In this second of two articles, authors Van Tyne and Walters examine defects that can occur in forgings because of microstructural irregularities or ductile fracture.
In the first of this two-part series on defects, we covered geometrical defects that can occur during a forging process – laps, folds, underfills and geometrical discrepancies. The use of simulation software to analyze the forging process can be an effective method to determine the underlying cause and solution for geometrical defects. In this final part, we will discuss the material-based problems that can lead to defects due to microstructure and ductile fracture.
It should be remembered that forging is an inherently sound process that can enhance the properties of the component being produced. Forging creates a component that is better than one created by a casting or machining process. Although defects are not intrinsic to the forging process, a forger may occasionally find a defect in the final product. It then becomes the responsibility of the forger to identify the cause of any defect that occurs and implement appropriate corrective actions.
When the press or hammer dies close, the workpiece will move based on the path of least resistance. For geometrical defects, it is imperative that both die and preform design create this least-resistance path so that a sound forging can occur. For material defects, we must understand the natural limitations that exist within the material and ensure that the forging operation is designed to work within the boundaries that these limitations impose.
There are three types of material-related defects that are primarily associated with the microstructure of the workpiece metal. These defects are:
- Dead zones
- Incipient grain-boundary melting
- Shear bands (flow localization)
There are other material-related defects associated with a ductile fracture process. These defects include:
- Surface cracking
- Internal fracture (chevron cracks)
- Shear or trim operations
The input material for a forging can range from cast metal through fully wrought stock. The exact choice depends on the alloy system, the size of the forging, the component application and customer requirements. For many alloys, especially non-steels – such as some aluminum alloys, nickel-based superalloys, titanium alloys and copper-based alloys – a minimum deformation (strain) is often required during the final forging operation(s) to achieve specified mechanical properties.
In contrast, most steels – excluding stainless grades – are forged into the correct geometrical shape and subsequently will be heat treated to obtain their final mechanical properties. Microalloyed forging steels are an exception, as their final properties are often obtained by controlled cooling of the component after it leaves the hammer or press. Minimum strain requirements are frequently imposed to ensure a forged microstructure throughout. With the proper microstructure, it’s possible to obtain appropriate properties.
Dead zones are regions in the forging where little or no deformation occurs. For example, in a direct extrusion process with a flat die opening, the material in the corner regions adjacent to the die but far from the opening does not experience much deformation as the ram moves forward. This corner region is a dead-zone area. Figure 1 shows a dead zone in a gridded half billet. In this figure the material in the corner regions has not experienced any deformation. Since the material in the dead-zone region may not have been deformed sufficiently, it’s possible that the microstructure and mechanical properties could be less than optimal.
Dead zones can be predicted in a process model when the effective strain values are very low. The redesign of the forging or extrusion die may create a situation in which an appropriate amount of strain is imposed to the material resulting in a good microstructure.
INCIPIENT GRAIN-BOUNDARY MELTING
During forming, deformation energy is converted to heat. This conversion process is often called adiabatic heating. When the forging temperature is initially high, large amounts of plastic strain can result in local temperatures becoming very high. In extreme cases, grain-boundary melting can be observed when the initial forging temperature is too high or when a severe case of flow localization exists.
Grain-boundary melting in steels is often called “burning” the steel. Even if actual melting is not observed, incipient melting can result in a local microstructure with unacceptable properties. For example, the result of incipient grain-boundary melting can reduce mechanical properties, the toughness or the fatigue life of a component.
The causes of incipient grain-boundary melting are the inherent compositional variations of any metal alloy. The grain-boundary regions tend to accumulate some of the tramp elements (or residuals) present in the metal. Because of the difference in local composition, the local melting point is often far below the melting point of the alloy as a whole. With localized heating, localized compositional variations and localized low melting point, a small region around the grain boundary can become weakened or even melt. In the worst-case scenario, a fracture or a crack can form in this region during the forging process. The mechanical properties of the finished forging – such as fatigue and fracture toughness – will be diminished if these regions are melted during deformation. To avoid this problem, the forging temperature must be properly controlled. It is often recommended that the maximum forging temperature for steels be at least 300°F below the melting point of the alloy.
