Page 20 - PR 2014 2016 10 Materials and Nanotechnology

296 Materials and Nanotechnology | Progress Report

sonable cost, but also high fatigue resistance. by residual stress measurements. The alumi-
Moreover, the Al alloy parts have a rather high num alloys studied are from the Al-Mg-Si and
residual value, because the aluminum is fully Al-Zn-Mg types (6005A, 6082 and 7108).
recyclable at the end of the useful life of the
part. Automotive cylinder liners are mechan- Ti-6Al-4V and γ-TiAl alloys
ical components with the function of being
an internal coating of automotive engine cyl- Ti-6Al-4V alloy has found applications at el-
inders. Among the different foundry casting evated temperatures and as biomaterial. The
methods, the centrifugal casting technique alloy Ti-6Al-4V is one of the most widely used
relies on a tubular die with a high rotational titanium alloy because of an excellent combi-
speed during pouring (1,700 rpm). The dynamic nation of properties like strength, toughness,
effect of the high rotational speed promotes corrosion resistance and chemical stability.
high centrifugal acceleration, reduces porosity Surface modification of the Ti-6Al-4V alloy
and promotes some density segregation effect has been carried out to improve mechanical
that cannot be achieved in static foundry pro- properties at elevated temperatures and fa-
cesses. The centrifugal casting process using tigue resistance. The surface modification that
the Al-Si alloys for cylinder liners can induce was carried out was plasma nitriding. The
some reduction in the silicon content, mainly nitriding was performed by varying param-
with low hypereutectic compositions com- eters as time, temperature and gas mixture
pared to the materials used in static process- in the atmosphere. The gas mixture of Ar: N :
2
es. The high centrifugal acceleration prompts H (0.49: 0.49: 0.03) at 700°C for 4 hours was
2
silicon to run mainly in the inner diameter the chosen condiction. Creep and hot tensile
direction, due to its lower density compared tests were performed to evaluate the mechan-
to that of the aluminum alloy, even in the ical properties at elevated temperatures and
liquid state. Thus, with low silicon content on fatigue test at room temperature. The phases
the outer side, it becomes easier to guarantee of the nitrided sample detected by XRD were
wear resistance in the cylinder/piston and ring the ε-Ti N and δ-TiN, in addition to the α-Ti
2
working surfaces, as well as the mechanical and β-Ti matrix phases. The layer thickness
properties requirements. The aluminum alloys of this sample was about 1 μm. Hot tensile
studied are Al-19%Si with additions of up to tests were performed in a temperature range
5%Mg and 5%Cu. from 500°C to 700°C in nitrided and no ni-
trided material and show an increase in the
Extruded aluminum alloys find applications strength of the nitrided alloy. The creep tests
as automotive components. For automotive were also performed in a temperature between
components, adequate mechanical strength/ 500°C and 700°C and stress range from 125 to
weight ratio at a reasonable cost is essential. 319 MPa. The result was an increase in creep
Moreover, these materials should be resistant resistance of the nitrided sample. This was
to fatigue when used as components exposed evidenced by both the decrease in secondary
to cyclic loads. Precipitation hardening is an im- creep rate and an increase in final creep time.
portant method used to increase the strength Fracture of the tensile tested specimens was
of some aluminum alloys. Shot peening is a predominantly ductile with microcavities. The
process to cold work the surface and is used to selected alloy after ion implantation by plasma
increase the fatigue life. This can be evaluated immersion was submitted to creep tests at

Instituto de Pesquisas Energéticas e Nucleares