
Hi Rumman,
There's two ways you can accomplish this. The first would be to rotate the structure with the columnar grains using the Rotate Sample Reference Frame filter (
http://www.dream3d.io/Filters/SamplingFilters/RotateSampleRefFrame/) before you use the Append Z Slice filter. Judging from the axes on your right image, you'd want to rotate the columnar structure 90 degrees about the Y axis (90 degrees about 010).
The other way to get the columnar grains aligned along Z is to enforce an orientation distribution of the columnar grain axes when you create the grain statistics in StatsGenerator (this is of course assuming you are creating synthetic microstructures). In StatsGenerator, when you create the rolled grain statistic, you can access the Axis ODF tab to adjust how the A/B/C axes of the columnar grains are oriented in space. Note that DREAM.3D considers A >= B >= C, so the A axis is always your "long" axis. Given this, you would want the A axis to be aligned along Z. You can achieve this by entering Euler angles of (0, 90, 90) into the Axis ODF tab, with a high weight value (something like 1e6, just to saturate the ODF). This will produce an ODF where the A axis (that is, the long axis) is strongly oriented along Z, the B axis is strongly oriented along X, and the C axis is strongly oriented along Y. See the attached screenshot for what the pole figures would generally look like.
hope that helps,
-Sean