Workers at Alcoa's Pinjarra, Wagerup, and Kwinana refineries have walked off the job following a breakdown in negotiations over pay and conditions.
Australian Workers' Union (AWU) members employed by the mining giant met in Pinjarra on Thursday before starting a four-day work stoppage.
AWU secretary Mike Zoetbrood said minesite workers had been taking industrial action for "some time", and continued to take action.
"The protected industrial action comes after more than 14 months of negotiations for a replacement Enterprise Agreement," he said.
"[This] is not the path preferred by the union and the membership.
"There are no unreasonable claims from the union."
Mr Zoetbrood said the outstanding issues concerning Alcoa workers revolved around job security and preserving current terms and conditions of employment.
"The union will continue to bargain in good faith to secure a fair and reasonable outcome with Alcoa in the best interests of our members," he said.
Negotiations between Alcoa of Australia and AWU workers have been ongoing since December, 2016.
An Alcoa spokesperson acknowledged the strike action on Friday and said sites affected by the work stoppages had contingency plans in place to ensure continued operations.