to volunteer

• 自愿原则

  principles of free will

• 根据自愿原则

  on a voluntary basis

• 自愿参加

  voluntary participation

• 出于自愿

  of one's free will

• 并非出于自愿

  not by choice

• 这是我自愿的。

  I'm doing this by choice.

• 他自愿前往。

  He volunteered to go.