| In any sheet forming operation such as stamping, drawing, ironing, or rolling, sliding contact may occur between the metallic sheet and the die or roll. Regardless of the surface finish or the lubricant employed, metal will eventually be transferred to the opposing surface. Nonuniform transfer results in formation of macroasperities, locally increased contact stresses, and galling of the alloy surface. For economic and aesthetic reasons, galling is not acceptable and must be minimized in production situations.; It is well known that nearly all aluminum alloys have a very strong propensity for severe galling during most drawing operations. The tendency for these alloys to gall appears to be related to several microstructural characteristics. While most aluminum alloys do exhibit galling, 3004 is nearly unique in that galling is practically nonexistant in most forming operations. Examinations of dies used to form 3004 reveal the absence of transferred aluminum which is necessary for the development of macroscopic asperities. The inability of 3004 to develop a transfer layer is believed to be related to the presence of a very finely dispersed MnAl(,6) second phase. 3004 is a relatively low strength alloy and not suitable for most structural applications. Therefore, an investigation was undertaken to determine the role of other precipitate particles in the adhesive transfer of aluminum to surfaces and provide a scientific basis for the design of new high strength nongalling aluminum alloys.; Three sets of alloys (Al-Mn, Al-Mg, and Al-Mg-Si) were examined to determine the role of various second phase particles on galling behavior. All alloys exhibited second phase precipitation reactions as observed by optical and TEM methods. The presence of second phase particles in the Al-Mg and Al-Mg-Si alloys raised the crack nucleation rate and adversely affected the galling response of the alloys. The Al-Mn alloy had a much finer dispersion of second phase particles and raised the crack nucleation rate the most. With a very high crack nucleation rate, adhesive transfer of material and build-up of macroasperities were prevented and galling did not occur.; In designing high strength, nongalling alloys, one method might be to produce a very fine second phase dispersion in the material to prevent adhesive build-up and galling. |
