search for a method to join 2 pieces of C45 steel together 

which doesn’t involve heat-input above 120c
which provides a join with similar strength to a spot weld
which needs little interface 4mm(squared) (very small attachment area)

The connection must stop movement in the X,Y,Z direction

You could sumarize by saying we require something that is exactly the
same
as spot welding but is not actually spot welding ! !

The reason for this need is to reduce costs and allow the 2 pieces of
metal to be painted before they are joined.

WE have considered but please provide extra information about, (all of
which have problems if the pieces are painted first:
Glue 
Velcro
Clamping 

Areas of interest:
Ultrasonic bonding

Operating conditions:
The created item (join) must be able to with stand a temperature range
of –40c to 120c

Date required 25th August.