Form data is empty while posting form through ajax using jquery in WordPress

Try using .serialize() instead of FormData

 function step1SaveData(){
    var formData = jQuery('#tpform1').serialize();
    console.log(formData);
    jQuery.ajax({
        type:"POST",
        url:'http://lexem.in/wp-admin/admin-ajax.php',
        data:{
            action:'tpartners',
            formdata:formData,
        },
        success:function(data){
            var insertedID = data.trim();
            if(insertedID!='fail'){
            }else{
                console.log('fail');
            }
        }

    });

}

or use .serializeArray() if you want your data in an array instead of a string.

EDIT – from the comments, remove processData:false and contentType:false from the ajax call