String replace is not performing exchange

I can not change the character of my string with the value of the map, if they are equal: key and character.

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class Mapa {

    public static Map<Character, Character> mapa;

    public static void main(String[] args) {

        mapa = new HashMap<>();

        mapa.put('á', 'a');
        mapa.put('é', 'e');
        mapa.put('í', 'i');
        mapa.put('ó', 'o');
        mapa.put('ú', 'u');

        String str = "cása";        
        for(int i=0; i < str.length(); i++) {
            Character ch = str.charAt(i);
            for(Entry<Character, Character> entry : mapa.entrySet()) {
                if(ch == entry.getKey()) {
                    str = str.replace(ch, entry.getValue());
                }
            }
        }

        // Aqui esta retornando a mesma string.
        System.out.println(str);
    }
}