VOL. 11 May ISSUE YEAR 2010
in Vol. 11 - May Issue - Year 2010
Residual Stresses and Their Origins in Manufacturing Processes
Figure 1: Residual stress vs. depth in an Electro-Discharge-Machined Inconel Bar
Figure 2: Residual stress map of friction-stir welded aluminum alloy plates
Figure 3: HR 1020 steel weldings before and after thermal stress relief
As manufactured products become more sophisticated, particularly in industries where there are concerns for user and public safety, there is increased interest in optimising designs to give reliable performance from all components of the final product. This interest may be accompanied by focus on other aspects, such as minimising manufacturing costs or weight. The natures of finished surfaces are crucial in managing potential mechanisms of fracture, corrosion and wear. Residual stresses, whether deliberately installed or coincidentally present after a manufacturing operation, are an integral part of the component, and need to be understood by designers and manufacturing engineers. Ideally, their understanding should be based on knowledge of the parameters of the component, including its residual stresses because of their impact on the component’s behaviour during manufacturing and its service life. Sophisticated products require more than dimensional checks and simple tests to demonstrate their quality, and process control parameters have to be shown to produce parts that consistently meet the designer’s intention.
In most cases, residual stresses in components are the result of non-uniform yielding, and are installed either deliberately (e.g. by peening), or as an unintended effect of a necessary process (e.g. mechanical forming, or heat treatment). Occasionally, however, residual stresses arise from less obvious sources, such as carburising in case hardened steels, or in situations where materials with dissimilar thermal expansion coefficients are joined as welded or brazed assemblies, or one of the materials is present as an applied coating.
Residual stresses in components are always in balance: if the balance is disturbed (e.g. by machining off a peened compressive layer from the surface), the remaining forces redistribute to restore the balance, changing the shape of the component in the process. For the same reason, the arc height of an Almen strip is a quantitative measure of the forces installed by peening one of its faces, and hence a measure of the stability of the peening process.
Residual Stresses from Manufacturing Operations
Metallic parts usually need some form of heat treatment to develop the component’s mechanical properties. Depending on the material, the significant heat treatment may be applied at an early stage in the manufacturing cycle, or late in the process. In the case of some Ni- and Ti- alloy aerospace alloys, the final heat treatment may be applied after investment casting or forging, while martensitic steels are commonly heat treated to an interim condition to facilitate machining, and the part will be fully heat treated later in their process. In heat treatments involving quenching (either to suppress precipitation or to promote hardening) the cooling rates at the surface are higher than at the center. The early thermal contraction at the surface is resisted by the incompressible core, and results in tensile yielding at the surface. As the temperature continues to fall, the core of the part contracts and draws the surface inwards, leading to the piece having hydrostatic tension in the core, and compression near the surface. In materials with a martensitic transformation, the associated volumetric change adds to the complexity and magnitude of the residual stress pattern, because the transformation and its associated volume change progress through the part as the local temperature falls. The orientation of the part during heat treatment, particularly during quenching, will influence the symmetry of the pattern of residual stresses.
These residual stresses are most likely to extend through the volume of the part, and are often referred to as body stresses. Body stresses are most likely to be observed as changes in the shape or dimensions of the part as machining progresses, and are most noticeable during turning the faces of thin, large diameter discs. These parts often have to be machined in small increments on each face so that the residual stresses are balanced and do not cause unmanageable distortion during machining. Long, thin-walled shafts are also susceptible to bending and changes in length as material is removed from the cylindrical surface.
Traditional metal removal processes include the chip forming techniques (turning, drilling, broaching, sawing, etc.). While each may appear to be a simple process, they include a wide range of basic cutting parameters (e.g. speed, feed and depth of cut). Residual stresses are influenced by the mechanical and thermal interactions between the tool and the work-piece so many other parameters are important in their development. The original shape and developing wear of the tool, the availability of coolant or lubricant at the points of contact, the precise orientation of the cutting edge under load, and many other events can radically change the near-surface residual stress pattern in the product. Metal cutting processes and the available tooling have been the subject of rapid and extensive development in recent years. These attractive developments have given faster cutting, longer tool life and other benefits, without deterioration of surface finish, without changing process stability or introducing detectable linear or point defects. The nature of a surface also includes its residual stress distribution, which can be measured and monitored during process development or tool proving, in post-production inspection and even while the part is still on the machine tool, with appropriate instrumentation.
