"Welcome back, Dad."
Well, it could also just be a case of talking to someone else about their father, for example. So when you want to ask "How is your father", you would probably say 「お父さんはげんきですか？」 or some such. When you ask your father how he is, I suppose it would be 「お父さん、げんきですか？」 and when you want to ask your sibling, say, how your own father is, then I guess it could be 「父はげんきですか？」 Mind you, I'm a learner myself, so take all this with a grain of salt.
ちち is used when referring to your own father お父さん is used when referring to another person's father ^ both cases are when you are talking with someone outside of your family. Using either ちちor お父さん when talking ABOUT your families establishes an easier way to know who you are referring to.
When talking to your own father you can use お父さん, 父さん,etc.
This is a good question. I feel that in Japanese, the person you are talking to or about is named first, but Duo has lots of these, "Welcome home, Father" type phrases where I want to type お父さん, おかえりなさい but they have it the other way around. I hope someone will be able to answer your question and clear it up! :)
お父さん (otōsan) is also father, but it's the formal, polite version. Have a read of the duolingo wiki if you're interested. I hope it helps! https://duolingo.fandom.com/wiki/Japanese_Skill:Family
No, one never uses "chichi" when addressing one's father. We always use otousan, or "tousan." You may, however, use "papa"! When children talk about their own dads, they may say "otousan," but by the time they're teens and certainly when they are adults, the humble "chichi" is used when talking ABOUT one's own dad, but never when talking TO him. It's all about circles. Inside the family circle, you're respectful to all persons older than you. When talking to someone outside your circle, you refer to yourself and your family with humble words. The same thing goes for the office circle, the friends circle, etc.