Steel concepts associated with an efficient use of resources are currently the subject of intensive research. Aluminum addition to steel offers advantages in terms of lightweight design and enhanced strength. In the present research, up to 6 wt.% Al was added to Fe-12Cr-9Ni-3Mn-0.3C (wt.%) stainless steel to assess its influence on the microstructure and mechanical properties after welding by the tungsten inert gas (TIG) process. Microstructures after autogenous welding and after welding with a filler metal were studied by light optical and scanning electron microscopy (SEM) examinations. The microstructures in the fusion zone (FZ), heat-affected zone (HAZ), and base material (BM) were correlated with thermodynamic calculations. Furthermore, hardness maps covering FZ, HAZ, and BM were generated and related to the corresponding microstructure. In steels containing up to 3 wt.% Al, the microstructure after autogenous welding remained almost fully austenitic. Hard zones indicative of embrittlement were absent, with hardness remaining below nearly 240 HV. In the case of autogenously welded steels with 4.5 and 6 wt.% Al, the presence of ferrite resulted in an increase in hardness to levels up to about 440 HV. Significant hardening due to ferrite was justified by the formation of B2-(Ni,Fe)Al intermetallic precipitates within ferrite. Hardness distribution across FZ, HAZ and BM in duplex microstructures was interpreted in terms of the thermal history, which in turn controls the ferrite fraction and the size and fraction of strengthening B2 precipitates. Welding with 308L as filler metal resulted in fully austenitic FZs exhibiting hardness values comparable to those of autogenously welded austenitic steels. In the case of steels with 4.5 wt.% and 6 wt.% Al, duplex microstructures were obtained in HAZ and BM. Accordingly, hardness values significantly exceeded those in the FZ. In summary, Al-added stainless steels in the present research exhibit an adequate weldability as long as the microstructure in FZ and HAZ remains austenitic or duplex with only small fractions of ferrite.