• Overview• Stainless Steel to Titanium joining (temp connection)• Copper to Titanium (pressure drop measurements)
ATLAS Upgrade dissimilar metal joining activities
Richard French, Paul Kemp-Russell: SheffieldKeith Birmingham: Aerobraze EuropeNeil Austin: VBC Group Ltd (brazing division)Trevor Smith: Firmachrome LtdPeter Cookson: Bodycote
Stainless VCR connector to Ti tube Used to make temporary test fitting stubs for cooling
circuits………… Method A = cheap idea
• Electroless Nickel Plating.• Electroless Nickel coating is an alloy of nickel
and phosphorous. Ability to work to close tolerances without post-plating grinding, whilst holding the original surface finish.
• Electroless Nickel can improve corrosion resistance, wear resistance, lubricity, solderability or be used to rectify and recover close tolerance undersize parts.
• The big advantage of elecroless (chemical) plating over electrolytic plating is it will adhere to Titanium, in a very controlled manner. We do not need to plate the entire circuit, just local areas as wherever the solution touches it will plate.
Method B = proven but expensive• Vacuum Brazing using an
aerospace proven method• Using a silver copper eutectic braze alloy, coat
the Ti with the alloy, assemble the Stainless VCR fitting to the tube (post electro-polishing) and place in furnace at a lower temperature somewhere around 850°C.
• Excellent joint, clean and pressure handling proven up to 250bar.
• 3.175mm Ti has heavy oxidisation that is proving difficult to remove. This is needed for the Cusil alloy to adhere to the Ti tube.
• Once tube is cleaned (glass bead blasting) good adhesion is found.
• Both methods are the “active” or direct joining of dissimilar materials with a braze filler metal (BFM). This is ideal for Ti as the BFM forms a strong permanent joint with the base materials.
• What we did not realise is that at certain temperatures the Ti can suddenly start taking on alloying abilities with the BFM. This should not have happened when correctly controlled.
• Titanium is a strong oxygen-getter, and thus will react with any oxygen that it can as it is heated from room temperature up to brazing temperature, therefore, "reacting" too early.
• This is with free oxygen or water-vapor in the furnace atmosphere, or with metal-oxides on the metal surfaces during heat-up (such as when the metals are not properly cleaned prior to brazing), then the so called “brazing” (joining/bonding) of alloy-to-metal may be completely prevented from happening.
Method A• During this process, the bonding
was be very successful but, the difference in the thermal expansion between the 6LV stainless steel – Nickel – Titanium which it is being joined caused premature cracking to develop in the brazed joint upon cooling. If we did not see the cracks at this stage they would present themselves during subsequent use in service.
• It is very important to try to match the expansion characteristics of the metals to filler to metal joint, so that huge stresses in the joints are not built up.
• During the furnace cycle (1100C) something really odd happened. High temp was down to a miscommunication.
• The braze alloy has a initial melting temp of around 400C. The furnace temp was in the region of 1100C. Therefore we managed to alloy Ni with Ti:
Stress cracking CTE mismatched
Ni-Ti alloyed tube
As the Ti reaches 600C the oxygen in the Ti starts getting thirsty and in the resulting exchange drags the Ni into the microstructure. As the assembly cools, it falls apart as the CTE mismatch is beyond what the structure can cope with.
GOOD NEWS – we don’t necessarily need to vac braze all our components and can do this with any induction furnace (have small tube furnace in lab ready). Ni plating works fine so will drop the cost of the heater block joining for pressure drop work.
Method B•For mechanical tolerance, achieve a good push fit in the Ti tube to VCR fitting. Micropolish fitting.•Using Cusil braze alloy from VBC simply clean components and plate the Stainless Steel component.•Mechanically clean Ti, chemically clean the Ti, assemble components and remember your nuts.•Place in furnace at 850C and cycle once allowing time to cool. •Bingo – one joint.•Items of weirdness to note:
•The VCR nut threads are silver plated to prevent gauling during assembly. This silver is reflowed during the furnace cycle.
1/8” Ti to VCR
1/8” Ti to VCR
2.275mm OD Ti to VCR
5Costs to date (A) Electroless nickel plating of Ti
Cost of initial trials on 1 off previously de-scaled tube Background research £60Materials £1610 litre solution make up £13processing trials £30
Sub total £119
Cost of initial trials pickling heavily scaled tube Background research £30Materials £810 litre solution make up £13processing trials £20
Sub total £71 Total to date £190
Immediate cost to incur in Feb Further cost of pickling trials (expected costs) Processing trials £100 If the trials are successful no further costs are expected for trial purposes
Best guess prices for processing based on tube length inc bendsupto 100mm £3.50100 to 250mm £5.00260mm + £2.50 per 100 mmelectropolishing small stainless steel components @£0.25 each
SCALE UP COSTS
This is the preferred method for joining the Cu heater blocks to the Ti and 316L tubes as we can carry out the brazing in-house using our mini tube furnace that will cycle at 700C to 1100C so 850C is attainable.
6Costs to date (B) Cusil vac brazing of Ti
Braze alloy:50.00 gms @ £8.86 per gramme0.9 mm Dia VBC ALLOY 4010 CUSILBS EN ISO 17672:2010 AG272V1£443.00 Delivery Charge £15.00Total £458.00
No costs incurred. All R&D free through good will.
Brazing and assembly:Furnace cycle charged at £500 per cycle (we will process all parts in one cycle)£10 per joint for assembly and cleaning.Copper plating MOC of £359 (currently a 7-10 day lead time)
SCALE UP COSTSPreferred solution for manufacture ot Ti to VCR test stubs for welding.One furnace cycle will make enough parts for 2012 to 2014 when correct OD tube selected.50g of alloy equates to ~8m length . 200mm length completes 30 joints= 1200 joints possible per furnace cycle.= ~ £ 3500 for more than we need!