Intake manifolds on both 4 and 6 cylinder engines are cast aluminum. The exhaust manifolds on the V6 are 409 stainless steel. I don't know what the 4 cylinder exhaust manifolds are made of as its been over 25 years since I've owned one.
409 stainless. You can weld them!! And, you don't need special wire. I just welded my forward exhaust manifold while still in the car. It was split between #4 and # 6, but more on number 6 all the way around. Might be caused by the weatherstrip thing that makes water drip on that area. If you use regular wire feed wire it will eventually rust where special stainless wire won't. I welded as much as I could from under the car and then did the top.... it was apita because you can only get at it from a certain angle. Turn the heat down because they are not super thick. After I finished welding it, I started it and made sure it was no longer leaking. There was a very very small pin hole that I could not get at with the welder which I rubbed in some "muffler weld putty stuff". Fixed it!!! Completely quiet!!! I was afraid of trying to replace the manifold due to the possibility of broken bolts.....then cradle drop and Rodney's drill kit or head removal all together. Hope this helps. Marshal
You can also braze them. Clean, brush-on flux helps, braze with a coated rod, or plain dipped in flux. Doesn't flow out that well, but if you can braze, you CAN do it. Less chance of cracking adjacent to the repair, hold up well, Standard precautions against warpege.