aluminum alloy. The diffraction pattern reveals a wealth of information on the sample material at the atomic scale. This information helps to under- stand the evolution of phase transformation and the prior thermal history the material has experienced to decode its heat treatment schedule and man- ufacturing processes in reverse engineering.
An elemental composition analysis is also feasible in an analytical TEM based on the physics of chromatic aberration of electrons when they pass through the thin sample. The interactions between the passing electrons and the con- stituent elements result in various levels of energy loss. An electron energy loss spectroscopy then forms an image showing a characteristic elemental map of the sample based on the atomic absorption of these interactions.
Table 5.4 summarizes the applications and limitations of the common ana- lytical techniques used in reverse engineering for material identification and process verification.
TablE 5.4
Applications and Limitations of Analytical Techniques
Technique Application Limitation
Composition Analysis Inductively coupled
plasma/optical emission spectroscopy (ICP-OES)
Primarily for the
determination of the major component concentrations
Some common elements such as hydrogen, boron, carbon, nitrogen, and oxygen are not detectable
Glow discharge mass
spectrometry Trace element analysis Specialized technology Interstitial gas analysis Determination of included
gases, e.g., hydrogen, nitrogen, and oxygen, in a solid sample up to 1 ppm
Specialized technology
DEXA Quick scanning of sample
constituent elements Qualitative or semiquantative, elements with an atomic number less than 11 (sodium) usually not detectable
WDS Accurate quantitative
element analysis Single element detection Microstructure Imaging
Secondary electron image Topographical analysis Lack of composition analysis capability
Backscattered electron
image Surface analysis and
elemental mapping Two-dimensional X-ray composition image Multielement composition
mapping showing the spatial distribution of the elements in a sample
Element of low concentration might be not detectable, particularly by EDXA
Microscopy
Transmission electron
microscopy (TEM) Grain morphology, crystallographic structure (diffraction), and chemical composition (analytical TEM)
Sample preparation is time- consuming, and data analysis is complicated, while area of analysis is small and might lack representation Scanning electron
microscopy (SEM) Topographical features, fractography, grain morphology, and chemical composition (when equipped with EDXA or WDS)
Surface analysis only
Optical microscopy General applications of microstructural analyses at low magnifications
Magnification is limited to 1,500×, no other analytical capability besides surface examination
and surface treatment until the final product is complete. To reverse engineer the manufacturing process is a very challenging task. The inherent complex- ity virtually prohibits a full replication of the original process. Advanced technologies have made the verification process relatively easier today. The end-product performance test also provides reasonable assurance of equiva- lent functionality between the OEM and reverse engineered parts. The three most utilized manufacturing processes to produce mechanical components are casting, product forming, and machining. Casting is a thermal process that melts and solidifies raw material. Product forming is a deformation process that molds raw material into a part by force. Machining is a shap- ing process that removes “extra” material to produce the part. To assemble individual parts together, many joint methods are used, such as soldering, blazing, and welding. To improve the strength and performance of the prod- uct, many engineering treatments, such as heat treatment and surface treat- ment, ranging from coating to shot peening, are commonly applied. These processes and treatments and the challenges of replicating them in reverse engineering are discussed below.
5.4.1 Casting
The casting process has two primary phases: melting and solidification. This process produces either an ingot with a specified composition for further application, or a semifinished part directly from the molten metal. A high- quality ingot requires multimelting in a controlled environment. A triple vacuum arc remelting (VAR) process is required to produce an aerospace- grade titanium alloy. This process (re)melts and solidifies the ingot in a vac- uum or inert gas atmosphere three times to minimize the brittle alpha phase.
The size of the ingot could also affect the product properties. Some forged titanium products starting with a larger titanium ingot size have a higher tendency to show deficiency in dwell time fatigue resistance. Different cast- ing processes, such as sand casting, permanent mold casting, and invest- ment deficiency in casting, will produce parts with distinct macro- and microstructures, and therefore different properties due to their different solidification processes. These distinct structural features provide valuable information in reverse engineering to verify the casting method used for the original OEM product. For example, Figure 5.15 shows the macrostructure of the aluminum base for an electric power transmission post. It has a course columnar grain morphology growing inward just beneath the edge and a coarse equiaxed grain morphology in the center. This is a typical direct chill cast structure where three layers of grain configurations are observed: fine equiaxed grains on the chilled surface and coarse equiaxed grains in the cen- ter, while columnar grains appear in between these two regions. The macro- structure of the aluminum base, as shown in Figure 5.15, reveals two out of three characteristic structural features of direct chill casting. It is also noted that direct chill casting is one of the most frequently used casting processes
for wrought aluminum alloys. However, a definite verification of the casting method requires more microstructure analysis.
