BILLET COMPRESSOR WHEELS

Billet versus Cast impellers – (MFS) Machined From Solid compressor wheels

Impeller durability is a key factor for all high horse power applications. The drive for greater horsepower and torque, plus the desire for high boost pressures up to 4,5/5:1 pressure ratio in single stage Truck- and Tractorpulling applications has led to the increase of rotational turbo rotor speeds and temperatures going up to and over the limits of the standard materials used.

Why billet impellers are a premium option? Read more....

Cast aluminium normally cannot achieve the same durability as wrought/forged aluminium for turbocharger impellers. Wrought alloys are mechanically processed, which breaks down any oxide defects that might be present and refines the grain structure to improve material fatigue properties. There is typically an order of magnitude difference in grain size of cast and wrought alloys. It is also possible to choose for a different (low silicon) aluminium alloy with higher contents of alloying elements to strengthen the material in a wrought aluminium component, which further improves its fatigue properties. The base material properties of solid wrought aluminium are measurably superior and more dependable than cast aluminium alloys.

Cast aluminium impellers often fail as a result of high temperature (at high boost pressure) and material creep. The impeller then ‘grows’ and deforms and touches the compressor housing contour. At such high boost pressures the turbo is running in/over its maximum allowable speed range. Such high rotational blade-tip speeds of the rotor cause undesirable extra vibrations and frequencies, in particular when rotor speed changes. This in turn can result in impeller breakage or blade failures, often destroying the entire turbocharger in split seconds.

When cast aluminium impellers fail, it mostly is the result from metal fatigue, traceable to casting defects such as oxides or other impurities. The material properties of cast aluminium are less suitable for top end boost pressures and resulting high material temperatures. Therefore a new method of manufacturing impellers has been developed.

Using forged aluminium 2618-T61alloy (t61 stands for ’aging heat treatment and hardened by cold working’) the ‘billet’ impellers are machined by a high speed five-axis CNC operation, which is needed to achieve the same intricate aerodynamic impeller blade profile and finish required for optimal aerodynamic and mechanical performance. This also means that wheel geometry is not subject to casting surface tolerances and foundry process variability over time. It furthermore means that no compromise has to be made to the sophisticated surface blade design.

As well as better performance, it brings an inherent life improvement, even at top end boost pressures and high compression inlet temperatures.

The comparably expensive 5-axis CNC milling technology versus the casting process, plus the more expensive forged aluminium alloy used, both come at an increased cost and therefore price.

But the higher price of the billet impeller over the cast version quickly pays back in durability, increased performance and long life.

At high boost applications it greatly reduces the risk of destroying a complete turbo caused by impeller wheel failure, so it is an investment well worth to consider.

Billet impellers currently are only available for certain turbo models and inducer diameters. Please check our basic HX program, where these options, next to the compressor wheel size, have been marked with a star (*).

Caboturbo will further extend this program to offer an even wider range of billet impeller options in future.

Some comparisons of Billet versus Cast material properties

Next you will find a comparison of the chemical composition of a widely used good quality aluminium die casting alloy for compressor wheels, like aluminium 354 and 413 compared with wrought/forged aluminium A2618-T61 showing the difference in percentages of other alloyed metal elements:

                                               Al 354 Al 413 Al 2618-T61

Silicon (Si)                            8.6-9.4          11.0-13.0      0.25 max

Iron (Fe)                               0.2                  2.0                  0.9-1.3

Copper (Cu)                         1.6-2.0          1.0                  1.9-2.7

Manganese (Mn)                 0.1                  0.35                -

Magnesium (Mg)                 0.4-0.6          0.10                1.3-1.8

Nickel (Ni)                            -                      0.50                0.9-1.2

Zinc (Zn)                               0.1                  0.50                -              

Tin (Sn)                             -              0.15                -

Titanium                               0.2                  -                      0.04-0.1

Others                                   0.15                0.25              0.15 max

Aluminium    balance        87-88 %        83 %             92-94 %        (approximately)

Aluminium forged 2618-T61 alloy is containing higher concentrations of the elements copper and magnesium and is age hardenable for good strength in applications that involve exposure to heat up to 300/575 degrees C/F. 2618-T61 is commonly used in the manufacture of aircraft engines. 2618-T61 contains a very low (0.25 max) percentage of Silicon (see next remarks).

Like all aluminium die casting alloys, both alloys 354 and 413 contain a high percentage (8.6-13.0) of Silicon for good fluidity and castability of the alloy. The disadvantage is the formation of mainly silicon oxides during the casting process, next to other casting impurities. Temperature limit for die cast aluminium is 240/465 degrees C/F.

                                   

This indicates a temperature limit advantage for billet versus cast of 25 %.

High boost Truck- and Tractorpulling applications often exceed these temperature limits by up to 20 %, but field experience learns, that these fairly short duration overheating conditions create no material problems in practice.

Also Physical data like Density, Gravity, modulus of Elasticity Tension and Torsion etcetera are far better. Furthermore the Mechanical data and properties like Shear-, Tensile-, Strain- and Compressive strength etcetera of 2618-T61 are comparably superior over any Aluminium die casting alloy.

As mentioned before in this chapter the grain structure of forged material is infinitely far superior to the anisotropic structure of a lump of cast metal.

Therefore the danger of dislocation movement, which is the mechanism by which plastic deformation of metals occurs, is brought well within acceptable limits.

Microscopic research of both these alloys has proven, that the grain size of the wrought alloy is nearly 10 times smaller than the cast grain structure. Grains are indicated by coulors in below pictures.

Microscopic pictures of material grains

            Wrought: magnification 900x            Cast: magnification 100x

                     

Pictures of impeller wheels

                           

              Billet wheel HX50                                    Cast wheel HX50                                                                          (also shot peened)