Did the union soldiers fought against slavery during the US Civil War?

No - no more than the Confederate rank-&-file (poor white trash) were fighting to defend slavery.

Most Northerners were not Abolitionists, and they were fighting to save the Union and regain the cotton revenues.

After Lincoln issued the Emancipation Proclamation (effective January 1863), Abolition became an official war-aim, but this was chiefly to do with keeping Britain and France from aiding the South.

The North did not respond enthusiastically to the Proclamation, and Union troops were not keen on admitting fugitive slaves into the Union army.

However, those same troops could see the point of freeing the slaves they came across in their Southern campaigns, to deprive the enemy of his workforce, and this ended the Southern way of life - and slavery - for ever.