  Recent observations suggest that some type Ia supernovae (SNe Ia) originate
from the merging of two carbon-oxygen white dwarfs (CO WDs). Meanwhile, recent
hydrodynamical simulations have indicated that the accretion-induced collapse
may be avoided under certain conditions when double WDs merge violently.
However, the properties of SNe Ia from this violent merger scenario are highly
dependent on a particular mass-accretion stage, the so-called WD + He subgiant
channel, during which the primary WD is able to increase its mass by accreting
He-rich material from a He subgiant before the systems evolves into a double WD
system. In this article, we aim to study this particular evolutionary stage
systematically and give the properties of violent WD mergers. By employing the
Eggleton stellar evolution code, we followed a large number of binary
calculations and obtained the regions in parameter space for producing violent
mergers based on the WD + He subgiant channel. According to these simulations,
we found that the primary WDs can increase their mass by ~0.10-0.45 Msun during
the mass-accretion stage. We then conducted a series of binary population
synthesis calculations and found that the Galactic SN Ia birthrate from this
channel is about 0.01-0.4*10-3 yr-1. This suggests that the violent WD mergers
from this channel may only contribute to ~0.3%-10% of all SNe Ia in our Galaxy.
The delay times of violent WD mergers from this channel are >1.7 Gyr,
contributing to the SNe Ia in old populations. We also found that the WD + He
subgiant channel is the dominent way for producing violent WD mergers that may
be able to eventually explode as SNe Ia.
