Local structure of metallic chips examined by X-ray microdiffraction

Highlights

• We present a detailed microstructure and phase analysis of chips produced by cutting.

• 3D analysis proved mixed nature of shear bands propagation to the material.

• We examine phase composition of the chips by focused X-ray beam.

• Crystallites in segment and shear band change their orientation up to 10°.

Abstract

Nickel-base alloys are used in high-temperature applications whenever steels or titanium alloys cannot be applied anymore. This class of alloys is furthermore used in low-temperature applications in the oil or gas industry in case the corrosion resistance of stainless steels in related liquid media is not sufficient and titanium alloys would be too expensive. Nickel-base alloys, however, due to their high strength and toughness can be machined only at low cutting speeds as otherwise poor surface quality and enhanced tool wear is observed. From all aspects influencing the machinability, the chip formation mechanism is the key factor and only a thorough understanding of this mechanism can lead to an optimisation of the cutting process.

In the current study, a detailed microstructure and phase analysis of Alloy 625 chips produced in an orthogonal cutting process at conventional cutting speeds is presented. Utilising hard monochromatic X-rays focused down to micrometre size, microstructural differences between distinct structural units of the chips, namely, the segments and shear bands, are investigated. Scanning cross sections of the chips with this small beam allowed us to determine misorientation between the segments and shear bands crystal lattices which as we found are not changing abruptly but continuously, with an absolute difference up to 10°.

Introduction

Nickel-base alloys are preferably used in high-temperature applications whenever steels (due to drop in mechanical properties at elevated temperature) or titanium alloys (due to insufficient oxidation resistance above 550 °C) cannot be applied anymore. In aerospace engine applications or in stationary gas turbines, excellent mechanical properties are needed at temperatures above 600 °C in combination with corrosion and oxidation resistance [1], [2].

Due to its excellent corrosion resistance, the class of Ni–Cr–Mo alloys like Alloy 625 with the chemical composition Ni (bal), Cr 20–23%, Fe < 5%, Si < 0.5%, Mn < 0.5%, Mo 8–10%, Ti < 0.4%, Co < 1%, Nb + Ta:3.15–4.15%, and Al < 0.4% (all numbers in wt.%) is not only applied in stationary gas or turbines for exhaust or piping systems. It is furthermore applied in low-temperature applications in the oil or gas industry wherever the corrosion resistance of steels in liquid media is not sufficient anymore and titanium is too expensive [3].

During the component manufacturing of related Alloy 625 products (e.g. turbine housings) in any application, up to 50% of the forged semi-finished parts have to be removed by different machining operations as during welding or brazing microstructural transformations might occur leading to increased notch-sensitivity of the parts or the transformation of γ″- to δ-phase [4], [5]. Due to the high strength and toughness of Alloy 625 only low cutting speeds can be applied during metal cutting operations as otherwise poor surface quality and enhanced tool wear is observed. In addition, the cutting process has to be interrupted as often as it is necessary to remove the long chips from the process zone. Automation especially of turning or drilling operations is therefore almost impossible. The following suggestions are given for machining of Alloy 718 (an alloy similar to Alloy 625 with respect to machinability) regarding the cutting speed: turning operations <80 m/min, drilling operations between 3 m/min and 5 m/min [6].

From all aspects influencing the machinability, the chip-formation process is the key factor and only a thorough understanding of this mechanism can lead to an optimisation of the cutting process. The chip formation process can be described as follows: At the beginning of the cutting action, the material is dammed in front of the tool and the plastic deformation is concentrated in a narrow zone, the primary shear zone, leading from the tool tip to the upper surface of the workpiece. Most of the energy used for the plastic deformation is transformed into heat in the primary shear zone. The rate of heat dissipation into the surrounding material strongly depends of the material properties (e.g. heat conductivity and flow curves) and the cutting parameters (e.g. the cutting speed, cutting depth and feed rate). In case the heat can dissipate quickly, the material is deformed homogeneously leading to the formation of a continuous chip with constant chip’s thickness. The cutting force remains almost constant as well. If the heat cannot dissipate quickly into the material surrounding the primary shear zone, the material locally softens and the deformation therefore localises. In the end, the material is deformed in a narrow zone of a few micrometres (the so-called adiabatic shear band, the width of the shear band in the investigated alloy lies between 2 μm and 20 μm) which leads to the formation of segmented, saw-tooth like chips [7], [8]. The (local) temperature in the shear bands can easily reach 1000 °C [9], [10].

In the current study, a detailed microstructure and phase analysis of chips produced in an orthogonal cutting process at conventional cutting speeds are presented. These results were used to verify related finite element simulations as presented in [11].