So many choices, so little time
Just thought I'd put my 2 cents in here as I've got some experience with most of the materials you have all suggested.
Steel would work for this, but the fabrication would drive any sane person nuts, and any of us even nuttier. But I've always thought that the Mjolnir armor looked like press molded steel...which makes sense why so many people are vacuum forming it. But looking at the Halo 3 screens it looks more, cast...like an engine block. Problem is you cant cast steel very thin conventionally because of the flow charatceristics and the fact that sand molds aren't terribly accurate in an application like this.
Steel is ruled out, unless you want to fabricate it by hand...and if you do, bless you, and say goodbye to your friends and family because they aren't going to see much of you for a long time. I fabricated my suit out of sheet polystyrene, then reinforced it all with fiberglass resin, so it can be done, it's just not very practical.
Speaking of practicality...why would you really need a Halo Armor suit that could stop a bullet? Planning a heist? Going to test the mjolnir design with real bullets? They'd just shoot you in your plastic faceshield! It just really isn't necessary for a costume.
As far as titanium...maybe if you're Bill Gates. The problem with Titanium is thats it's damned difficult to cut, weld, and cast. It's melting temperature is higher than most ductile metals and it's ungodly expensive and hard as diamonds, but very brittle. But you can get it in sheets, so if you had like 5 years to spare, you could potentially do this. Tig weld everything, and grind and sand it til it looks rounded and smooth. Then call the Guiness Book people.
Aluminum is doable. Cast it in investment molds and sand it smooth. Still it's not very strong or maleable. Better be sure that your patterns are perfect because you're not going to be able to bend it much.
And finally ceramic plates...Ceramic materials can be engineered to be super stong, bulletproof, and lightweight. But engineering ceramic is a hugely expensive process...more expensive than titanium, thats why only the military is really using engineered ceramics to their potential. I can't even begin to tell you of the multiple layers of problems with making something for yourself in this material...and I went to the New York State College of Ceramics. Engineered ceramic is just not doable unless you have access to all the equipment and expertise already.... In which case, could you loan me like $25,000, cause you'd have to be rich and gullible to try this.
Listen to these guys, they know what they are talking about. Vacuum form it in polystyrene or cast it in resin. Doing it any other way is going to cost you like 10 times as much and really won't be useful in any recognizable way.
