Response of Ti microstructure in mechanical and laser forming processes

Abstract

Microstructural deformation mechanisms present during three different forming processes in commercially pure Ti were analysed. Room temperature mechanical forming, laser beam forming and a combination of these two processes were applied to thick metal plates in order to achieve the same final shape. An electron backscatter diffraction technique was used to study the plate microstructure before and after applying the forming processes. Substantial differences among the main deformation mechanisms were clearly detected. In pure mechanical forming at room temperature, mechanical twinning predominates in both compression and tensile areas. A dislocation slip mechanism inside the compression and tensile area is characteristic of the pure laser forming process. Forming processes which subsequently combine the laser and mechanical approaches result in a combination of twinning and dislocation mechanisms. The Schmid factor at an individual grain level, the local temperature and the strain rate are factors that determine which deformation mechanism will prevail at the microscopic level. The final microstructures obtained after the different forming processes were applied are discussed from the point of view of their influence on the performance of the resulting formed product. The observations suggest that phase transformation in Ti is an additional microstructural factor that has to be considered during laser forming.

In Situ Digital Image Correlation Observations of Laser Forming

“…These IFP maps show grain directions parallel to the plate surface normal in a central part of the laser track (top) and under the LHAZ area (bottom). The microstructural changes within the LHAZ (starting from the top towards the bottom) are evident and similar to that reported by [8] showing that these changes can be slip, grain fragmentation, recovery, grain boundary motion due to twinning, alpha-to-beta transformation, grain growth, phase transformation and recrystallization. The LHAZ in Figure 8 shows a fine-grained acicular alpha structure on the top of the LHAZ followed by columnar grains elongated in the direction perpendicular to the surface.…”

“…Although the experimental data was used to calibrate the FEM and the FEM was used to determine the dependence of the maximum temperature on the depth of the sample for different laser power and scanning speeds, it should be noted that the FEA was used as primary purpose of serving as supporting tool in order to understand the underlying physical phenomena of the laser process and not as an exact measure for validating the results obtained with the laser scanning speed. Similar results for the sample depth have been reported by [8] in which only an approximate equation for heat transfer in titanium has been used. Therefore it can be concluded that the size of the LHAZ observed by EBSD is identical to the volume undergoing phase transformation.…”

“…CP Ti specimens were prepared for EBSD analysis as described by [8] using the same field emission gun, EBSD camera, EBSD data analysed software techniques and acceleration voltage to determine the crystallographic orientations. A step size of 0.7 µm and hexagonal grid was used for measurements.…”

“…The physical properties of the material and surface such as the absorption coefficient, crystallographic orientation, thermal conductivity and thermal expansion also play a vital role in the final desired shape of any material that will undergo LF. The selection of the above-mentioned processing parameters are therefore crucial as previously reported by [7,8]. More recent studies deal with laser forming involving, e.g., the edge effect [9], three dimensional laser forming [10,11] and modelling of multiple scans [12].…”

“…An advantage of DIC is that it can also be used to evaluate any sudden movement during the heating and cooling period. Furthermore, commercially pure (CP) titanium (Ti) experiences an allotropic phase transformation throughout the LF process as the material is heated above the alpha to beta region which is associated with a volume change and therefore influencing the LF process as reported by [8]. This phase transformation always needs to be taken into account when laser processing of Ti or its alloys is studied [13][14][15].…”