Seal elements for establishing fluid seals in a wellbore may be constructed of expandable metal materials. For example, an elongated gland of expandable metal may replace some or all the components in a packing stack of a locking mandrel. The gland may be recessed within the locking mandrel, or narrower than a seal bore of a landing nipple, such that the gland is protected from damage as the locking mandrel passes through landing nipples to reach a target landing nipple. The expandable metal may be induced to chemically react with a brine or other water-based fluid to form metal hydroxides and may thus create a seal with the target landing nipple. Once an intended purpose of the locking mandrel has been achieved, a pressure reversal may be employed to break the seal and/or a mild acid may be introduced to accelerate dissolving of the expandable metal seal.