Eliminate Aluminum Casting Porosity, Shrinkage via Liquid HIPping.

A cousin to traditional gas HIPping, this process has a reduced cycle time, making it more economical as part of a standard cast component production cycle.

The demand on foundries to reduce the cost and lower the weight of their cast components is a daily struggle. End-users, due to fuel economy standards and the need to provide an economical end-product to their customers, are putting the onus on their suppliers to implement technology to provide components that maintain the necessary mechanical and physical properties while also achieving the weight and cost savings.

For the automotive industry, the result of end-user demands has been the conversion of many cast iron and steel components to aluminum. However, high-cost raw materials and production processes and strict quality requirements have made these conversions high-cost solutions. When components such as those made for automotive applications are cast in aluminum, there is little margin for error in regard to mechanical properties such as tensile and fatigue strength. In addition, there is little room for component defects such as porosity.

In pursuit of a new technology to improve aluminum casting quality, the Hot Isostatic Pressing process (HIPping) was developed as a value-added service to eliminate porosity from aluminum castings.

HIPping

In commercial use, the HIPping process has been reserved for aerospace and auto racing cast components due to the cost this process adds to components. In HIPping, castings are placed in a chamber and slowly heated while pressure is increased on an inert gas that surrounds the castings. This pressure, which then is exerted on the casting, helps collapse any of the internal porosity (inclusions or shrinkage) left in the component after casting. After the process, the components are cooled to room temperature.

This process hasn't become a standard for aluminum castings because of its low ratio of cycles per hour (typically 8-hr cycles) and resultant high cost. This was the impetus for developing the Liquid HIPping (LHIP) process.

As its name implies, LHIP uses a liquid media instead of a gas to apply the isostatic pressure on the cast components during treatment. This liquid media is able to reach every outside surface area of the casting and apply uniform pressure around the casting. The liquid media used are molten salt mixtures that are routinely used by heat treaters. A specific salt is selected based upon its working temperature range.

Because the LHIP temperatures are in the range of the solution heat treatment for aluminum, economy is gained by combining those two operations. A water quench routine follows the LHIP process to complete the solutioning cycle. As a result, foundries do not have to utilize additional energy to heat components for LHIPping.

In production, the components are placed in the high-pressure chamber filled with the molten salt liquid media. The cover to the chamber is closed. A plunger from the cover then is introduced to compress the liquid. A short stroke by the plunger via hydraulic pressure will make the liquid pressure increase drastically. Once the hydraulic force pushing the plunger is equally contrasted by the reaction of the liquid, the plunger stops moving as the ideal liquid pressure is reached and can be held for the required length of time.

The following process parameters have been used to achieve successful results:

* pressure: 1100-1200 bar;

* solution temperature: 860-996F (460-535C) (depending upon the alloy);

* cycle time: 200 sec (including loading and unloading of the system.

Mechanical Properties Achieved via LHIP

The combination of heat and pressure on the castings can eliminate shrinkage and hydrogen ([H.sub.2]) porosity. Shrinkage porosity will collapse under the force of the pressure and the [H.sub.2] porosity will go into solution due to the heat and pressure. However, nitrogen ([N.sub.2]) porosity resulting from gas entrapment in the mold cannot be reduced by this process because [N.sub.2] cannot go into solution in aluminum alloys as once the pressure is relieved the porosity opens up again. In addition, surface cracks and exposed shrink cavities cannot collapse in the process because the LHIP liquid will fill the voids without effect.

In the initial development of the process, a safety-critical A356 suspension arm was chosen as a test specimen to compare components that were and were not LHIPped. Tables 1 and 2 detail the bending and tensile strength differences respectively. In Table 1, note the increase of deformations before cracks and the decrease of standard deviation. As a result, the designer would have more reliable properties to work with on a LHIPped component. Table 2 shows little increase in the ultimate tensile strength and yield tensile strength after LHIPping, however, elongation increased 41%.

In a fatigue bench test that measures load on the component as in use, the LHIPped suspension arm withstood a 30% increase in the load beyond that which would occur in actual issue. These results are stated in Table 3. In terms of actual production, the LHIP increased fatigue lift by 35%. Since the suspension arm being tested was a low-pressure permanent mold casting, these results demonstrate that a redesign of the component is possible using reduced wall thickness.

Component density was another property checked before and after LHIP treatment. Fig. 2 shows the results.

The true economic benefits of the LHIP process won't be realized until further testing is performed on aluminum castings produced via high-production sand processes. The use of LHIPping in conjunction with low-pressure permanent mold may not be as cost-conducive as using the process in conjunction with the lower cost sand casting.

                 Bending Test Results on a Low-Pressure
                        Cast A356 Suspension Arm
                    Production    Production     Increase
                     'f' (mm)   w/LHIP 'f' (mm)
Zone 1                 8.5           10.5         25.3%
Zone 2                  9            10.6         17.8%
Zone 3                 8.3           10.9         31.3%
Zone 4                 8.1           10.5         29.6%
Average                8.48          10.62        25.2%
Standard Deviation     0.38           0.2           --
                Tensile Strength Property Comparison of
                an A356 Suspension Arm With and Without
                                  LHIP
                                 Production  Production
                                               w/LHIP
Ultimate Tensile Strength (Mpa)     307         309
Yield Tensile Strength (Mpa)        251         261
Elongation (%)                      7.8          11
                  Fatigue Bench Test Comparison of an
               A356 Suspension Arm With and Without LHIP
                    Production      Heat Treat
                      w/T-6      Production w/T-6
                                Heat Treat and LHIP
Cycles to Failure    410,000          550,000
Standard Deviation   100,000           70,000

[Graph omitted]