Residual stress formation in H13 tool steel produced by Additive Manufacturing

 
Ryan Cottam (Swinburne) and Vladimir Luzin (ANSTO)
 
Additive Manufacturing is an expression for a group of technologies that can build engineer components up in a layer-by-layer fashion. It is sometimes also known as 3-D printing.  There are several advantages to this group of processes, including a reduced amount material wastage and the ability to change the design digitally, which allows for customisation of an initial design to meet the needs of a customer.
 
Additive Manufacturing of metallic components is receiving world-wide research attention. Both the metallurgical characteristics of the alloy being deposited and the processing parameters used influence the quality and character of the finished product.
 
The repeated heating and cooling that occurs as the layer are deposited results in the formation of residual stresses. Residual stresses can be beneficial to the properties of the material, if they are compressive in nature, as they can improve the structural integrity of the component. However if the residual stresses are tensile in nature,  they weakens the structural integrity, resulting in the material being prone to cracking in service, particularly fatigue cracking.
 
The production of dies for die casting and forging is a potential application of Additive Manufacturing, as customised designs can be produced with shorter lead times with reduced material wastage, when compared with the traditional machining method.
 
H13 is a tool steel that has been receiving attention from several research groups around the world [1-4]. While it has been demonstrated that H13 can be successfully deposited by Additive Manufacturing, free from cracks and porosity, the metallurgical character of the deposits has not been investigated, and in particular the residual stress distributions that form. 
 
Additive Manufacturing fig 1
Figure 1: Residual stress maps of a H13 tool steel-wedge measured by the KOWARI strain scanner, The Bragg Institute, ANSTO, Sydney, Australia.
 
Neutron diffraction measurements have been made to calculate the residual stress maps of the H13 tool steel block shown in Figure 1. The wedge geometry was chosen to elucidate the effect section thickens on the residual stress distribution. Microstructural characterisation of the wedge after residual stress measurements was also conducted, including microhardness traverses (see Figure 2), scanning electron microscopy and optical microscopy.
 
 
Additive Manufacturing fig 2
Figure 2: Microhardness map (Vickers) along the central section of the H13 tool steel wedge, where the tip of the wedge is on the right hand side.
 
As can be seen from the residual stress profile in Figure 1, the top 4mm of the wedge is in compression at a value of around 250-300MPa. This is ideal for die casting or forging dies as the compressive residual stress will resist the formation of thermal fatigue cracks, a common problem for tool steel dies.
 
The hardness map also reveals that the hardness in the top four millimetres is around 650 Vickers which is as hard as when produced by conventional heat treatment. Therefore dies made using Additive Manufacturing can avoid the heat treatment step in the manufacturing of a die, hence saving manufacturing costs.
 
The results of this investigation suggest the possibility that the bulk of the die could be built using a cheaper material (like mild steel) and that the top four millimetres of the die could be built using H13, thus saving on material costs. 
 
For a more in-depth analysis please see Ref. 5. 
 
References
 
  1. Maziasz, P.J., et al., Resdiual stresses and microstructure of H13 steel formed by combining two different direct fabrication methods. Scripta Materialia, 1998. 39: p. 1471-1476.
  2. Mazumder, J., et al., The direct metal deposition of H13 Tool Steel for 3-D Components. JOM, 1997. May: p. 55-60.
  3. McHugh, K.M., et al., Influence of cooling rate on phase formation in spray-formed H13 tool steel. Materials Science and Engineering A, 2008. 477: p. 50-57.
  4. Pinkerton, A.J. and L. Li, Direct additive laser manufacturing using gas- and water atmoised H13 tool steel powders. International Journal of Advanced Manufacturing Technology, 2005. 25: p. 471-479.
  5. R. Cottam, J. Wang and V. Luzin, Characterization of microstructure and residual stress in a 3D H13 tool steel component produced by additive manufacturing, J. Mater. Res. 29, 1978-1986 (2014).