M. Burak Toprakoğlu - 030070082 - 10th week

Radiographic Testing (00:17 - 14.04.2011)

Radiographic testing (RT) uses X rays or gamma (g) rays that are passed through the weld and expose a photographic film on the opposite side of the joint. X rays are produced by high voltage generators, while g rays are produced by atomic disintegration of radioactive isotopes.

Whenever radiography is used, precautions must be taken to protect workers from exposure to excessive radiation. Safety measures dictated by the Occupational Safety and Health Administration (OSHA), the National Electrical Manufacturer’s Association (NEMA), the Nuclear Regulatory Commission (NRC), the American Society of Nondestructive Testing (ASNT), and other agencies should be carefully followed when radiographic inspection is conducted. Radiographic testing relies on the ability of the material to pass some of the radiation through, while absorbing part of this energy within the material. Different materials have different absorption rates. Thin materials will absorb less radiation than thick materials. The higher the density of the material, the greater the absorption rate. As different levels of radiation are passed through the materials, portions of the film are exposed to a greater or lesser degree than the rest. When this film is developed, the resulting radiograph will bear the image of the cross section of the part. A radiograph is actually a negative. The darkest regions are those that were most exposed when the material being inspected absorbed the least amount of radiation. Thin parts will be darkest on the radiograph. Porosity will be revealed as small, dark, round circles. Slag is also generally dark, and will look similar to porosity, but will be irregular in its shape. Cracks appear as dark lines. Lack of fusion or underfill will show up as dark spots. Excessive reinforcement on the weld will result in a light region.

Radiographic testing is most effective for detecting volumetric discontinuities: slag and porosity. When cracks are oriented perpendicular to the direction of the radiation source, they may be missed with the RT method. Tight cracks that are parallel to the radiation path have also been overlookedwith RT.

Radiographic testing has the advantage of generating a permanent record for future reference. With a “picture” to look at, many people are more confident that the interpretation of weld quality is meaningful. However, reading a radiograph and interpreting the results requires stringent training, so the veracity of a radiograph depends to a great degree on the skill of the technician.

(Manufacturing Engineering Handbook, H. Geng, p. 21.40)

Fin Rolling (00:49 - 14.04.2011)

The rolling of fins on tubing to increase its heat transfer capability is an old process. The fins are helical and normally produced by three skewed annular die assemblies. In some systems the die assemblies are used in a three-cylindrical-die through feed machine and the dies drive the tube. In other systems, particularly for rolling small diameter fin tube, the free wheeling die assemblies are mounted in a three-cylindrical-die skewed axis head which is rotationally driven while the tube is fed axially through the system while being rotationally constrained. In both cases a support mandrel is often used.

(Manufacturing Engineering Handbook, H. Geng, p. 22.32)

High-Speed Roll Sizing (00:52 - 14.04.2011)

The use of rolling machines and attachments to roll finish a variety of high volume of automotive,appliance, and electronic parts has proven the speed and reliability of that process. However, it has not been practical to achieve sizing improvements at the same time. The development of ultrastiff die systems and of size control by the feed back of true die position, which can be precisely controlled, opens up new roll sizing opportunities. This new capability, combined with the use of annular preformed surfaces on the parts, now makes it practical to size bearing mounts and other precision diameters to total tolerance ranges of 0.0003 in at production rates of as high as 30 per minute.

(Manufacturing Engineering Handbook, H. Geng, p. 22.60)

Diffusion Bonding: (02:33 - 16.04.2011)

Diffusion bonding, a method used to join materials, occurs in three steps (Figure 5.15). The first step forces the two surfaces together at a high temperature and pressure, flattening the surface, fragmenting impurities, and producing a high atom-to atom contact area. As the surfaces remain pressed together at high temperatures, atoms diffuse along grain boundries to the remaining voids; the atoms condense and reduce the size of any voids in the interface. Because grain boundry diffusion is rapid, this second step may occur very quickly. Eventually, however, grain growth isolates the remaining voids from the grain boundries. For the third step – final elimination of the voids – volume diffusion, which is comparatively slow, must occur. The diffusion bonding process is often used for joining reactive metals such as titanium, for joining dissimilar metals and materials, and for joining ceramics.

(The Science and Engineering of Materials, Donald R. Askeland, 3rd S.I Edition, p.130-131)