Combining Precipitation Hardening with Laser Shock Peening on a Die-Casting Tool Steel
Laser Shock Peening (LSP) is an innovative  mechanical surface engineering technique used for improving the mechanical fatigue resistance of metal components by inducing significant surface compressive residual stresses. During LSP, the surface of the treated component, usually protected with a thin layer of an absorbent coating and covered with a transparent confining medium, is exposed to nanosecond-long laser pulses of intense energy. The material in the interaction area is vaporized by the laser beam and transforms into plasma, which generates pressure on the surface by transmitting shock waves into the treated material . The mechanical effect of shock waves that exceed the metal's dynamic yield strength can generate compressive residual stresses in the treated surface to depths as great as to 1 mm.
Since the beginning of its development in the early 1960s [3, 4], researchers have investigated the effects of LSP on the surface and mechanical properties of numerous metals, such as stainless steels, aluminium alloys, titanium alloys, and Inconel. However, very little is known about the effects of LSP on the surface integrity and mechanical properties of tool steels. Studies in recent years [5-8] have shown that SP, by generating compressive residual stresses and inducing strain hardening, can improve the wear resistance and thermo-mechanical fatigue resistance of tool steels, such as AISI M2, D2, and H13. In the study presented in this article, the effects of LSP on a maraging die-casting steel were investigated by analysing the surface integrity before and after different combinations of the laser treatment and precipitation-hardening stage.
Maraging steel X2NiCoMo18-9-5 (DIN 1.6358), which can be used as a structural steel for added value applications as well as a tool steel for die-casting, was the chosen material for our study. Maraging square-shaped specimens with 9.5 mm thickness were solution-annealed for 1 h at a temperature of 820 °C. Air quenching was followed by precipitation hardening, performed for 3 h at a temperature of 480° C. The ground surface of the heat-treated maraging specimens was exposed to LSP, conducted with a Q-switched Nd: YAG laser operating at a wavelength of 1.064 ?m, a laser pulse energy of 2.8 J, a laser pulse duration of 10 ns, and a laser spot size of 1.5 mm. In one case, LSP was carried out between the solution-annealing stage and the precipitation-hardening stage. To evaluate the surface integrity of untreated and LSP-treated specimens, topography, roughness, surface residual stresses, and in-depth microhardness were analysed.
Topography and surface roughness
Surface roughness was analysed by measuring the arithmetical mean deviation Ra and maximum peak-to-valley height Rt of the surface profile. A Surtronic 3+ contact profilometer was used for this purpose.
Ra and Rt measurements results of the untreated and LSP-treated maraging steel specimens are represented in Figs. 1 and 2. When using the same LSP-processing parameters, the generated surface roughness between the annealed specimens and hardened specimens was nearly the same. Ra and Rt of the ground specimens, using 800 grit paper, amounted to 0.2 ?m and 3 ?m respectively. The increase in surface roughness after performing LSP is significant. The maximum surface roughness was achieved on the precipitation-hardened specimen treated with a laser pulse density of 2500 cm-2. As can be observed in both figures, the greater scattering of Ra measurement results at the highest pulse density indicates a less uniform surface roughness. Differences in surface topography between untreated and LSP-treated specimens can be observed in Fig. 3: a-c. The parallel grinding scratches on the laser-untreated specimen (Fig. 3a) disappeared after LSP, which generated numerous small craters caused by a combination of ablation and plasma pressure. The distinctive pattern and the longitudinal direction of the laser beam can be observed in Fig. 3c, which shows the surface of a maraging specimen treated with a 2.5 mm laser spot diameter and 900 cm-2 pulse density. By reducing the spot diameter and increasing the pulse density, which also leads to a coverage increase, the lines of the laser path become barely visible (Fig. 3b).
Surface residual stress
The residual stresses in the thin surface layer of untreated and LSP treated specimens, using the 2500 cm-2 pulse density, were analysed in accordance with the standard hole-drilling strain-gage method ASTM E837. In this measurement technique, a small hole is drilled through the geometric centre of a strain gage rosette. The material removal allows residual stresses, present in the surface layer, to relax and strains to occur. The strain gage rosette, properly attached to the investigated surface area, is employed for detecting strains, which occur during material removal while drilling a hole into the surface using a milling guide. Afterwards, the residual stresses and their orientation are calculated from the measured strains.
The residual stress measurement results before and after performing different sequences of precipitation hardening and LSP are represented in Fig. 4. Each bar chart indicates the difference between the measured maximal and minimal principal stress at the corresponding depth. As can be observed, the slight compressive state of the solution-annealed specimen (SA) almost disappeared after precipitation hardening (SA+PH). Low residual stresses are a consequence of relaxation due to a 3h exposure to the precipitation-hardening temperature. According to results, compressive residual stresses were successfully generated after LSP in both the solution-annealed specimen (SA+LSP) and the precipitation-hardened specimen (SA+PH+LSP). The results show that compressive residual stresses are present even when LSP is performed between solution annealing and precipitation hardening (SA+LSP+PH). Despite a decrease after precipitation hardening, the surface compressive residual stress of SA+LSP+PH is comparable to the surface compressive residual stress of SA+PH+LSP.
The microhardness profile in the surface layer of untreated and LSP-treated specimens was analysed by measuring the Vickers microhardness. The indentation force was performed with a 200 g load and 15 s load time. The obtained microhardness profiles of untreated and LSP-treated solution-annealed specimens and precipitation-hardened specimens are represented in Figs. 5 and 6 respectively. The average microhardness of the solution-annealed specimen before LSP amounted to 340 HV0.2. After the laser treatment, the microhardness increased to 368 HV0.2, indicating the presence of strain hardening caused by the mechanical effect of shock waves. The microhardness increase is also present after laser peening the maraging specimens in the precipitation-hardened state, as can be observed in Fig. 6. After LSP, the microhardness of the precipitation-hardened specimen increased from an average of 669 HV0.2 to 742 HV0.2, the maximum measured profile value belonging to the maraging specimen treated with 2500 cm-2 pulse density. A comparison of the microhardness profiles of the precipitation-hardened state indicates that a greater strain-hardening effect occurs when using a higher LSP pulse density. The microhardness increase remains present even after the LSP-treated solution-annealed specimen undergoes precipitation hardening (SA+LSP+PH), achieving a maximum of 716 HV0.2.
Different sequences of the laser shock peening and precipitation hardening on a maraging die-casting tool steel were investigated. Measurements results indicate that LSP successfully induced compressive residual stresses into the surface layer. LSP, performed after solution annealing, generated the highest compressive residual stresses. The surface compressive stress achieved after the precipitation hardening of the LSP-treated solution-annealed specimen was comparable to the surface stress of the LSP-treated precipitation-hardened specimen. The analysis of the surface integrity indicates a significant increase in the surface roughness after LSP when using higher laser pulse densities. Therefore, optimal peening parameters must be investigated in order to avoid extensive surface finishing stages, which can induce undesirable residual stress in the surface layer.
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Luca Petan(a), Researcher
Prof. Dr. José Luis Ocaña(b)
Prof. Dr. Janez Grum(a*)
(a) University of Ljubljana, Faculty of Mechanical Engineering, Slovenia
(b) Polytechnic University of Madrid, Laser Center, Spain
(*) Corresponding Author