A new breed of turbochargers constructed of super tough alloys operates at higher temperatures and rotational speeds than ever before, resulting in greatly increased output in a smaller package for gas and diesel engines alike.
Turbochargers contain three main components—the turbine, compressor, and central housing. Exhaust gas is used to spin the turbine, which drives a compressor that forces fresh air into the engine’s combustion chamber. Cast iron has long been the material of choice for the turbine manifold and housing, but with turbochargers now spinning at up to 350,000 rpm and reaching temperatures greater than 1300°C, more heat-resistant metals have become necessary.
One of these is GX40CrNiSi25-20, an austenitic stainless steel composed of 25% chromium, 20% nickel, small amounts of silicon and manganese, and iron. Due to its excellent corrosion resistance and mechanical strength at elevated temperatures it has recently become a favorite for use in “warm side” turbocharger components. The downside of GX40CrNiSi25-20 is that it is very difficult to machine, and the manufacturing cost of the raw casting is quite high.
The challenges of turbocharger machining extend beyond the metal’s toughness. The central housing is awkwardly shaped but contains several round features that some manufacturers opt to turn rather than machine. This makes workholding difficult and limits spindle speed. A “V-band” on the exhaust side of the housing requires special grooving inserts and an engineered tool to reach around the face to machine the backside, while the inside of the housing has several close-tolerance bearing bores and sealing surfaces.
Proper carbide and toolholder selection is critical for tool life with tough metals such as this. Sandvik Coromant recommends shock-resistant CVD-coated GC2025 for dry machining in roughing operations and GC1010 PVD-coated carbide or equivalent for finishing with coolant. For boring, a Silent Tool or comparable vibration dampening bar should be used. In either case, a positive rake CCMT-style insert generally provides the best tool life—customer tests demonstrated up to 13 minutes of cut time per edge at 100 m/min surface speed, a feed rate of 0.10 mm, and 0.5-mm DOC.
Provided you have a rigid setup, custom engineered form cutters are a good solution for single operation machining of internal features such as spot faces and sealing surfaces in both turning and milling applications. This approach substantially shortens cycle times, which considering Honeywell’s prediction that 200-million+ turbocharged vehicles will be produced by 2019, is an important consideration for those making high volumes of turbo components.
High pressure coolant (HPC) is another important consideration. Turning tests showed a seven-fold improvement in tool life when finishing GX40CrNiSi25-20 at HPC pressures of 10 MPa, compared to standard machine tool coolant pressure of 1.4 MPa or less.
For milling the various mounting faces on exhaust manifolds and central housings, cost per insert edge is critical to machining profitability. Roughing and semifinishing operations using a double-sided button style face mill provide predictable tool life, often completing dozens of parts per edge between indexes, while a positive rake, wiper-style PVD-coated insert should be used for finishing.
Considering the tensile and fatigue strength of GX40CrNiSi25-20 and other high-temp, nickel chromium alloys, the tool failure modes during machining are unsurprising: flank wear due to the material’s extreme toughness, and edge chipping caused by vibration and end mill hammering. Cutting speeds in the 80 to 110 m/min range are a good starting point, with light depths of cut and moderately high feed rates (assuming a rigid setup). As with most superalloys, cutting forces are high, making short robust toolholders with minimal overhang a must. Hydraulic clamping is likewise recommended, as is program optimization to provide a smooth, jerk-free toolpath and consistent rates of metal removal.
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