Well, From my experince Inventor is not good in FEA try to use Ansys or Visual Nastran, i don't know anything about inventor 2010 , but i work with inventor for almost 5 years & the version i'm using now is inventor 2009, so they may enhance some feautres about FEA in Inventor 2010 ,
inventor is Mainly CAD program & not FEA Program, that's why it can't solve welded parts or 3d contact parts
i hope you found this information useful,
if you have updated information about this issue please don't hesitat to share it with me
have a good day
I've Searched The Inventor Forume, Some Suggestions Where About Adjusting The Mesh Setting "Average Element Size". Change from the default 0.1 to 0.075 Or Even Smaller.
I Dont Know If This Will Help, Right Now I Dont Have Inventor So Its Strange To Answer Without Trying!