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Vol. 14
May Issue
Year 2013
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Science Update


in Vol. 14 - May Issue - Year 2013
Residual Stress in Machined Surfaces



A portable XRD residual stress system performs measurements at electro-polished locations on the turned surface of a large turbine disc

Introduction

Critical components are found in many industrial applications; they are those components whose failure could cause a catastrophic event of much greater significance than the failure of the component alone. Critical components are usually assigned a safe service life, at which time they are replaced whether they appear fit for further service or not. Examples include the timing belt of an automobile engine (whose fracture could cause pistons to meet valves, leading to severe internal damage to the engine), and the major rotating components in an aircraft engine (whose fracture could cause structural damage leading to loss of control of the aircraft). The manufacturing process of these parts is usually subject to strict control, to ensure that parts made in the future are the same as parts that were tested, usually at significant expense, to establish the safe service life.

Originally intended to prevent the use of lower standards for materials and inferior processes, Process Control can actually hinder the adoption of legitimate process improvements in the production of future parts. The controls are intended to ensure the repeatability of components, part after part, batch after batch, and year after year. For metal components, the controls may cover materials and processes from melting and casting, all the way to machining and subsequent finishing operations.
Manufacturing Engineering has recently seen significant advances in the design of cutting tools for metals, and in the machine tools used to manufacture critical components. Cost savings and greater reproducibility in the product are attractive to manufacturers, while maintaining older equipment and methods is definitely unattractive. There are other reasons to incorporate current methods, including changes inspired by increased attention to workplace safety. Chemical products, including coolants once considered benign, are removed from the marketplace when the true risks associated with their use are recognized.

What can Industry do to take advantage of these developments and improved methods in existing products? One way is to take advantage of modern scientific tools to compare the product of modern and established processes.

Taking a simple example from a machining operation, this article looks at some of the methods that can be used to demonstrate similarity in parts manufactured by an existing controlled process with parts made by a new or modified process.

Machining Processes

An Example: Point Turning
 
One of the most accessible machining methods is point turning. In the simplest terms, a sharp tool is made to traverse the part, defining a new shape by removing material as swarf or chips.

Obvious parameters applied at the machine in this type of process are feed, depth of cut and speed. Less obvious  but equally important is the supply of cooling fluid, and the state of wear of the tool’s cutting edge. The manufacturing engineer will have designed the process with a specific cutting tool or insert in mind, to accommodate the machining characteristics of the material, the desired surface topography of the finished part, designed radii, access to the part’s surface for the tool and coolant, and for chip removal.

The characteristics of the machining centre itself are also important, including its rigidity and any resonances it may be subject to. There will be an outline of the operation sequence, defining the tool paths and parameters for preliminary and finishing cuts. Depending on the nature of the starting material, the engineer may have to consider shape changes in the workpiece during machining because residual stresses locked in during earlier processing or heat treatment re-distribute to accommodate the removal of stressed material near the surface. Tool or insert changes may be scheduled to provide a new or lightly used cutting edge for the final finishing passes of each part.

Events at the cutting edge

When all these parameters have been defined, and the tool meets the metal, what takes place near the cutting edge? The tool and the workpiece can be considered separately, and two kinds of interactions take place simultaneously, thermal and mechanical.

Mechanical interactions include friction where the tool and the workpiece are in contact, deformation of the chip and the sub-surface material ahead of the tool, and some smearing of the new surface under the tool.

In the tool, the mechanical interactions should not cause plastic deformation, but the cutting forces can deflect the tool elastically. There may be some wear or build-up of material from the workpiece where the chip and workpiece contact it. The forces acting on the tool can be measured by the appropriate use of strain gauges mounted on the tool and its holder.

Thermal processes in the workpiece include adiabatic heating of the workpiece and chip as they are deformed, and frictional heating of the workpiece and chip where they are in contact with the tool. These effects are highly localized and the local temperature can increase enough to significantly modify the local mechanical properties during cutting. Heating occurs in the area where the workpiece is being cut, and in some materials, causes damaging metallurgical transformations at the newly created surface. The thermal effects on the workpiece are otherwise transient: individual points on the surface are only affected as the tool passes, and the severe thermal gradients in very small volumes of material allow the heat to dissipate rapidly.

In the tool, processes generating heat are limited to frictional heating where the tool is in contact with the workpiece. The effects of frictional heating on the tool are severe because the tool is in contact with the workpiece and chip all the time it is cutting. The overall temperature of the tool may be much higher than that of the workpiece, and the added effect of localized high temperature near the cutting edge can accelerate deterioration of the tool, as well as influence heat flow between the tool and workpiece. Tool temperatures can be monitored by the use of thermocouples embedded in the tool or attached to the surface, or by non-contact methods. Temperature excursions could indicate a change in the events taking place at the cutting edge.

The thermal and mechanical interactions of the tool and workpiece are complicated by the very high strain rates of the localized plastic deformation preceding the fracture that creates the new surface. Conventionally measured material properties do not usually apply in the extreme and dynamic mechanical and thermal conditions near the tool’s cutting edge. Mathematical models of machining processes are improving, but do not yet predict the nature of the new surface well enough to demonstrate the equivalence of machining process parameters for a specific component.

