Laser Peening Enhances the Life of Die Casting Dies

Introduction:

Die casting has evolved to become one of the major manufacturing methods for component production. It is a metal casting process that forces molten metal under high pressure into a mold cavity. Once the part is formed and cooled, the part is ejected, and the die is quickly prepared to cast the next part. Because of the rapid cycling, the die experiences a thermal-mechanical cycle that usually leads to die failing after producing a set number of parts.

This Industry continuously investigates various approaches to extend the die life to reduce cost and down time. These includes different die materials or material manufacturing to extend die life. Other approaches include post-processing techniques to enhance the life of dies. The most commonly used technique is Nitriding but other approaches include surface coatings applied by vapor depositions or plasma spray are used to enhance die life.

The Die Casting industry has recently demonstrated the significant benefit of Laser Peening as a surface enhancement process for mitigating cracking in die cast dies.

Laser Peening – How It Works:

Laser peening, a well-proven process for enhancing the fatigue life of metallic components, has recently shown an outstanding ability to enhance the thermal mechanical fatigue (TMF) life of die casting dies. This process uses a high-energy short-pulse laser beam to create a shock wave on the surface of a part that cold works the component surface. This shock wave produces a deep compressive residual stress profile at the surface that is thermally stable and hinders the propagation of surface-initiated fatigue cracks. Laser peened die cast dies outperformed the majority of die materials and post-processes up to 10 times. The laser peening process is localized to the critical failure location of the dies, mitigating the need for full component coverage.

“Tests conducted at Case Western Reserve University that were funded by the North American Die Casters Association (NADCA) demonstrated that laser peening is capable of almost mitigating any crack initiation even after 20,000 thermal cycles. This result demonstrates the significant potential laser peening has to enhance the TMF life of die casting dies,” says Micheal Kattoura, Materials Research Engineer at LSP Technologies, Inc. (LSPT).

Recently, die casting companies have started to utilize additive manufacturing to produce die cast dies due to its greater flexibility this process offers to produce intricate features and cooling channels within them. However, additive manufactured components suffer from lower fatigue life. Laser peening has provided significant fatigue life enhancement to other additively manufactured components and can be used to extend the life of die cast dies.

You can read more about the benefits of laser peening on additive manufactured parts here: “Laser Peening to Enhance Addictive Manufacturing”

Testing Methodology – Dunker Test and Die Casting Die Failures:

The Dunker Test was designed more than 50 years ago at Case Western Reserve University as a highly repeatable test that mimics the thermal-mechanical fatigue cycling experienced by die casting dies (Benedyk, 1969). Since its design, the Dunker Test has been utilized by the NADCA to characterize the performance of various die casting materials and surface enhancement processes to quantify their performance enhancement over the baseline H13 tool steel.

The Dunker Test uses a 2” x 2” x 7” rectangular sample to investigate new surface enhancement processes or new die cast die materials.  In this test, the long corners of the sample are evaluated for specific defect types.  For evaluating surface enhancement processes the samples is made from standard H13 according to NADCA requirements. The center of sample in hollow to allow flowing cooling water within the sample to mimic the die casting thermal gradients and cycling. The sample is cycled between immersion onto a molten aluminum bath for a specified time and withdrawn from the aluminum bath to allow cooling which includes spraying it to prevent aluminum sticking all of which induces thermal cycling. The sample is then inspected every 5000 cycles for thermal mechanical fatigue (TMF) cracks at the corners by polishing the corners and the measuring crack length and crack densities.  The test is continued until 20,000 cycles have been accumulated.

There are two features that are measured on the specimen corners to characterize die life; crack length and crack area.  These two main failure modes seen in die casting dies are characterized as follows:

1. Die cast dies fail due to deep cracks that initiate and grow to a critical length and cause die failure. These crack lengths are measured, and the parameter “Average Maximum Crack Length” is used to characterize this failure mode.
2. Die cast dies form high density of cracks that causes sticking of the cast part to the die preventing part ejection.  In this case, the high density of cracks formed due to heat checking which allows molten metal to penetrate the cracks and hold the cast part to the die. The area of these cracks is measured, and the parameter “Total Crack Area” is used to quantify this failure mode.

Laser Peening Results:

Laser peening was applied to a baseline Dunker Test H13 sample using two laser peening conditions. Two corners of the sample were processed with the typical processing conditions used for die casting dies and the other two corners were process with an experimental processing condition aimed at reducing the process time. It is worth noting that laser peening did not change the surface roughness of die sample.

The graphs below show 1) the Historic data (Benedyk, 1970) used to compare the performance of the new die casting materials and processes, 2) the average data from 13 samples representing new materials and process technologies, and 3) the average of all 4 corners of the laser peened sample. The laser peened sample did not show any cracks up to 15,000 cycles for the two process conditions. No cracks were detected on the typical processing conditions even at 20,000 cycles. Three micro-cracks (around 20 um in length each) were detected on the experimental processing condition at 20,000 cycles. This result demonstrates there is a slight benefit of the typical processing conditions over the experimental processing condition.  The average of these two laser peening conditions is plotted in the graphs.

In comparison to the Historic data (Benedyk 1970), laser peening provided 77 times reduction in crack length and 17,500 times reduction in crack area after 20,000 cycles. The quality of today’s H13 die cast material has dramatically improved over the years due to improved manufacturing processes and has performed better than the initial H13 baseline data originally generated by Benedyk. In comparison to the new die cast materials, laser peening has provided 10 times reduction in crack length and 4,580 times reduction in crack area at 20,000 cycles. This test demonstrates that laser peening significantly outperformed the newer die materials and other surface enhancement technologies.

These results indicate laser peening drastically improves the life of die cast dies and addresses both modes of die failure, crack length, and crack density. Laser peening provides high magnitude and high depth compressive residual stresses that counteract the TMF from the die casting process resulting in enhanced fatigue life. The compressive stresses generated by laser peening are thermally stable within the die cast material allowing a larger portion of these residual stresses to be retained even when operating at elevated temperatures.

Benefits to the industry

Laser peening significantly improves the thermal mechanical fatigue life of die cast dies.  It provides the die casting industry with the ability to reduce maintenance costs and equipment down time. Laser peening can be applied to a large array of die sizes from small die inserts to large die assemblies and a variety of die geometries.

Who is using Die Casting Dies?

Die casting is a very common process used across a number of industrial sectors. Some of the main industrial sectors that rely on die casting are automotive, aerospace, heavy machinery, firearms, consumer appliances, construction, and oil and gas. To support this large array of industries and a huge market, there are many die casting companies that support different aspects of the industry from material suppliers, to die cast die manufacturers to die equipment manufacturers.

North American Die Casters Association  

The North American Die Casting Association (NADCA) represents the die casting industry within North America. NADCA is committed to promoting industry awareness and actively support research to promote new technologies to advance the state of the art for die casting industry. 

Case Western Reserve University – Case Metal Processing Laboratory 

The Case Metal Processing Laboratory (CMPL) at Case Western Reserve University collaborates with Federal Agencies, Industry, Professional Societies and Associations, National Laboratories, and other Universities to promote and advance metal processing technologies by conducting cutting edge research and development. 

LSP Technologies 

LSP Technologies is the trusted leader in laser peening for metal forming and fatigue mitigation for customers around the world. In addition to a robust laser peening services business, LSPT provides laser peening equipment and custom-engineered laser peening solutions.