Lai-Zhe Jin

Machinability data applied to materials selection

Abstract In an attempt to collect information appropriate to materials selection, machinability data for engineering alloys are assessed by means of the chip-breaking property and the relative rate of material removal. The additions of alloying elements and free-machining additives influence the distribution of inclusions, thus the chip-breaking property and material removal rate. It is found that the beneficial effects of calcium deoxidation on machinability can be achieved by either carbide or high-speed steel tools, regardless of the cutting speed used. The level of improvement on machinability due to the addition of free-machining additives differs between operations, being significant in continuous machining operations. It is not obvious that results from one operation can be related to another. Two machining properties at different levels of precision are proposed for a description of the machining behaviour of engineering alloys: chip characteristic rating and nominal machinability rating. The first property describes the chip-breaking and the surface finish characteristics and the second the relative rate of material removal in turning, milling and drilling operations. Nominal machinability ratings for carbon and alloy steels, martensitic and precipitation-hardening stainless steels are associated with the hardness by a polynomial regression model. The ratings for austenitic and duplex stainless steels, aluminium, copper, and magnesium alloys are related to the conditions of heat treatment. These material types are divided into 46 material groups. Nominal machinability ratings are assigned for each group and chip characteristic ratings for individual aluminium alloys. The data are presented in such a manner that they are directly comparable between different types of materials.

Evaluation of machinability data

Systematic materials selection is essential to fulfill the design criteria. Reliable information on material properties, in turn, is a vital factor for approaching such an objective. The machinability of engineering metals, owing to the marked influence on the production costs, has to be taken into account in the process of materials selection. In an attempt to develop a method for estimating the machinability of engineering metals, machinability data collected from laboratory and literature are assessed. A rating system derived from the metal removal rate is proposed for estimating the relative machinability of carbon and alloy steels, stainless steels, and aluminum, copper, and magnesium alloys.

Modelling of creep in friction stir welded copper

Abstract Copper canisters for storage of nuclear waste will be exposed to creep. The canisters will be closed with friction stir welding (FSW). To describe the creep behaviour of the welds, uniaxial creep tests have been performed. A previously developed fundamental creep model for parent metal is applied to the different weld zones. The differences in microstructure and yield strength between the weld zones are taken into account. Creep strain versus time curves for the weld zones have successfully been predicted without the use of any adjustable parameters. It should be noted that the temperature range of interest of 50–100°C is deep down in the power law break down regime with Norton exponents between 25 and 100. The constitutive equations are used in FEM computations of creep in the canister weldments.

Finite element modelling of temperature distribution in friction stir welding process and its influence on distortion of copper canisters

The Swedish model for final disposal of nuclear fuel waste is based on copper canisters as a corrosion barrier with an inner pressure holding insert of cast iron. One of the methods to seal the copper canister is to use the Friction Stir Welding (FSW), a method invented by The Welding Institute (TWI). This work has been focused on characterisation of the FSW joints, and modelling of the process, both analytically and numerically. The first simulations were based on Rosenthal’s analytical medium plate model. The model is simple to use, but has limitations. Finite element models were developed, initially with a two-dimensional geometry. Due to the requirements of describing both the heat flow and the tool movement, three-dimensional models were developed. These models take into account heat transfer, material flow, and continuum mechanics. The geometries of the models are based on the simulation experiments carried out at TWI and at Swedish Nuclear Fuel Waste and Management Co (SKB). Temperature distribution, material flow and their effects on the thermal expansion were predicted for a full-scale canister and lid. The steady state solutions have been compared with temperature measurements, showing good agreement. Microstructure and hardness profiles have been investigated by optical microscope, Scanning Electron Microscope (SEM), Electron Back Scatter Diffraction (EBSD) and Rockwell hardness measurements. EBSD visualisation has been used to determine the grain size distribution and the appearance of twins and misorientation within grains. The orientation maps show a fine uniform equiaxed grain structure. The root of the weld exhibits the smallest grains and many annealing twins. This may be due to deformation after recrystallisation. The appearance of the nugget and the grain size depends on the position of the weld. A large difference can be seen both in hardness and grain size between the start of the weld and when the steady state is reached.