FLOW LOCALIZATION/SHEAR BANDS
During deformation, material follows the path of least resistance, and it is subject to adiabatic (deformation) heating. Flow localization can occur due to the geometrical or thermal restraints of the surrounding material. The resulting defect can appear as a lap, fracture, incipient grain-boundary melting, or material with poor mechanical properties or microstructure. Shear banding is more frequent in materials that exhibit flow softening during deformation such as aluminum. Flow localization is intentionally utilized to allow trimming, blanking and machining operations.
Figure 2 shows a sequence in a forging operation in which flow localization occurs. Because of the inherent material properties of the workpiece, the deformation becomes localized and a “snowball” effect occurs. The localization causes the local temperature to rise, causing the material in the local region to be softer. This results in more localized deformation, causing the local temperature to rise and a shear band to form. Typically, an appropriate redesign of the preform or die cavity will allow you to avoid this material-limited localized flow.
Ductile fracture is a crack or tear that exists after plastic deformation. No single law of physics or fracture model describes all modes of ductile fracture, whereas brittle fracture occurs before or shortly after yielding. Research has demonstrated promising models for ductile fracture, but no complete model exists.
Ductile-fracture models are an active area of ongoing research. Most of these models are based on the concept of mechanical energy accumulation, which quantifies the energy that is internally stored within the metal. Once this accumulation reaches a critical material-dependent value, a fracture will occur. Debate still exists as to the nature of the accumulation function and the proper the critical value.
For forgers, three areas of ductile fracture are of particular interest:
- Surface cracking
- Internal fracture (chevron cracks)
- Intentional shear, trim and machining operations
Surface cracking is an issue with materials that are difficult to forge. Optimum process conditions – temperature, stress state, strain rate, input material – can maximize the deformation prior to fracture. Figure 3 shows severe fractures on both the outside and inside of an automobile component.
Internal fractures have been a topic of research for decades. A variety of fracture models have been developed for these cases. Figure 4 depicts two versions of a transmission shaft that experienced internal fractures. The left side shows the chevron crack, and the right side shows the improved process.
While fracture is reputed to be a defect, controlled fracture is part of most processes. Blanking, shearing and trimming are all well-known processes that take advantage of controlled flow localization and ductile fracture to separate flash, punch-outs and others. Figure 5 demonstrates a blanking/trimming process.
Most forging defects can be understood and quantified based on fundamental principles. The material will follow the path of least resistance. The forging-die designer needs to ensure that die cavities and preforms cause the least resistant path of material flow to correspond to a path that produces the desired final shape. Forged metals also have intrinsic limitations. The forging process must be carefully designed to remain within these material-related boundaries.
Although forging is a process that produces inherently superior products, defects can occasionally occur. Defects can be present when:
- Die and preform designs are not optimum.
- Process control is inconsistent.
- Process is not clearly understood.
- Equipment is not well maintained.
Human error or equipment failures can also result in defects.
Although some defects are not fully understood and require additional fundamental research at universities and government labs, leading companies will have systems in place to prevent, understand and promptly resolve any potential problems before they get out of hand.
In this two-part series on forging defects, we have covered both geometrical defects (FORGE, April 2007) and material-related defects. The cure of geometrical defects such as laps, folds, underfills and geometrical discrepancies requires a change in the design of the forging die or preform shape. Simulation software can be very helpful in determining the causes of and solutions for these defects.
The material-related defects due to microstructure and fracture behavior are covered in this article. In order to avoid these types of defects, the forging engineer must design the process to work within the limitations inherent to the material itself. These solutions require that the forger become familiar with the metal workpiece behavior and the metallurgical limitations that exist for each individual alloy that is used.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is made possible by the dedicated team of professionals from the Department of Defense and industry. These teammates are determined to ensure that the nation’s forging industry is positioned to meet the challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). The material presented herein by Scientific Forming Technologies Corporation and the FORGE-IT team was originally prepared for the Forging Fundamentals 101 course.
This article is featured in FORGE's 10th Anniversary: 10-Top Forging Industry Articles. Read the other featured articles and vote for your favorite!