"Non-conventional" machining processes include a growing number of metal removal methods that are increasingly finding applications replacing more traditional methods, or carrying out tasks that were not possible by conventional means. Some of them (e.g. electro-discharge machining (EDM), laser drilling and cutting, and plasma cutting) leave behind a re-solidified surface and a modified sub-surface layer of material that may require extensive characterisation and subsequent processing. An example of the residual stress vs. depth in an EDM processed Inconel bar can be seen in Figure 1. It should be noted that both tensile and compressive residual stresses can result (depending on the direction of residual stress considered/measured) from this process and can vary with machining parameters employed. Other processes (e.g. electro-chemical milling and electro-polishing) may not even leave a residual stress field associated with the new surface. Nevertheless, these surfaces will also be different from the more conventionally produced surfaces they replace, and consideration should be given to developing, characterising and applying a treatment to enhance these surfaces.
There are many welding processes, of varying complexity, from the "Thermite" process and gas or Acetylene torch welding, to manual and automatic arc welding, and electron-beam and plasma welding. Then there are the modern techniques of friction welding and friction-stir welding. An example of the residual stresses in friction-stir welded aluminum alloy plates can be seen in Figure 2. It can be seen that significant tensile residual stresses can result from the application of this welding process. Not only are there numerous techniques, but they are used to join a vast range of alloys, to similar alloys and to alloys that are quite different in composition, sometimes without an added filler material and sometimes with one. Add to these the complexities introduced by the range of section sizes joint geometries and bead shapes, and then the effects of single and multiple passes. And of course, the welder may be assembling a new part, or repairing a damaged one.
Despite their diversity, welding processes do have some things in common. All welding processes raise the local temperature of the materials to be joined to about or above their melting points. They fuse together, and the source of heat is removed (or moves on along the joint). Then the temperature falls as the heat is removed from the new joint. The cooling metal contracts, developing residual stresses in and around the joint, with or without local yielding. In turn, these stresses are reacted in the rest of the structure, possibly causing it to distort. Stress-relieving heat treatments elevate the temperature to the point at which the materials can flow to lower the local stresses, but it is rarely possible to completely remove residual stresses by thermal treatments alone, unless a phase change is involved (e.g. re-austenitising and re-hardening a martensitic steel). An example of the residual stresses in an HR 1020 steel welding before and after thermal stress relief can be seen in Figure 3. In this case the thermal stress relief parameters selected were very effective in relieving the residual stresses due to welding in this sample. It is also important to note that the residual stress fields in and around a weld are almost certainly three dimensional, and vary through the thickness of the material. Stress gradients in the weld bead and heat affected zones will be steep. Evaluations of residual stresses around a weld need to take the three-dimensional nature of the weld into consideration. Modern x-ray systems designed for residual stress measurement can provide the excellent spatial resolution needed to capture these gradients, and their associated software can be used to compute the principle stresses from multiple measurements and map the 3D stresses, at least at the surface.
Almost all manufacturing processes include cleaning operations, for use either within the process, or in preparation for delivering the product. Some processes, including apparently cosmetic brushing and light blasting operations can modify the material at the extreme surface enough to mask the pattern of residual stresses in the component.
Residual stresses are a universal feature of the things we make. They are not as visible as a surface texture or a colour, or as easy to specify as a dimension or a hardness limit. We have looked at some of the common manufacturing processes and seen how they introduce residual stresses. They can have a vital role in determining the ease or difficulty of subsequent manufacturing operations. Their presence or absence can cause or prevent reworks and repairs, and they can be used to extend a component’s service life and functionality.