Manganese is a strong deoxidize. If it isn't "consumed", i.e., needed for deoxidation, it acts as an alloy addition to the iron and increases the hardness and strength.

Manganese is also usefull to counter the ill affects of sulfur. In general, the manganese content should be on the order of ten times the sulfur content.

I don't believe the typical amounts of manganese found in carbon steel filler metals reduces the weldability of steel unless the manganese content is on the order of 4% in which case you have a high alloy steel that can be work hardened.

Best regards - Al

Deciding if you have too much Mg in your wire is dependent upon the application. I agree with what Al posted but in my case our shop fabricates piping for Sour Service applications. Our WPS's comply with ASME Sec IX and NACE RP0472. Excessive Mg in the wire will increase weld metal hardness which is undesirable in this enviornment. I restrrict Mg content for our carbon steel wires (Er70s) to no more that 1.6% This does reduce fluidity but not enough to cause a problem. The result is an "as welded" deposit with about a 180 BHN. My point is that you can't say that elevated Mg levels are good or bad without considering the enviornemt the finished weld will be subjected to.