Here is a DIY metal 3D printer using a MIG welder. http://www.3dprinterworld.com/articl...ed-mig-welders The output is not exactly high res but there may be scope for refinement.

I had seen the mini metal maker that uses metal clay from a syringe and relies on firing in a kiln after printing. Parts shrink heaps with firing. This thing is really only good for making jewellery. I doubt it would scale economically or technically to normal reprap scale parts.

I have also read of someone making an ABS female mould and casting low melting temperature metal into it, then removing ABS with acetone. Messy but do-able for decorative (but not particularly durable) metal parts.

I know that it isn't exactly DIY, but using your 3D printer to make PLA parts then having them turned into metal castings at an investment casting shop is a damn fine alternative to direct 3D metal printing. I have recently used this path to make some replica parts in brass for a vintage car. Finished parts come out exactly the same (but 2% smaller due to shrinkage) including detail of 3D printing surface finish. You could make parts in any metal that your local casting shop handles - aluminium, bronze, brass, stainless steel, cast iron. It won't replace normal processes for investment casting of parts in significant numbers, but for one or two off it is fantastic.