Remove Vowels - Global Variables

#include <iostream>
#include <string>
using namespace std;
string word, word2; //string


void substr()
{
    for (int i = 0; i <= word.size(); i++)
    { //removes vowels
        if (word[i] != 'a' && word[i] != 'A' && word[i] != 'e' && word[i] != 'E' && word[i] != 'I' && word[i] != 'i' && word[i] != 'o' && word[i] != 'O' && word[i] != 'u' && word[i] != 'U')
        {
            word2 += word[i];
        }
    }
}

int main()
{
    cout << "please enter a word: ";
    cin >> word;
    substr(); //calls to remove vowels
    cout << "your new word is: " << word2;
    return -0;
}