at elevated temperatures. During the related component manufacturing for gas turbines up to 50% of the material has to be removed by metal cutting operations like milling, turning or drilling. As a result of high strength and toughness the machinability of Alloy 625 is generally poor and only low cutting speeds can be used. High-speed cutting of Alloy 625 on the other hand gets more important in industry to reduce manufacturing times and thus production costs. The cutting speed represents one of the most important factors that have influences on the tool life. The aim of this study is the analyses of wear mechanisms occurring during machining of Alloy 625. Orthogonal cutting experiments have been performed and different process parameters have been varied in a wide range. New and worn tools have been investigated by stereo microscopy, optical microscopy and scanning electron microscopy. Energy-dispersive X-ray analyses were used for the investigation of chemical compositions of the tools surface as well as the nature of reaction products formed during the cutting process. Wear mechanisms observed in the machining experiments included abrasion, fracture and tribochemical effects. Specific wear features appeared depending on the mechanical and thermal conditions generated in the wear zones.