You can either say أهلا و سهلا or مرحبا and adding يا to each of them sounds more warm and used usually with people u know ( someone you have been waiting for ) because without يا is very formal and rarely used with people u know يا مرحبا يا اهلا و سهلا

No ,"اهلا بك " Does not express heartily greeting. it is very formal. you can say يا أهلا وسهلا this يا gives it that "heartily".

Hello / hi = مرحباً / السلام عليكم welcome = أهلاً وسهلاُ / أهلاً بك / أهلاً

you can say أهلاً وسهلاُ

you can express it in many situation: أهلاً بك -to welcome someone. -also mean hello, hi when some one say (hi, welcome you) they 'll say أهلاً وسهلاُ / أهلاً بك / أهلاً