The effectiveness of microstructure analysis in reverse engineering can be further exemplified by earlier micrographs (Figure 3.1a and b). Even though the figure shows two vastly different microstructures, both samples are alu- minum alloys, and each has the same chemical composition. The different microstructures result from different solidification processes. Figure 3.1a shows a typical equiaxed microstructure and provides some evidence that the material was processed by traditional casting. The fine microstructure of Figure 3.1b indicates that this material was solidified at a much higher cooling rate, which can only be achieved by a rapid solidification process.
The directionality of the microstructure in Figure 3.1b further implies that it was processed by extrusion or another similar product-forming process. The product-forming process will be discussed in more detail below.
5.4.2 Product Forming
Product forming is the shaping of the raw material, such as an ingot or bil- let, into a product form, such as a turbine disk or axial shaft, by forging, rolling, or extrusion. The deformation of solid raw material during product forming usually results from the force applied to the solid when it is heated up, like a blacksmith working on a horseshoe. The process parameters from the applied force to operating temperature are often traceable because of their “imprinted” effects on the product characteristics and properties. The grain texture reveals the direction of the external force during rolling, extru- sion, or forging. The microstructural features, such as subgrain structure
FIgurE 5.15 (See color insert following p. 142.) Aluminum casting macrostructure.
from dynamic recovery, grain shape, and size, all provide a hint in reverse engineering to decode the prior forming process that the original part was subject to. The forming process can also drastically affect the product prop- erties, thus providing more information about the fabrication of the original part using reverse engineering.
5.4.3 Machining and Surface Finishing
Machining refers to the manufacturing process that shapes the part geometry by removing material from the workpiece. The traditional machining pro- cesses remove materials either by cutting, such as turning, milling, and drill- ing, or abrading, such as grinding. The advanced machining processes remove materials electrically or chemically instead of mechanically, such as electrical discharge machining (EDM), laser cutting, and chemical etching. In recent years the technologies of laser cutting and EDM have made great strides. They are increasingly becoming the favorite methods for complex, fragile, and tiny parts. In the medical filed, EDM is a particularly preferred method. Many of the cooling holes in jet engine turbine blades are laser drilled.
A cutting process involves a close interaction between the workpiece and the cutting tool. The mechanics of the cutting process are complicated and yet to be fully understood. This process involves mechanical force, mate- rial yielding, elastic and plastic deformation, chip formation, and breaking.
Choosing a cutting tool and cutting speed is often based on corporate knowl- edge and experience. Due to the lack of theoretical modeling that can trace the exact cutting process utilized by the OEM, the analytical data based on the finished part become pivotally critical when reverse engineering a cut- ting process. The postmachining examination is primarily focused on macro appearance and micro characteristics. The key parameters are surface fin- ishing, subsurface microstructure, grain morphology, and surface residual stress. The precise quantitative measurements of these parameters can be very challenging. Making the task even more complex, many of the foot- prints that are critical to substantiate the operating conditions and cutting parameters used for an OEM part might have been eliminated during the sequence of processes. The microstructure analyses are addressed in other sections of this book. The following will focus on surface finishing, that is, surface roughness and surface residual stress analysis.
A nice surface finishing conjures up the image of a shiny part that bright- ens the appearance and often the value of a part. During part fabrication, raw materials will be stamped, ground, machined, or heat treated to trans- form them to the finished parts. These deformation processes shape the mate- rial into a part; they also introduce burrs, contamination, scales, and tooling marks on the part surface. The surface finishing can significantly affect part fatigue life; the surfaces of many heavy-duty mechanical springs are elec- tropolished for fatigue life improvement. A good surface finishing also pro- tects the part from corrosion attack by minimizing embedded contamination