Influence of pH value on chromate-free conversion coating for magnesium alloys

Many factors were found to have effects on the conversion coatings for AZ31 alloy, alloy the most important one in producing high quality conversion coatings is found to be the control of the pH value. The influence of pH value on the conversion coating including color, thickness, adhesion and surface morphology was studied. The performance of conversion coating was examined by cross cut test, SEM method and salt immersion. The results show that the variation of pH value causes surface treatment process unstably. The conversion coating can obtain as pH value ranging from 3.0 to 5.0, while it presents dark, thick and bad adhesion under lower pH value. The conversion coatings have good combination of thickness and adhesion when pH value ranging from 4.0 to 4.5, and it exhibits a good corrosion resistance.

EBSD analysis on aluminum brazing sheet for automotive heat exchangers

Grain boundary character distribution (GBCD) in the vicinity of clad-core interface in annealed aluminum brazing sheet was investigated by EBSD technique. The results show that, with the increase of annealing temperature, the frequency of special boundaries increases, but the random boundary network can not be fragmented by special boundaries. This means that Si diffusion in annealed sample may not be reduced during brazing. The results also indicate that the modified microstructure of the core, with coarse grains distributed parallel to the rolling direction and broken random boundary network by high frequency of special boundaries, would enhance the brazeability by suppressing the excess Si grain boundary (GB) diffusion from the cladding to core material.

Effect of minor RE addition on microstructures and mechanical properties of AZ31 magnesium alloy

AZ31 alloy with minor RE addition was investigated. The material was homogenized, hot-rolled and annealed. Optical microscopy, scanning electron microscopy and X-ray diffractometry were employed to characterize the microstnictures of AZ31 alloy with RE addition. And micro-hardness test was employed to measure mechanical properties of annealed material. The results show that minor RE addition (0.3% is mass fraction) has little effect on the grain size of as-cast AZ31 alloy, and only Mg17(Al,Zn)12 phase was found in the microstructure of as-cast AZ31RE alloy. The result of X-ray diffraction shows that supersaturation of alloying elements has little effect on the lattice parameters of theα-Mg in homogenized sample; but owing to RE solute with high melting point, AZ31RE alloy exhibits better heat resistance than AZ31 alloy.

Microstructural evolution of aluminum alloy 3003 during annealing

The microstructural evolution of cold-rolled aluminum alloy 3003 during annealing was investigated by means of micro-hardness measurement, electrical resistivity measurement, optical microscopy and transmission electron microscopy. The interaction of recrystallization and precipitation of aluminum alloy 3003 was also discussed. The results show that the recrystallized grain size of cold-rolled aluminum alloy 3003 is strongly affected by precipitation during annealing. When precipitation occurs prior to recrystallization at low temperature(300℃), the grain structure becomes coarse, and the precipitation process is affected by the presence of lattice defects, i.e. high cold reduction results in a large number of precipitates. When annealing at 500℃, however, for the recrystallization is prior to precipitation, the precipitation is independent of cold deformation reduction and a fine, equiaxed grain structure is obtained.