HOW WE HOT PRESSED A HEMISPHERE FROM A SHEET OF Ti-6Al-4V

Our success in welding Ti-6Al-4V was noticed by Defense Research and Development Organization(DRDL) in Hyderabad. They had a need for spherical titanium gas bottles for their missile applications. DRDL was importing these bottles from France. The hemispheres were forged in France and after machining the two hemispheres were welded by Electron Beam welding. When DRDL approached R&D of BHPV, we told DRDL that we don't have forging facilities and we have no plans to install an Electron Beam Welding machine but we may able to hot press Ti-6Al-4V hemispheres. machine it to the required contour and weld the hemispheres by conventional TIG welding. We also guaranteed that we will meet their specifications regarding hydraulic test,increase in volume during hydraulic test and burst pressure. DRDL reposed confidence in us and gave us a trial order.
If I look back I wonder the self confidence we had to tackle such a complex problem. We had to develop whole lot of technologies from scratch. We had to develop tools for hot pressing, machining the hemispheres on a lathe to close tolerances and weld the hemispheres. Possibly because our team members were young and full of enthusiasm and self confidence. We will tell you later how we went about the development.

WELDING Ti-6Al-4V

Welding Ti-6Al-4V

Having mastered welding of commercially pure titanium we decided to continue our work on Ti-6Al-4V alloy of titanium which is called the work horse alloy in defence and aerospace industries.

This alloy has the following mechanical properties:

Ultimate tensile strength: 950MPa or 138kpsi

Yield strength: 880MPa or 128kpsi

Elongation: 10 to 14 percent.

These are typical values. Since Ti-6Al-4V is a heat treatable alloy the above strengths can be increased by suitable heat treatment.

For aerospace applications, the above alloy containing very low oxygen and nitrogen designated as ELI grade(Extra low interstitial) is used.

The welding of Ti-6Al-4V is not different from that of commercially pure titanium except that we used ultra pure argon gas to keep the interstitial low. Ti-6Al-4V is welded by TIG welding using commercially pure titanium filler wire or by matching composition filler wire or by the so called half alloy which is Ti-3Al-2V filler wire. The selection of wire depends upon the final property desired in the final weld.

We welded Ti-6Al-4V in annealed condition, solution annealed and solution annealed and aged condition. We also used commercially pure filler wire, matching composition filler wire and also half alloy wire. Welded test coupons were subjected to extensive testing. Test results were tabulated so that we have data bank to select what type of filler wire to use for a specific application.

The data so collected by us was used for defence application about which I will tell later.

Organising R&D in BHPV

I was selected to establish R&D in BHPV. We trnaferred some postgraduate engineers from various department to form the nucleus. Full freedom was given to me to organise the R&D.

We decided to form four groups in consultations with the user groups, namely,

1. Chemical enginering group.

2. Cryogenic engineering group.

3. Manufacturing technology group.

4. Welding technology group.

Chemical engineering group concentrated on the develolpment of computer programmes for the design of various equipment such as heat exchangers, columns etc.

Cryogenic engineering group concentrated on the development of cryogenic equipment such as cryogenic storage systems, liquid nitrogen and oxygen transfer sysytems etc,

welding titanium

This goes back to 1976. When welding technologists in India were finding it difficult to weld alloy steels we were developing welding technology for commercially pure titanium.

Titanium is a highly reactive material which has great affinity for oxygen and nitrogen. Titanium cannot be welded by shielded metal arc welding. Similarly, at that time neither submerged arc welding or MIG welding. In recent times Russians have developed submerged arc welding technique. The preferred method of welding is by gas tungsten arc welding also called as TIG welding.

Welding titanium is different from welding stainless steel. While welding stainless steel by TIG welding the gas shielding is provided to the weld pool from the gas flowing through the torch and a separate gas flow is provided to the bottom side of the weld (called back purge) to prevent the weldment getting oxidized.

While welding titanium in addition to the the gas flow through the torch and back purge you have what is called a travelling shield.The travelling shield is an attachment fixed to TIG torch which blankets the molten side of the pool as the torch moves forward and also on either side of center of the weld. This is because titanium continues to absorb oxygen and nitrogen right down to temperature down to 500 deg. celsius. When nitrogen and oxygen are absorbed the weldment becomes brittle. The UTS of weldment will be high and hardness increases.

The sucecess of welding titanium depends upon the clealiness you maintain and the design of the trailing shield. We developed a method of designing the trailing shield based on calculated temperature distribution for a given welding parameter. We had developed trailing shields for butt weld, tube to sheet welding, corner welding etc. Using the technolgy developed in
R&D we manufactured titanium lined urea reactor, titanium heat exchangers, titaniun anodes and number of equipments of titanium.