Comparing Machined Surfaces

Surface Integrity studies offer a practical approach to evaluating and comparing machined surfaces. The comparisons should go beyond the requirements of routine Quality Control inspections, where the objective is to establish the absence of recognized ‘defects’. Surface Integrity studies search for and identify differences between the past and future products. These comparisons need to be carried out without further processing of any of the parts involved.

Measurements of surface roughness, penetrant inspections and side-by-side visual comparisons with typical past product provide basic data. Additional comparisons may include enhanced visual examination, using specific illumination techniques and magnifying loupe or stereo-optical microscope evaluations of predefined areas. When differences are found, their origins can be investigated and their significance determined. These techniques are all applied at the surface of the component. They do not address the thermal and mechanical events that occurred beneath the surface during machining.

Comparisons of Sub-Surface Material

Sub-surface comparisons usually require parts to be sacrificed for metallographic examination. Near-surface metallographic examination of suitably prepared cross-sections can reveal distortion of the material resulting from machining, and can identify phase changes resulting from localized and/or severe overheating.

Direct examination of the machined surface can be performed with a metallographic microscope if the surface is flat and smooth enough, but scanning electron microscopy offers much greater depth of focus and a greater range of magnifications, if such detailed examination is needed. Progressive electro-polishing and chemical etching of the surface can reveal microstructural changes resulting from machining. However, these techniques are better suited to the detailed characterization of surfaces known to be altered, than to determining whether the surfaces are altered.

Residual Stress Depth Profiles to Compare Machined Surfaces

In general, residual stresses in components result from non-uniform plastic deformation, and from the localized thermal expansion and contraction resulting from non-uniform heating and cooling. These are precisely the conditions that exist during machining operations. Residual stresses may be expected near the component’s surface as a result of machining. Further, the profiles of residual stress with depth are expected to be a sensitive witness of the thermal and mechanical conditions present during machining.

Residual stresses can be measured in crystalline materials (including most metals) using the distances between layers of atoms in the crystals as a strain gauge. Tensile stresses increase these distances, whereas compressive stresses push the crystal planes towards each other. X-ray diffraction can be used to measure the changes in crystal plane spacing very precisely, and the stresses can be calculated from these measurements. The X-rays used for the measurements have low energy, so they are easily absorbed by typical engineering materials, making the measurements very surface sensitive. Each measurement usually represents the top few tenths of a thousandth of an inch of the sample, and can often be completed in a few minutes. Also, the equipment used to perform the measurements can be available in a portable version, allowing measurements to be performed on site, even in the chuck of the machining centre!

Electro-polishing may be used to remove thin layers from the surface, without altering the remaining residual stresses. In this way a series of residual stress measurements can be performed through the layer affected by machining to define a residual stress depth profile. It is important to continue the profile until the residual stress stabilizes at the core stress. A profile that appears to stabilize at a different level than others in the set may indicate that an earlier stage in the manufacturing process was significantly different. Like a fingerprint, the residual stress depth profile characterizes the processes that created the surface.

It is not even necessary for this purpose to determine the details of what happened during machining, it is only necessary to determine whether the new process falls within the range defined by past experience. The comparison of residual stress depth profiles representing machining processes can be as simple as overlaying the new profiles onto charts displaying the range of profiles measured at the same location and in the same direction on the product of the existing process. Comparisons are not limited to turned surfaces; the products of any pair of machining or surface finishing processes can be compared by reviewing the residual stress depth profiles.

Conclusion

Changes to the manufacturing process of certain critical components, including improvements, must be shown not to reduce the life of the part in service. Clearly, there are many reasons to make changes to an existing process. Some are necessary, for example: the removal of a chemical product from the market for safety reasons, or the irreparable breakdown of a necessary piece of equipment. Others may be desired as part of a wider plan, such as transferring work to a different factory, or to phase out a particular outdated lathe in favour of a modern machining centre. Entire manufacturing processes may be updated to take advantage of the latest equipment and methods available in the rapidly developing world of manufacturing technology.

The measurement of residual stress profiles is a particularly powerful tool in the objective comparison of manufactured surfaces. The X-ray diffraction method is documented in SAE and ASTM publications, and general guidelines can be found in the ASM Handbook (Vol. 11, 2002). The data generated by the method provides a permanent record of the surface condition, and results can be compared with data from other times (past and future) in a manufacturing history.

Whatever the reason for a process change, one of the requisites of critical components is that the component must meet the stated life, and process changes must be shown not to introduce significant changes in the product. Applying tools that assess the surface integrity of existing and new product allow the effects of changes to be measured and monitored. Even where differences are found, the results of surface integrity evaluations can be used in further refinement of the process, and to manage additional testing needed to demonstrate the life of the new product.




Author: Glyn Richards
For Information: Proto Manufacturing Ltd.
2175 Solar Crescent, Oldcastle,Ontario
Canada, N0R 1L0
Tel. +1.519.737.6330, Fax +1.519.737.1692
E-mail: proto@protoxrd.com
www.protoxrd.com