The following are our recommendations for preparing aluminum metallographic specimen preparation and testing according to common characteristics. We will cover both the four step and five step methods for preparing aluminum as well as some helpful tips during specimen preparation and testing.
Etching
Aluminum is a soft, ductile metal. Deformation induced damage is a common preparation problem in the purer compositions. After preparation, the surface will form a tight protective oxide layer that makes etching difficult. Commercial grades contain many discrete intermetallic particles with a variety of compositions. These intermetallic particles are usually attacked by etchants before the matrix. Although the response to specific etchants has been used for many years to identify these phases, this procedure requires careful control. Today, energy dispersive analysis is commonly performed for phase identification due to its greater reliability.
Five and four step practices for aluminum alloys are presented in Table 6.1 and 6.2 with expected results as in Figure 6.1. While MgO was the preferred final polishing abrasive for aluminum and its alloys, it is a difficult abrasive to use and is not available in very fine sizes. Colloidal silica has replaced magnesia as the preferred abrasive for the final step and is finer in size.
For color etching work, and for the most difficult grades of aluminum, a brief vibratory polish may be needed to completely remove any trace of damage or scratches. The five step practice is recommended for super pure (SP) and commercially pure (CP) aluminum and for wrought alloys that are difficult to prepare.
Grinding & Polishing
Either 240 [P280] or 320 [P400] grit SiC waterproof paper may also be used for the planar grinding step. An UltraPol cloth produces better surface finishes than an UltraPad cloth, but the UltraPad cloth has a longer useful life. A ChemoMet cloth is recommended when edge retention is critical.
Pure aluminum and some alloys are susceptible to embedment of fine diamond abrasive particles, especially when suspensions are used. If this occurs, switch to diamond in paste form, which is much less likely to cause embedding.
SP and CP aluminum can be given a brief vibratory polish (same products as last step) to improve scratch control, although this is generally not required. MasterPrep alumina suspension has been found to be highly effective as a final polishing abrasive for aluminum alloys, however, the standard alumina abrasives made by the calcination process are unsuitable for aluminum.
For many aluminum alloys, excellent results can be obtained using a four step procedure, such as shown below in Table 6.2 with expected results as shown in Figure 6.2. This procedure retains all of the intermetallic precipitates observed in aluminum and its alloys and minimizes relief.
Synthetic napless cloths may be used for the final step with colloidal silica and they will introduce less relief than a low or medium nap cloth, but may not remove fine polishing scratches as well.
For very pure aluminum alloys, this procedure could be followed by vibratory polishing to improve the surface finish, as these are quite difficult to prepare totally free of fine polishing scratches.
Helpful Hints for Aluminum
Many low alloy aluminum specimens are difficult to polish to a perfect finish using standard methods. A vibratory polisher can be used quite effectively with MasterMet colloidal silica to produce deformation free, scratch free surfaces for anodizing or color etching and examination with polarized light or Nomarski DIC.
For more information on metallographic specimen preparation for aluminum and other metals, refer to the Buehler SumMet Guide.
