Aluminium and Copper Joints

A. Heat-up at joint area is because the bolts and nuts, I guess you use ss.

A. Probably, oxide on the aluminum surface resulted in electrical resistance causing heating and further oxidation... I will presume that the 3 aluminum bars are interleaved between 4 copper bars (minimum for 4000 Amps), perhaps in a right angle connection, and that this joint is compressed together with a single set of 6 bolts.

Ideally, the aluminum contact areas are plated with silver.
A less expensive alternative is to clean and protect the contacts as follows: Remove the oxide from contact areas, either chemically or by very fine grit sanding, apply electrical joint compound (EJC) such as Alcoa No. 2, then use a stainless steel brush or emery cloth to lightly abrade the aluminum surface working through the EJC layer(to expose bare metal while preventing oxidation). Also clean the copper contacts. Then, the joint should be assembled with a clamping pressure of 1000 psi. This reduces the contact resistance to 0.5 micro-ohm/sq. inch according to Figure 53 in the Alcoa Aluminum Bus Conductor Handbook. Presuming a 2 x 3 bolt pattern in an 8x4 inch contact area, use 1/2-inch bolts tensioned to 6000 lbs along with heavy flat washers and stainless steel Belleville washers (which maintain the load over time). If available, use high-strength aluminum fasteners lubricated with EJC; if not, use ASTM A325 steel or equivalent.

See also Letters #7897, 29456, 39557, 40211 dealing with aluminum bus bars.