147 Common Defect Structures

There are two common defect structures that can and have occurred in all laser-based rapid manufacturing processes. These are porosity and lack-of-fusion (LOF). In some cases porosity may result in a chemical reaction between different feedstocks. For example, if a high-carbon ferrous-based substrate is used in combination with a copper powder feedstock high in oxygen, then carbon may react with the oxygen and form CO and CO2 gas, which can then be entrapped as porosity. Porosity can also be caused by out gassing of powder during processing [29]. Figure 14.6 shows an LOF defect in a Ti-6Al-4V deposit; such defects are hard to detect by nondestructive means. Process control must be such that the formation of LOF defects is completely avoided.

Of the common defects, LOF is the most damaging since it may act as a crack under certain loading conditions. A single LOF defect can lead to component failure. Excessive porosity (either size of individual pores or the number density of pores) can also be damaging particularly to tensile ductility.

Other defects that will be specific to the deposition process and material include various types of cracking such as hot shortness, strain age, and tearing during solidification. Cracking must be controlled by careful selection of deposition material and process parameters.

FIGURE 14.6 Lack-of-fusion defects in Ti-6Al-4V.
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