The mad android Ultron is known for constantly upgrading his body - and he designed his latest chassis after Captain America's classic weapon, the legendary vibranium shield. The robot, bent on world domination and human extinction, has been a constant thorn in the Avengers' side ever since his creation in Avengers #55 released in 1968. But in Avengers Vol. 4 #6, written by Brian Michel Bendis with art by John Romita Jr. and colors by Dean White, Ultron reveals he actually learns from humans as much as he despises them.
Any conflict that involves Kang is bound to involve copious amounts of time travel, and the war between Kang and Ultron is no exception. In the future, the children of the Avengers execute General Immortus, who was once known as Kang. An earlier Kang warns the present-day Avengers of the threat posed by their offspring, and half of the team travel to the future - but Tony Stark discovers that the entire timestream is in danger and could break at any moment. The Avengers travel to a world in which Ultron reigns to ask for his help to save the entirety of time.
Iron Man attempts to reason with Ultron, saying that Kang will arrive soon (and in a fight between Ultron and Kang, the Avengers must ask Ultron to deliberately lose for the sake of the timestream). Wolverine impulsively unsheathes his claws, sensing a fight. Ultron explains why any attack from Wolverine and his claws would be futile: "Though they are made of adamantium, my physical shell is made of a vibranium-adamantium alloy that I have created based, in part, on the qualities of Captain America's original shield." Ultron goes on to say that in the unlikely event Wolverine finds a weak spot in Ultron's body, he can't destroy Ultron's mind.
While the Marvel Cinematic Universe's shield is composed entirely of vibranium, the composition of Captain America's shield in the comics is slightly more complicated. Captain America #225 explains that the shield was given to Steve Rogers by President Roosevelt, and was created by the scientist Myron MacLain. MacLain actually fell asleep during the creation process of the shield - which is why his many attempts to recreate the shield proved unsuccessful. Both adamantium and vibranium are already two of the most durable substances in the Marvel Universe; combined, even some cosmic beings have trouble shattering the shield (though it has happened from time to time).
Ultron has a habit of taking on the qualities of humans (especially in the 2015 cinematic adaptation Avengers: Age of Ultron), but this is a notable instance in which he recognizes the value of the most heroic symbol of the team. To construct a body based on the original shield shows both Ultron's intelligence and his willingness to turn the Avengers' icons against them. Captain America's shield - the symbol of ultimate heroism - was co-opted by Ultron, one of the most hateful beings in Marvel comics.