with pandas create column from two existing columns on two conditions

Question

I want to fill a new column by merging two existing columns on specified conditions.

create example df

data = { 'T1': [1,1,1,1,1,2,2,2,2,2],
         'T2':[1,2,3,4,1,2,3,4,1,2]}
df = pd.DataFrame(data)

create empty new column

df['new']=''

tried it with a function

def merge(row):
    if row['T1']==1 & row['T2']==1:
        val='1_1'
    elif row['T1']==1 & row['T2']==2:
        val='1_2'  
    elif row['T1']==1 & row['T2']==3:
        val='1_3'
    elif row['T1']==1 & row['T2']==4:
        val='1_4'
    elif row['T1']==2 & row['T2']==1:
        val='2_1'
    elif row['T1']==2 & row['T2']==2:
        val='2_2'
    elif row['T1']==2 & row['T2']==3:
        val='2_3'
    else:
        val='2_4'
    return val

df.loc[:,'new']=df.apply(merge, axis=1)

but getting wrong results

T1  T2  new
1   1   1_1
1   2   2_4
1   3   1_1
1   4   2_4
1   1   1_1
2   2   2_2
2   3   2_2
2   4   2_4
2   1   2_4
2   2   2_2

Can anybody see what I'm doing wrong? Or maybe there is a better way?