A heat exchanger comprising: a first core having a first end and a second end, comprising; a first plurality of first layers, wherein each of the first plurality of first layers comprises a first channel having a first channel inlet at the first end and a first channel outlet at the second end configured to convey a first fluid from the first channel inlet to the first channel outlet; a first plurality of second layers, wherein each of the first plurality of second layers comprises a plurality of fins that define a first plurality of second channels, and wherein each of the first plurality of second layers is brazed to at least one of the first plurality of first layers; wherein the first plurality of first layers and the first plurality of second layers are arranged in an alternating stack; a first port disposed at the first end of the first core, wherein the first port provides the first fluid to the first core; and a first plate disposed at and surrounding the first channel inlets of the first channels of the first plurality of layers, wherein the first plurality of first layers of the first core and the first plate are joined via a first joint that comprises a friction-stir weld devoid of brazing material; and wherein the first plate and first joint collectively define a barrier that fluidically isolates the first port and the first plurality of second layers. a first core having a first end and a second end, comprising; a first plurality of first layers, wherein each of the first plurality of first layers comprises a first channel having a first channel inlet at the first end and a first channel outlet at the second end configured to convey a first fluid from the first channel inlet to the first channel outlet; a first plurality of second layers, wherein each of the first plurality of second layers comprises a plurality of fins that define a first plurality of second channels, and wherein each of the first plurality of second layers is brazed to at least one of the first plurality of first layers; wherein the first plurality of first layers and the first plurality of second layers are arranged in an alternating stack; a first port disposed at the first end of the first core, wherein the first port provides the first fluid to the first core; and a first plurality of first layers, wherein each of the first plurality of first layers comprises a first channel having a first channel inlet at the first end and a first channel outlet at the second end configured to convey a first fluid from the first channel inlet to the first channel outlet; a first plurality of second layers, wherein each of the first plurality of second layers comprises a plurality of fins that define a first plurality of second channels, and wherein each of the first plurality of second layers is brazed to at least one of the first plurality of first layers; wherein the first plurality of first layers and the first plurality of second layers are arranged in an alternating stack; a first port disposed at the first end of the first core, wherein the first port provides the first fluid to the first core; and a first plate disposed at and surrounding the first channel inlets of the first channels of the first plurality of layers, wherein the first plurality of first layers of the first core and the first plate are joined via a first joint that comprises a friction-stir weld devoid of brazing material; and wherein the first plate and first joint collectively define a barrier that fluidically isolates the first port and the first plurality of second layers. <EOS>