i used this code in 4-5 projects and working perfectly, Check if helps
$objPHPExcel->getActiveSheet()->setTitle('Customers');
$objPHPExcel->setActiveSheetIndex(0);
header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
header('Content-Disposition: attachment;filename=Customers-"' . date('d-m-y-h-i-s', time()));
header('Cache-Control: max-age=0');
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save('php://output');
exit;